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Accurately assessing cyclone intensity changes due to global warming is crucial for predicting and mitigating sequential hazards. This study develops a high-resolution, fully coupled air–sea model to investigate the impact of global warming on Supertyphoon Mangkhut (2018). A numerical sensitivity analysis is conducted using the pseudo–global warming (PGW) technique based on multiple global climate models (GCMs) from phase 6 of Coupled Model Intercomparison Project (CMIP6). Under ocean warming scenarios, the increasing average sea surface temperature (SST) by 2.26°, 2.44°, 3.45°, and 4.53°C results in reductions in the minimum sea level pressure by 9.2, 10.6, 15.7, and 19.4 hPa, respectively, compared to the original state of Typhoon Mangkhut. Rising SST increases the turbulent heat flux; to be specific, an average SST increase of 2.26°–4.53°C changes the turbulent heat flux into 177%–272% of the original value. Besides, stronger winds enhance SST cooling, including upwelling and entrainment, leading to an increase in the mixed layer depth (MLD). Tropical cyclone heat potential (TCHP) tends to be enhanced under the combined influences as the SST rises. An average increase in the SST of 2.26°, 2.44°, 3.45°, and 4.53°C leads to an increase in the TCHP of 9.94%, 9.85%, 14.67%, and 15.30%, respectively. However, future changes in atmospheric temperature and humidity will moderate typhoon intensification induced by ocean warming. Considering atmospheric conditions, the maximum wind speed decreases by approximately 10% compared to only considering ocean warming. Nevertheless, typhoon intensity is projected to strengthen under future climate change. Significance Statement: This study examines the role of global warming in typhoon intensity and the response of typhoon events to changes in the oceanic thermal structure. Sensitivity experiments considering future warming climates are conducted using a fully coupled air–sea numerical model. An average increase in sea surface temperature (SST) by 4.53°C can lead to a reduction in the minimum sea level pressure by 19.4 hPa. Ocean warming enhances oceanic mixing, potentially increasing the availability of heat energy for typhoon's development (i.e., an average increase in SST by 4.53°C leads to a 15.30% increase in heat energy). However, future changes in atmospheric temperature and humidity will moderate the intensification of typhoons induced by ocean warming. These results are expected to provide information for assessing the future changes in typhoon intensity under a warming climate, which is important for predicting and reducing sequential risks. [ABSTRACT FROM AUTHOR]

We present an improved approach to generating moist baroclinically unstable background states for f-plane-channel simulations via potential vorticity (PV) inversion. Previous studies specified PV distributions with constant values in the troposphere and the stratosphere, but this produces unrealistic static-stability profiles that decrease sharply with height in the troposphere. Adding moisture to such environments can yield unrealistically large values of convective available potential energy (CAPE) even for reasonable relative humidity (RH) distributions. In our modified approach, we specify a PV distribution that increases with height in the troposphere and the stratosphere, yielding background states with more realistic values of static stability and CAPE. This modification produces environments that are better suited for representing moist processes, namely, deep convection, in idealized extratropical-cyclone simulations. Also, we present a method for introducing moisture that preserves a specified RH distribution while maintaining hydrostatic balance. Our approach allows for a large degree of control over the initial conditions, as background states with different jet strengths and shapes, average temperatures, moisture contents, or horizontal shears can easily be obtained without changing the underlying PV formula and inadvertently producing unreasonable values of static stability or CAPE. We demonstrate the characteristics of idealized extratropical cyclones developing in our background states by adding localized perturbations that represent an upper-level trough passing over a low-level frontal zone. In particular, we illustrate the impacts of horizontal shear, moisture, and grid spacing on baroclinic-wave development. [ABSTRACT FROM AUTHOR]

Exposure to weather extremes, such as heatwaves, can cause discomfort, harm, or death in grazing cattle in pastures. While the Australian Bureau of Meteorology issues sheep graziers alerts when there is an exposure risk to chill for livestock, there is no equivalent alert for heat stress for Australian cattle. Before any such alert system can be developed, a robust assessment and comparison of relevant cattle thermal stress indices is required. This study evaluates and compares the multiyear climatology of three cattle thermal heat stress indices across Australia in the warm season months (October–March). The same indices are then used to assess historical Australian heat events where cattle died from heat exposure. These events are based off official records and survey responses from northern Australian graziers. In the seven historical heat events studied, high relative humidity combined with low wind speeds, or high solar exposure combined with high surface temperatures, exacerbated the impact of heat stress on cattle. In the two historic events where multiple compounding weather factors combined (e.g., high humidity, low winds, and high solar exposure), the cattle mortality levels were significantly high. These events were characterized by rainy conditions followed by a rapid warming, meaning cattle were likely unable to acclimatize to such dramatic temperature changes. This study highlights the need for using more than one thermal stress index when verifying cattle heat stress events and, importantly, calls for further research on standardizing the risk classifications of these thermal indices for cattle in Australia's variable climate. Significance Statement: Cattle across Australia's northern tropical and semiarid regions often experience extreme hot and humid conditions in the summer months, which increases the risk of heat stress. This is the first study of its kind to evaluate observations of cattle heat stress across Australia using indices that describe the combined effects of solar exposure, wind speed, relative humidity, and surface temperatures. These cattle heat stress indices can be used to evaluate historical cattle mortality events in feedlots and in grazed pastures. This study lays the groundwork for the development of Australian-wide cattle heat stress forecast products on the 7-day to multiweek time scales. [ABSTRACT FROM AUTHOR]

A major challenge encountered in the development of systems exposed to weather stressors, such as autonomous vehicles and unstaffed aerial vehicles, is to ensure their proper functioning under adverse rain or snow conditions. Since the sensing of the surroundings by these vehicles relies on optical sensors such as lidars and cameras, it is essential to ensure the robustness of these systems from the early stages of the project. In this respect, experiments in climatic wind tunnels provide a solution for simulating the operating conditions that the autonomous vehicles will confront. This work proposes a method based on field measurements and unsupervised machine learning to faithfully reproduce in controlled environments real weather conditions captured during wintertime in Ontario, Canada. The purpose of this paper is not to investigate correlations between observed weather conditions and the characteristics of the precipitation encountered, but rather to establish a consistent method based on outdoor disdrometer data to identify critical parameters to be simulated in climatic wind tunnels. To achieve this goal, weather data such as temperature, relative humidity, and droplet size distribution were recorded at General Motors’s McLaughlin Advanced Technology Track (MATT) using an FD70 disdrometer andWXT530 weather transmitter, both manufactured by Vaisala, installed on a car provided by the Automotive Center of Excellent (ACE) team of the University of Ontario Institute of Technology. The implementation of the proposed method allowed the identification of precipitation clusters characterized by parameters of a theoretical model for particle size distributions fitted to the collected data. [ABSTRACT FROM AUTHOR]

The negative effects of relative humidity to measurements of particulate matter (PM) due to hygroscopic growth are often not inherently handled by low-cost optical particle counters (OPCs). This study presents a new approach in constructing a miniaturized diffusion dryer, for use with an OPC mounted on an uncrewed aircraft system (UAS), namely, the DJI S900 (weight of 7.5 kg and flight endurance of 20 min) for short-term measurements under humid conditions. In this work, an OPC of type N3 (Alphasense) was employed alongside the dryer, with experiments both in the laboratory and outdoors. Evaluation of the dryer's performance in a fog tank showed effective drying from almost saturated air to 41% relative humidity for 35 min, which is longer than the endurance of the UAS, and therefore sufficient. Changes in the flow rate through the OPC-N3 with the dryer showed a 17% reduction compared to an absent dryer, but the measured PM values remained unaffected. Airborne measurements were taken from four hovering flights near a governmental air pollution station (Mannheim-Nord, Germany) under humid conditions (88%–93%) where the system gave agreeable concentrations when the dryer was in place, but significantly overestimated all PM types without it. At a rural area near the Boundary Layer Field Site Falkenberg (Lindenberg, Germany), operated by the German Meteorological Service (DWD), vertical profiles inside a low-altitude cloud showed sharp increase in concentrations when the UAS entered the cloud layer, demonstrating its capability to accurately detect the layer base. [ABSTRACT FROM AUTHOR]

Anomalies in atmospheric moisture transport are critical for drought formation, with East Asian monsoon anomalies identified as the primary driver of droughts in East Asia. However, the drought mechanism for the border region between monsoon and nonmonsoon land areas remains unclear due to complex interactions between latitudinal circulation patterns. Using a Lagrangian framework, we quantify moisture supply (MS) during spring and summer drought events within the border region, specifically Northeast China (NEC). Our results reveal that decreased MS from monsoon land areas is more crucial than nonmonsoon land areas and local areas for NEC droughts, with the local water recycling playing a more substantial role in summer than in spring. On average, summer droughts are 4 times more intense than spring droughts. Contributions to MS deficits from monsoon land areas, nonmonsoon land areas, and local areas during spring (summer) droughts are 43.2% (43.8%), 25.1% (19.2%), and 22% (33.8%), respectively. The weakened atmospheric circulation from source regions to NEC contributes to over 80% of MS deficits during droughts. Atmospheric wave trains triggered by North Atlantic sea surface temperature anomalies (SSTA) gradients, along with the weakening of the East Asian subtropical westerly jet in spring and a positive phase of the polar–Eurasian teleconnection in summer, contribute to spring and summer droughts, respectively. A sustained, record-breaking positive SSTA along the western European coast from the spring to summer excited a wave train, and resulted in the extreme 2017 spring–summer drought. These findings provide valuable insights into the drought mechanism within the interaction zone of circulation systems from different latitudes. Significance Statement: The complex interactions between atmospheric circulation patterns at different latitudes have led to an insufficient understanding of the mechanisms causing extreme drought in the border region between the monsoon and nonmonsoon land areas. This study aims to investigate the drought mechanisms in the border region from the perspective of moisture tracking. Results indicate a moisture supply deficit from the monsoon region contributes the most (approximately half) to droughts, although the majority of air parcels reaching this region originate from the nonmonsoon region. The weakening of circulation contributes to more than 80% of the moisture supply deficit during droughts, far exceeding the impact of reduced evaporation from the moisture source regions. These findings are conducive to understanding the mechanisms of extreme drought. [ABSTRACT FROM AUTHOR]

This study evaluates the methods of identifying the height zi of the top of the convective boundary layer (CBL) during winter (December and January) over the Great Lakes and nearby land areas using observations taken by the University of Wyoming King Air research aircraft during the Lake-Induced Convection Experiment (1997/98) and Ontario Winter Lake-effect Systems (2013/14) field campaigns. Since CBLs facilitate vertical mixing near the surface, the most direct measurement of zi is that above which the vertical velocity turbulent fluctuations are weak or absent. Thus, we use zi from the turbulence method as the "reference value" to which zi from other methods, based on bulk Richardson number (Rib), liquid water content, and vertical gradients of potential temperature, relative humidity, and water vapor mixing ratio, are compared. The potential temperature gradient method using a threshold value of 0.015 K m−1 for soundings over land and 0.011 K m−1 for soundings over lake provided the estimates of zi that are most consistent with the turbulence method. The Rib threshold-based method, commonly used in numerical simulation studies, underestimated zi. Analyzing the methods' performance on the averaging window zavg we recommend using zavg = 20 or 50 m for zi estimations for lake-effect boundary layers. The present dataset consists of both cloudy and cloud-free boundary layers, some having decoupled boundary layers above the inversion top. Because cases of decoupled boundary layers appear to be formed by nearby synoptic storms, we recommend use of the more general term, elevated mixed layers. Significance Statement: The depth zi of the convective atmospheric boundary layer (CBL) strongly influences precipitation rates during lake-effect snowstorms (LES). However, various zi approximation methods produce significantly different results. This study utilizes extensive concurrently collected observations by project aircraft during two LES field studies [Lake-Induced Convection Experiment (Lake-ICE) and OWLeS] to assess how zi from common estimation methods compare with "reference" zi derived from turbulent fluctuations, a direct measure of CBL mixing. For soundings taken both over land and lake; with cloudy or cloud-free conditions, potential temperature gradient (PTG) methods provided the best agreement with the reference zi. A method commonly employed in numerical simulations performed relatively poorly. Interestingly, the PTG method worked equally well for "coupled" and elevated decoupled CBLs, commonly associated with nearby cyclones. [ABSTRACT FROM AUTHOR]

There is no simple explanation for the spatial structure of near-surface relative humidity over land. We present a diagnostic theory for zonally and temporally averaged near-surface relative humidity (RH) over land based on energy budgets of an atmospheric column in radiative–convective equilibrium. The theory analytically relates RH to the surface evaporative fraction (EF), has no calibrated parameters, and is quantitatively accurate when compared with RH from a reanalysis, and with cloud-permitting simulations over an idealized land surface. The theory is used to answer two basic questions. First, why is RH never especially low (e.g., 1%)? The theory shows that established lower bounds on EF over land and ocean are equivalent to lower bounds on RH that preclude particularly low values, at least for conditions typical of the modern Earth. Second, why is the latitudinal profile of RH over land shaped like the letter W, when both specific humidity and saturation specific humidity essentially decline monotonically from the equator to the poles? The theory predicts that the latitudinal profile of RH should look more like that of water stored in the soil (which also exhibits a W-shaped profile) than in the air (which does not). [ABSTRACT FROM AUTHOR]

What determines the vortex size at the small-amplitude stage of spontaneous tropical cyclogenesis remains unclear. A doubly periodic domain is a standard setup for numerically studying this problem, but the convectively coupled standing waves inherent to the setup could directly trigger vortices, rendering an unrealistic path for tropical cyclogenesis. We increase the Coriolis parameter to suppress the wave and double the longwave radiative feedback to make the more realistic moisture–radiation instability dominant. Experiments show that the moisture–radiation instability has a short-wavelength cutoff due to the smoothing effect of convective dynamics, which includes the nonlocal convective triggering by cold pools and the nonlocal longwave radiative effect of anvil clouds. By approximating the spread of convective activity as a Gaussian filter on the column humidity, we derive a bulk convective spreading length lcp+av to parameterize the combined effect of cold pools and anvils. Using a novel diagnostic method, lcp+av is shown to be around 10 km. The contribution of cold pools and anvil clouds to convective spreading is comparable in the doubled radiative feedback experiments. An extrapolation to the normal radiative feedback state shows the anvil clouds play a smaller yet nonnegligible role. [ABSTRACT FROM AUTHOR]

Accurately simulating the Madden–Julian oscillation (MJO), which dominates intraseasonal (30–90 day) variability in the tropics, is critical to predicting tropical cyclones (TCs) and other phenomena at extended-range (2–3 week) time scales. MJO biases in intensity and propagation speed are a common problem in global coupled models. For example, the MJO in the Navy Earth System Prediction Capability (ESPC), a global coupled model, has been shown to be too strong and too fast, which has implications for the MJO–TC relationship in that model. The biases and extended-range prediction skill in the operational version of the Navy ESPC are compared to experiments applying different versions of analysis correction-based additive inflation (ACAI) to reduce model biases. ACAI is a method in which time-mean and stochastic perturbations based on analysis increments are added to the model tendencies with the goals of reducing systematic error and accounting for model uncertainty. Over the extended boreal summer (May–November), ACAI reduces the root-mean-squared error (RMSE) and improves the spread–skill relationship of the total tropical and MJO-filtered OLR and low-level zonal winds. While ACAI improves skill in the environmental fields of low-level absolute vorticity, potential intensity, and vertical wind shear, it degrades the skill in the relative humidity, which increases the positive bias in the genesis potential index (GPI) in the operational Navy ESPC. Northern Hemisphere integrated TC genesis biases are reduced (increased number of TCs) in the ACAI experiments, which is consistent with the positive GPI bias in the ACAI simulations. [ABSTRACT FROM AUTHOR]

The mid-Pliocene (approximately 3.3–3.0 Ma) was one of the past geological warm periods. Since this past warmer climate was in many respects comparable to near future warming projections, how the regional monsoonal rainfall changed during the mid-Pliocene is an important scientific and socioeconomic concern. Based on phase 2 of the Pliocene Model Intercomparison Project (PlioMIP2), this work examines the simulated changes of Sahel summer rainfall during the mid-Pliocene. The results show the considerable intermodel uncertainty of the simulated mid-Pliocene Sahel rainfall changes in the PlioMIP2 multimodel ensemble, which is due to the uncertainties of both the simulated dynamic and thermodynamic responses to this past warmer climate. In particular, we find that the intermodel spread in the simulated northern North American warming is a major source of the uncertainty of the mid-Pliocene Sahel summer rainfall changes through two processes. One is a direct dynamic process: a stronger warming over northern North America could enhance the meridional temperature gradient between the extratropical and tropical regions, inducing an interhemisphere energy imbalance of the atmosphere. This could lead to a northward shift of the intertropical convergence zone, strengthening the Western African summer monsoon (WASM) circulation and Sahel summer rainfall. Another is an indirect thermodynamic process: the strengthened WASM circulation could further induce anomalous moisture convergence over the Sahel region, increasing local atmospheric moisture at the low-level troposphere, in favor of a wetter Sahel. Our results suggest that an improved warming simulation over northern North America is essential for the hydrological cycle simulation around the Sahel in the mid-Pliocene warmer climate. Significance Statement: The Sahel summer rainfall change in a global warmer climate is a widespread scientific and socioeconomic issue because of its important impacts on regional agriculture, ecosystems, food security, water resources, and even cultural environment. However, the present study identifies a nonnegligible intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene (one of the most recent geological warm periods in Earth's history) in the Pliocene Model Intercomparison Project phase 2 (PlioMIP2) multimodel ensemble. In particular, such an intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene is attributed to that of the simulated northern North America warming via both the direct dynamic process and the indirect thermodynamic process. This is quite different from the uncertainty source of near-future projections of Sahel summer rainfall changes. The present results improve our understanding of the underlying physics of the hydrological cycle change around the Sahel region in the mid-Pliocene warmer climate, with implications for the future projections of regional monsoonal rainfall. [ABSTRACT FROM AUTHOR]

This study classifies 407 developing disturbances (DEV) and 2309 nondeveloping disturbances (NONDEV) over the western North Pacific into five large-scale circulation patterns, namely the pre-existing cyclone (PC), easterly wave (EW), zonal wind convergence (CON), zonal wind shear line (SL), and mixed zonal wind convergence and shear line (CON-SL) patterns. The SL pattern has the highest TC yield percentage, followed by the CON-SL, while the EW is the least favorable pattern. The composite analysis shows that upper-level divergence, midlevel relative humidity, and surface heat flux (SHF) growth are crucial to the disturbance development in all the five patterns. Besides, large lower-level barotropic kinetic energy conversion and a well-developed primary circulation are good indicators for disturbance development in the PC, EW, and CON rather than in the SL and CON-SL patterns. Furthermore, for the PC, EW and CON patterns, the DEV features strong and rapidly growing SHF and mesoscale convective systems (MCS) closer to the disturbance center, which allows deep-layer warming and moistening, and drives a deep secondary circulation. Interestingly, due to an environment with high lower-level vorticity, the SL and CON-SL patterns typically foster a relatively mature primary circulation with strong SHF and MCS concentrated close to the center, especially for the NONDEV at the pre-genesis stage. However, a drier mid-to-upper-level environment for the NONDEV inhibits deep convection, which may explain its shallow secondary circulation and therefore poor potential to develop further. [ABSTRACT FROM AUTHOR]

Rapid intensification (RI) of tropical cyclones (TCs), which refers to an explosive increase of TC intensity exceeding a certain threshold [e.g., 30 kt (1 kt ~ 0.51 m s-1) in 24 h] within a short time period, poses a great challenge to both forecasting and disaster prevention efforts. Recent studies have documented a significant increase in the magnitude of TC RI (RIM; measured as the mean intensification rate of all TC RI records in a TC season) over the western North Pacific (WNP) since 1979, and have attributed it to the impacts of global warming. In this study, results from statistical analyses show that the TC RIM over the WNP during 1951-2021 exhibits significant interdecadal variability, which is found to be closely related to the Atlantic multidecadal oscillation (AMO). Further analyses indicate that the response of the local thermodynamic conditions to the AMO plays a dominant role in shaping this relationship. The positive AMO phase fosters a high TC RIM over the WNP by producing significant warm sea surface temperature (SST) anomalies, which in turn enhances TC heat potential and the midtropospheric relative humidity in the main region of TC RI occurrence. Results from both data analyses and numerical model experiments demonstrate that the AMO modulates thermodynamic conditions over the WNP, such as SST and ocean heat content, by affecting local heat fluxes and the Ekman heat transport in the WNP via the modulation of Walker circulation from the Atlantic to the Pacific. [ABSTRACT FROM AUTHOR]

Previous studies have investigated how sea surface temperature (SST) affects the potential intensity of tropical cyclones (TCs). However, this is an upper bound only on the maximum near-surface azimuthal winds and does not fully account for the effects of atmospheric moisture. Potential intensity might not vary in the same way as the total kinetic energy (WKE) of a TC would with changing SST. The term WKE is related, via the conceptualization of the TC as a heat engine, to TC mechanical efficiency. We investigate how TC mechanical efficiency varies with SST in a series of moist, axisymmetric, radiative-convective numerical experiments with constant SSTs ranging from 295 to 307.5 K. We find a -2.1% K-1 decrease in the mechanical efficiency with SST. While the increase in the net heat energy gained by the TC heat engine acts to increase WKE, the mechanical efficiency still decreases with SST due to the effects of moisture on WKE and on the total heat input to the TC. Moist convection in an unsaturated atmosphere is associated with substantial irreversible entropy production, which detracts from the energy that the TC can use to power its winds. The increasing moisture content in a warmer atmosphere predicted by Clausius-Clapeyron scaling leads this irreversibility to increase in an unsaturated atmosphere, presenting a larger penalty on WKE and decreasing the mechanical efficiency. Our results highlight the importance of giving full consideration to the effects of moisture on the TC heat engine in studies of how climate affects TCs. [ABSTRACT FROM AUTHOR]

Water vapor supersaturation in clouds is a random variable that drives activation and growth of cloud droplets. The Pi Convection–Cloud Chamber generates a turbulent cloud with a microphysical steady state that can be varied from clean to polluted by adjusting the aerosol injection rate. The supersaturation distribution and its moments, e.g., mean and variance, are investigated for varying cloud microphysical conditions. High-speed and collocated Eulerian measurements of temperature and water vapor concentration are combined to obtain the temporally resolved supersaturation distribution. This allows quantification of the contributions of variances and covariances between water vapor and temperature. Results are consistent with expectations for a convection chamber, with strong correlation between water vapor and temperature; departures from ideal behavior can be explained as resulting from dry regions on the warm boundary, analogous to entrainment. The saturation ratio distribution is measured under conditions that show monotonic increase of liquid water content and decrease of mean droplet diameter with increasing aerosol injection rate. The change in liquid water content is proportional to the change in water vapor concentration between no-cloud and cloudy conditions. Variability in the supersaturation remains even after cloud droplets are formed, and no significant buffering is observed. Results are interpreted in terms of a cloud microphysical Damköhler number (Da), under conditions corresponding to Da ≲ 1 , i.e., the slow-microphysics regime. This implies that clouds with very clean regions, such that Da ≲ 1 is satisfied, will experience supersaturation fluctuations without them being buffered by cloud droplet growth. Significance Statement: The saturation ratio (humidity) in clouds controls the growth rate and formation of cloud droplets. When air in a turbulent cloud mixes, the humidity varies in space and time throughout the cloud. This is important because it means cloud droplets experience different growth histories, thereby resulting in broader size distributions. It is often assumed that growth and evaporation of cloud droplets buffers out some of the humidity variations. Measuring these variations has been difficult, especially in the field. The purpose of this study is to measure the saturation ratio distribution in clouds with a range of conditions. We measure the in-cloud saturation ratio using a convection cloud chamber with clean to polluted cloud properties. We found in clouds with low concentrations of droplets that the variations in the saturation ratio are not suppressed. [ABSTRACT FROM AUTHOR]

Intense tropical cyclones can form secondary eyewalls (SEs) that contract toward the storm center and eventually replace the inner eyewall, a process known as an eyewall replacement cycle (ERC). However, SE formation does not guarantee an eventual ERC, and often, SEs follow differing evolutionary pathways. This study documents SE evolution and progressions observed in numerous tropical cyclones, and results in two new datasets using passive microwave imagery: a global subjectively labeled dataset of SEs and eyes and their uncertainties from 72 storms between 2016 and 2019, and a dataset of 87 SE progressions that highlights the broad convective organization preceding and following an SE formation. The results show that two primary SE pathways exist: "No Replacement," known as "Path 1," and "Replacement," known as the "Classic Path." Most interestingly, 53% of the most certain SE formations result in an eyewall replacement. The Classic Path is associated with stronger column average meridional wind, a faster poleward component of storm motion, more intense storms, weaker vertical wind shear, greater relative humidity, a larger storm wind field, and stronger cold-air advection. This study highlights that a greater number of potential SE pathways exist than previously thought. The results of this study detail several observational features of SE evolution that raise questions about the physical processes that drive SE formations. Most important, environmental conditions and storm metrics identified here provide guidance for predictors in artificial intelligence applications for future tropical cyclone SE detection algorithms. [ABSTRACT FROM AUTHOR]

Syukoro (Suki) Manabe's Nobel Prize in Physics was awarded largely for his early work on one-dimensional models of "radiative–convective equilibrium" (RCE), which produced the first credible estimates of Earth's climate sensitivity. This article reviews that work and tries to identify those aspects that make it so distinctive. We argue that Manabe's model of RCE contained three crucial ingredients. These are (i) a tight convective coupling of the surface to the troposphere, (ii) an assumption of fixed relative humidity rather than fixed absolute humidity, and (iii) a sufficiently realistic representation of greenhouse gas radiative transfer. Previous studies had separately identified these key ingredients, but none had properly combined them. We then discuss each of these ingredients in turn, highlighting how subsequent research in the intervening decades has only cemented their importance for understanding global climate change. We close by reflecting on the elegance of Manabe's approach and its lasting value. [ABSTRACT FROM AUTHOR]

Ice habit diagrams published prior to 2009—and many since—do not accurately describe in situ observations of ice shapes as a function of temperature and moisture. Laboratory studies and analysis of field observations by Bailey and Hallett in a series of papers in 2002, 2004, and 2009 corrected several errors from earlier studies, but their work has not been widely disseminated. We present a new, simplified diagram based on Bailey and Hallett's work that focuses on several ice growth forms arising from the underlying surface processes by which mass is added to a crystal: tabular, columnar, branched, side branched, two types of polycrystalline forms, and a multiple growth regime at low ice supersaturations. To aid interpretation for a variety of applications, versions of the ice growth diagram are presented in terms of relative humidity with respect to water as well as the traditional formats of relative humidity with respect to ice and vapor density excess. These diagrams are intended to be understandable and useful in classroom settings at the sophomore undergraduate level and above. The myriad shapes of pristine snow crystals can be described as the result of either a single growth form or a sequence of growth forms. Overlays of data from upper-air soundings on the ice growth diagrams aid interpretation of expected physical properties and processes in conditions of ice growth. [ABSTRACT FROM AUTHOR]

This work examines the diurnal and seasonal variability of near-surface temperature and humidity at several large areas with high population density within the Maritime Continent using the Bureau of Meteorology Atmospheric Regional Reanalysis (BARRA) 12-km-resolution dataset that covers the period 1990–2019. The diurnal cycle is examined in detail, with a key feature being the relatively small diurnal variation of the wet-bulb temperature TWB when compared with the temperature and dewpoint temperature TD. The diurnal variability is strongly modulated by the monsoons with their increased rainfall and cloud cover. The near-surface signals associated with the Madden–Julian oscillation across the domains are relatively weak. Dry and humid temperature extremes are examined for regional and seasonal variability. The dry and moist variable extremes occur at different times of year, but each have spatially coherent structure. Significance Statement: This paper examines the climatological variations of near-surface temperature and humidity and their extremes in four locations in the "Maritime Continent." This is important because there are significant variations potentially affecting human and ecosystem health and its resilience to climate change. [ABSTRACT FROM AUTHOR]

Global Historical Climate Network (GHCN) data are used to characterize changes in large-scale fall extreme precipitation in the mid-Atlantic and the northeast United States. Days with the highest regional extreme precipitation total [extreme precipitation (EP) days] are sorted into weather types based on tropical cyclone (TC), atmospheric river (AR), and extreme integrated water vapor transport (IVT) influences. Increased cumulative precipitation from EP days is attributed primarily to three sources. First, over the mid-Atlantic states, Pennsylvania, and eastern New England, large increasing trends are found in precipitation occurring on EP days attributed to TC-related weather types. These increases are due to a late-1990s increase in TC frequency, which manifests primarily as increased TC remnants in the mid-Atlantic area. Second, over New York State and central and northern New England, there are increasing trends in EP day precipitation from AR-related weather types. Finally, there is evidence of increasing extreme IVT-related precipitation in the absence of ARs and TCs. However, when taking into account prior TC influences, it is found that a combination of TC-related weather types accounts for much of the increasing EP day precipitation trend. These trends are then compared to EP day synoptic changes relating to atmospheric moisture content and transport. Results indicate that fall EP days have become warmer and moister, but that this does not necessarily translate to higher IVT because wind speeds have stayed the same or slowed. This is consistent with fall climatological changes during the 1979–2019 analysis period, including higher atmospheric water content and slowed westerlies in the vicinity of the mid-Atlantic. Significance Statement: This paper sorts days with high extreme precipitation totals in the mid-Atlantic and northeast United States fall into categories based on the presence of factors such as tropical cyclones (TCs), atmospheric rivers (ARs), and other extreme moisture transport. These days have become more frequent between 1979 and 2019, in part due to events with TC influences. However, the synoptic environment on extreme days has also changed, becoming warmer and moister. Many of the most extreme events involve combined AR and TC influences. This is important because it shows that 1) large-scale extreme precipitation has become more common and has changed synoptically and 2) interactions between tropical and non-tropical systems are a key factor for extreme events in this part of the country. Future work might investigate how changes in TC frequency, moisture transport, and jet stream patterns will affect these events in a future climate. [ABSTRACT FROM AUTHOR]

The northeastern Tibetan Plateau is located in a climatic junction, which is considered an ideal region to explore the interactions between the summer monsoons and the westerly circulation patterns. However, to date, the needed long-term precipitation-based isotopic dataset is too limited to predict the interactions and patterns. This paper presents an evaluation of hydrometeorological processes and climate dynamics in the northeastern Tibetan Plateau based on a 7-yr precipitation isotope dataset covering the summer monsoon periods from 2012 to 2018. Results illustrated remarkable seasonal isotopic variability, characterized by lower δ18O and δ2H values in June with an average of −10‰ and −66.7‰, respectively. Higher δ18O and δ2H values in July averaged −6.7‰ and −39.5‰, respectively. This clear isotopic variability is largely related to seasonal changes of moisture sources and hydrometeorological processes. These precipitation isotopic values were primarily determined by the amount of precipitation, relative humidity, and convective activity, but showed no correlation with air temperature. Backward trajectory model results showed that Xinjiang, northern China, the Arctic, central Asia, and the South China Sea (SCS) were the primary sources of precipitation for the study site with varying seasonal contributions. The maritime moisture source of the SCS primarily resulted in the lowest precipitation δ18O values during the prevailing summer monsoon, which is mainly as a result of the strong convective activity and rainout processes along the air trajectory. The higher average deuterium excess (d-excess) value of precipitation in September indicated continental sources from central Asia (e.g., 75.4%) as land vapor recycling increases d-excess concentration in the atmosphere. These findings provide further insights into the main factors of precipitation isotopic variability related to atmospheric processes along the trajectory and the relevant factors in the monsoon regions. Significance Statement: Recently, scientists and policy makers have become aware that Tibetan hydroclimate variability provides evidence of changes in regional and global circulation patterns that may result in the intensification of climate-driven extremes. However, these studies largely depend on crucial paleoclimate records of past precipitation isotopes in monsoon regions, which contain great uncertainties because of the complex relationship between climatic variability and precipitation isotopes. This study first presented a 7-yr isotopic dataset to understand the hydrological processes and climate dynamics controlling the isotopic variability in the northeastern Tibetan Plateau. The findings reveal important factors on the isotopic variability associated with atmospheric processes and their key climatic variables, which can enhance our interpretation of the paleoclimate records in monsoon regions. [ABSTRACT FROM AUTHOR]

The Madden–Julian oscillation (MJO) identified by Madden and Julian in the early 1970s has been well recognized as the most prominent intraseasonal signal in the tropics. Its discovery and its relationship with other weather phenomena such as tropical cyclones (TCs) are among the most significant advancements in modern meteorology with broad and far-reaching impacts. The original study by Madden and Julian used radiosonde data on Canton Island, and their spectral analysis revealed the signal of a 40–50-day oscillation. It has come to our attention that an earlier study by Xie et al. published in a Chinese journal documented an oscillatory signal of a 45-day period using radiosonde data from several stations between 70° and 125°E in the tropics. The 40–50-day signal found by Xie et al. is strikingly evident without any filtering. Xie et al. identified that occurrences of TCs are correlated with the 40–50-day variation of low-level westerlies at these stations. The original figures in Xie et al.'s article were hand drawn. Their results are verified using data from a longer period of 1958–70. The 40–50-day oscillation in the monsoon westerlies and its relationship with the occurrence of TCs are confirmed and further expanded upon. This study serves the purpose of bringing recognition to the community of the identification of a 40–50-day signal published in Chinese in 1963 and the discovery of the correlation between MJO phases and TC genesis three decades earlier than studies on this subject published outside China. [ABSTRACT FROM AUTHOR]

This paper examines the physical controls of extratropical humidity and clouds by isolating the effects of cloud physics factors in an idealized model. The Held–Suarez dynamical core is used with the addition of passive water vapor and cloud tracers, allowing cloud processes to be explored cleanly. Separate saturation adjustment and full cloud scheme controls are used to consider the strength of advection–condensation theory. Three sets of perturbations to the cloud scheme are designed to test the model’s sensitivity to the physics of condensation, sedimentation, and precipitation formation. The condensation and sedimentation perturbations isolate two key differences between the control cases. First, the sub-grid-scale relative humidity distribution assumed for the cloud macrophysics influences the location and magnitude of the extratropical cloud maxima, which interrupt the isentropic transport of moisture to the polar troposphere. Second, within the model’s explicit treatment of cloud microphysics, re-evaporation of hydrometeors moistens and increases clouds in the lower troposphere. In contrast, microphysical processes of precipitation formation (specifically, the ratio of accretion to autoconversion) have negligible effects on humidity, cloudiness, and precipitation apart from the strength of the largescale condensation and formation cycle. In addition, counterintuitive relationships}such as cloud condensate and cloud fraction responding in opposing directions}emphasize the need for careful dissection of physical mechanisms. In keeping with advection–condensation theory, circulation sets the patterns of humidity, clouds, and precipitation to first order, with factors explored herein providing secondary controls. The results substantiate the utility of such idealized modeling and highlight key cloud processes to constrain. [ABSTRACT FROM AUTHOR]

Globally, extreme rainfall has intense impacts on ecosystems and human livelihoods. However, no effort has yet been made to forecast the extreme rainfall indices through machine learning techniques. In this paper, a new extreme rainfall indices forecasting model is proposed using a random forest (RF) model to provide effective forecasts of monthly extreme rainfall indices. In addition, RF feature importance is proposed in this study to identify the most and least important features for the proposed model. This study forecasts only statistically significant extreme rainfall indices over Bangladesh including consecutive dry days (CDD), the number of heavy rain days (R10mm; rainfall ≥ 10 mm), and the number of heavy rain days (R20mm; rainfall ≥ 20 mm) within 1–3 months of lead time. The proposed model uses monthly antecedent CDD, R10mm, and R20mm including atmospheric parameters and ocean–atmospheric teleconnections, namely, convective available potential energy (CAPE), relative humidity (RH), air temperature (TEM), El Niño–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), and North Atlantic Oscillation (NAO), as the inputs to the model. Results show that the proposed model yields the best performance to forecast CDD, R10mm, and R20mm with only the antecedent of these indices as input. Ocean–atmospheric teleconnections (IOD, ENSO, and NAO) are useful for CDD forecasting, and local atmospheric parameters (CAPE, RH, and TEM) are useful for R10mm and R20mm forecasting. The results suggest that adding atmospheric parameters and ocean–atmospheric teleconnections is useful to forecast extreme rainfall indices. [ABSTRACT FROM AUTHOR]

The genesis potential index (GPI) has been used widely to estimate the influence of large-scale conditions on tropical cyclone (TC) genesis. Here we find that two GPIs, the Emanuel–Nolan GPI (ENGPI) and the dynamic GPI (DGPI), show opposite skills in quantifying decadal variability of TC genesis in the western North Pacific (WNP). During 1979–2020, ENGPI shows a reverse decadal variation to the WNP TC genesis with a significant negative correlation of −0.61, while DGPI can reasonably reproduce the decadal variation of the WNP TC genesis with a significant correlation of 0.66. The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis induced by a WNP anomalous cyclonic circulation that controls the decadal variation of TC genesis. On the one hand, the cyclonic circulation leads to favorable dynamical conditions including ascending motion, cyclonic vorticity, and weakened vertical shear, and thus tends to increase the DGPI. On the other hand, the cyclonic circulation leads to unfavorable thermodynamical conditions (decreased maximum potential intensity and midlevel humidity) that tends to decrease the ENGPI. As a result, the DGPI and ENGPI are reversely evolved and eventually lead to their opposite correlation between TC genesis. The significant positive correlation between DGPI and TC genesis suggests a critical role in the large-scale dynamical control of the decadal variability of the WNP TC genesis. Significance Statement: Tropical cyclones (TCs) account for one-third of the deaths and economic losses from weather-, climate-, and water-related disasters. Understanding variations in TC activity from the perspective of large-scale conditions is of great importance to seasonal forecasting and disaster mitigation. Here we find that two genesis potential indexes (GPIs), the Emanuel–Nolan GPI (ENGPI) and dynamic GPI (DGPI), show opposite skill in quantifying decadal variability of TC genesis in the western North Pacific (WNP). The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis. The result suggests a critical role of large-scale dynamic control in the decadal variability of TC genesis in the WNP. [ABSTRACT FROM AUTHOR]

Throughout 2020, a persistent marine heatwave (MHW) event occurred in the northeast (NE) Pacific, which exhibited a record-breaking intensity and long duration. Three peaks of sea surface temperature (SST) anomaly are identified in April, July, and November 2020, respectively, all of which are caused by the surface latent heat flux (LH) term. A Taylor expansion analysis for the LH term reveals that positive LH anomaly (LHA) played the decisive role in increasing the latent heating effect, whereas the mixed layer depth anomaly was not important in the 2020 MHW. The positive LHA was primarily caused by the southerly wind anomalies, which advected more humid air from lower latitudes to the NE Pacific and thus reduced the difference between the saturated specific humidity at sea surface and actual surface specific humidity. This reduced sea–air humidity difference was conducive to less evaporation, leading to positive LHA. Using an atmospheric general circulation model forced by observed SSTs, we find that although the atmospheric circulation anomalies associated with the 2020 MHW can be partly constrained by both tropical Pacific SST and local SST over the North Pacific, the role of internal atmospheric variability in the 2020 MHW's formation cannot be overlooked. The prediction skill for the 2020 MHW in the North American Multimodel Ensemble models is limited to one month, indicating the challenge of accurately predicting MHW in the NE Pacific. The finding about the contribution of sea–air humidity difference to the LHA provides a new insight into the formation mechanism of MHW. Significance Statement: An exceptionally strong and persistent marine heatwave occurred in the northeast Pacific throughout 2020, with three peaks in April, July, and November, causing considerable damage to local fisheries and ecosystems. This study found that the evolution of this marine heatwave was due to the less-than-normal release of latent heat at the ocean surface. This was mainly caused by the reduced sea–air humidity difference, which was attributed to the increased near-surface moisture transported by southerly wind anomalies. Our model experiments demonstrated that the associated anomaly fields in the atmosphere partly stemmed from stochastic processes. The state-of-the-art dynamic models can only predict this marine heatwave one month in advance, indicating the necessity of improving prediction skill for marine heatwave's evolution in current models. [ABSTRACT FROM AUTHOR]

This study investigates the impacts of grid spacing and station network on surface analyses and forecasts including temperature, humidity, and winds in Beijing Winter Olympic complex terrain. The high-resolution analyses are generated by a rapid-refresh integrated system that includes a topographic downscaling procedure. Results show that surface analyses are more accurate with a higher targeted grid spacing. In particular, the average analysis errors of surface temperature, humidity, and winds are all significantly reduced when the grid size is increased. This improvement is mainly attributed to a more realistic simulation of the topographic effects in the integrated system because the topographic downscaling at higher grid spacing can add more details in a complex mountain region. From 1 km to 100 m, 1-12-h forecasts of temperature and humidity are also largely improved, while the wind only shows a slight improvement for 1-6-h forecasts. The influence of station network on the surface analyses is further examined. Results show that the spatial distributions of temperature and humidity at a 100-m space scale are more realistic and accurate when adding an intensive automatic weather station network, as more observational information can be absorbed. The adding of a station network can also reduce forecast errors, which can last for about 6 h. However, although surface winds display better analysis skill when more stations are added, the wind at the mountaintop region sometimes encounters a marginally worse effect for both analysis and forecast. The results are helpful to improve the analysis and forecast products in complex terrain and have some implications for downscaling from a coarse grid size to a finer grid. [ABSTRACT FROM AUTHOR]

Human heat stress depends jointly on atmospheric temperature and humidity. Wetter soils reduce temperature but also raise humidity, making the collective impact on heat stress unclear. To better understand these interactions, we use ERA5 to examine the coupling between daily average soil moisture and wet-bulb temperature (Tw) and its seasonal and diurnal cycle at global scale. We identify a global soil moisture–Tw coupling pattern with both widespread negative and positive correlations in contrast to the well-established cooling effect of wet soil on dry-bulb temperature. Regions showing positive correlations closely resemble previously identified land–atmosphere coupling hotspots where soil moisture effectively controls surface energy partition. Soil moisture–Tw coupling varies seasonally closely tied to monsoon development, and the positive coupling is slightly stronger and more widespread during nighttime. Local-scale analysis demonstrates a nonlinear structure of soil moisture–Tw coupling with stronger coupling under relatively dry soils. Hot days with high Tw values show wetter-than-normal soil, anomalous high latent and low sensible heat flux from a cooler surface, and a shallower boundary layer. This supports the hypothesis that wetter soil increases Tw by concentrating surface moist enthalpy flux within a shallower boundary layer and reducing free-troposphere-air entrainment. We identify areas of particular interest for future studies on the physical mechanisms of soil moisture–heat stress coupling. Our findings suggest that increasing soil moisture might amplify heat stress over large portions of the world including several densely populated areas. These results also raise questions about the effectiveness of evaporative cooling strategies in ameliorating urban heat stress. Significance Statement: The purpose of this study is to provide a global picture of the relationship between soil moisture anomalies and a heat stress metric that includes the joint effects of temperature and humidity. This is important because a better understanding of this relationship will help improve the prediction of extreme heat stress events and inform strategies for ameliorating heat stress. We find a widespread positive correlation between soil moisture and heat stress, in contrast to studies relying on temperature alone. This raises the possibility that, over much of the world, and in the most populous regions, strategies like irrigation or "greening" that can reduce temperature might be ineffective or even harmful in reducing heat stress with humidity incorporated. [ABSTRACT FROM AUTHOR]

This work explores the modulation of the Pacific decadal oscillation (PDO) on the relationship between the occurrence position of rapid intensification (RI) events of tropical cyclones (TCs) over the western North Pacific (WNP) in boreal autumn and El Niño–Southern Oscillation (ENSO). From the warm to cold phase of the PDO, the occurrence position of WNP RI events experiences a significant westward shift of 5.5° in El Niño years and a significant northward shift of 4.5° in La Niña years. The strengthening of thermodynamic conditions west of 160°N plays a dominant role in the westward shift of RI events in El Niño years, and the northward shift in La Niña years is associated with the expansion of areas with warm sea surface temperature, high tropical cyclone heat potential and midlevel relative humidity, strengthening of relative vorticity north of 20°N, and weakening of dynamic conditions within 10°–20°N. During the PDO cold phase, the descending branch of the Walker circulation over the western Pacific is weak and shifts west of 140°E in El Niño years, whereas it is much stronger in La Niña years. In addition, the Hadley circulation over the WNP shows little change during El Niño, but the ascending branch around 10°N expands to 20°N during La Niña. These trends reflect the changing responses of the WNP environment to ENSO variation and are consistent with the changing distribution of WNP RI events. Moreover, during the PDO cold phases, SST over the north Indian Ocean is much warmer, and anomalous anticyclonic circulation occurs in the WNP in boreal spring (summer and autumn) during the developing phase of El Niño (La Niña) years, which may also contribute to strengthening the thermodynamic conditions over the WNP. [ABSTRACT FROM AUTHOR]

Environments associated with severe hailstorms, compared to those of tornadoes, are often less apparent to forecasters. Understanding has evolved considerably in recent years; namely, that weak low-level shear and sufficient convective available potential energy (CAPE) above the freezing level is most favorable for large hail. However, this understanding comes only from examining the mean characteristics of large hail environments. How much variety exists within the kinematic and thermodynamic environments of large hail? Is there a balance between shear and CAPE analogous to that noted with tornadoes? We address these questions to move toward a more complete conceptual model. In this study, we investigate the environments of 92 323 hail reports (both severe and nonsevere) using ERA5 modeled proximity soundings. By employing a self-organizing map algorithm and subsetting these environments by a multitude of characteristics, we find that the conditions leading to large hail are highly variable, but three primary patterns emerge. First, hail growth depends on a favorable balance of CAPE, wind shear, and relative humidity, such that accounting for entrainment is important in parameter-based hail prediction. Second, hail growth is thwarted by strong low-level storm-relative winds, unless CAPE below the hail growth zone is weak. Finally, the maximum hail size possible in a given environment may be predictable by the depth of buoyancy, rather than CAPE itself. [ABSTRACT FROM AUTHOR]

In this study, a new way to assimilate clear-sky Advanced Himawari Imager (AHI) surface-sensitive brightness temperature (TB) observations over land is investigated for improving quantitative precipitation forecasts (QPFs) in eastern China. To alleviate problems arising from inaccurate surface temperature in radiance simulations, surface station observations of land surface skin temperature (LSST) together with conventional and AMSU-A observations are assimilated to improve AHI surface-sensitive TB simulations of the Community Radiative Transfer Model (CRTM) before AHI data assimilation. First, the Gridpoint Statistical Interpolation (GSI) three-dimensional variational (3DVar) system is updated with the additional control variable of surface temperature and its background error covariances. Second, surface temperature and emissivity sensitivity checks are designed for the quality control of the surface-sensitive AHI channels. Finally, the impacts of a two-time data assimilation strategy are assessed for a local convection rainfall case and a synoptic-scale precipitation case. The experiment in which AHI data are assimilated after assimilating LSST data (ExpL2) outperforms the traditional experiment in which the LSST is not updated (ExpL) in terms of its 24-h QPF skill score. A better description of atmospheric instability and moisture convergence forcing is obtained in ExpL2 than in ExpL. Both experiments show additional low-level temperature and humidity adjustments compared to the experiment that does not assimilate AHI surface-sensitive channels (ExpNL). Lower AHI TB simulation biases are found in the ExpL2 experiment, which improve the analyzed field and subsequent QPFs. The results in this study suggest the importance of proper utilization of LSST observations for AHI surface-sensitive TB assimilations over land. [ABSTRACT FROM AUTHOR]

The societies of the coastal regions of the Greater Horn of Africa (GHA) experience two distinct rainy seasons: the generally wetter "long" rains in the boreal spring and the generally drier "short" rains in the boreal fall. The GHA rainfall climatology is unique for its latitude in both its aridity and for the dynamical differences between its two rainy seasons. This study explains the drivers of the rainy seasons through the climatology of moist static stability, estimated as the difference between surface moist static energy hs and midtropospheric saturation moist static energy h*. In areas and at times when this difference, hs 2 h*, is higher, rainfall is more frequent and more intense. However, even during the rainy seasons, hs 2 h*, 0 on average and the atmosphere remains largely stable, in line with the GHA's aridity. The seasonal cycle of hs 2 h*, to which the unique seasonal cycles of surface humidity, surface temperature, and midtropospheric temperature all contribute, helps explain the double-peaked nature of the regional hydroclimate. Despite tropospheric temperature being relatively uniform in the tropics, even small changes in h* can have substantial impacts on instability; for example, during the short rains, the annual minimum in GHA h* lowers the threshold for convection and allows for instability despite surface humidity anomalies being relatively weak. This hs 2 h* framework can help identify the drivers of interannual variability in GHA mean rainfall or diagnose the origin of biases in climate model simulations of the regional climate. [ABSTRACT FROM AUTHOR]

This study investigates the evolution of temperature and lifetime of evaporating, supercooled cloud droplets considering initial droplet radius (r0) and temperature (Tr0), and environmental relative humidity (RH), temperature (T∞), and pressure (P). The time (tss) required by droplets to reach a lower steady-state temperature (Tss) after sudden introduction into a new subsaturated environment, the magnitude of DT = T∞ 2 Tss, and droplet survival time (tst) at Tss are calculated. The temperature difference (ΔT) is found to increase with T∞, and decrease with RH and P. DT was typically 1-5 K lower than T∞, with highest values (~10.3 K) for very low RH, low P, and T∞ closer to 08C. Results show that tss is <0.5 s over the range of initial droplet and environmental conditions considered. Larger droplets (r0 = 30-50 mm) can survive at Tss for about = s to over 10 min, depending on the subsaturation of the environment. For higher RH and larger droplets, droplet lifetimes can increase by more than 100 s compared to those with droplet cooling ignored. Tss of the evaporating droplets can be approximated by the environmental thermodynamic wet-bulb temperature. Radiation was found to play a minor role in influencing droplet temperatures, except for larger droplets in environments close to saturation. The implications for ice nucleation in cloud-top generating cells and near cloud edges are discussed. Using Tss instead of T∞ in widely used parameterization schemes could lead to enhanced number concentrations of activated ice-nucleating particles (INPs), by a typical factor of 2-30, with the greatest increases (≥100) coincident with low RH, low P, and T∞ closer to 08C. [ABSTRACT FROM AUTHOR]

Deep-layer vertical wind shear and midtropospheric relative humidity (RH) are explored in and around environments of all intensifying North Atlantic tropical cyclones (TCs) between 1980 and 2021 using reanalysis data. Shear and RH are averaged within the standard environmental annulus of 200–800 km, along with a 100–600-km annulus, and a 0–250-km radius to represent the inner core and TC itself. Distributions of shear and RH at onset along with a time series of evolution from 48 h prior to and after onset of three different intensification rates, slight [5–10 kt (24 h)−1; 1 kt ≈ 0.51 m s−1], moderate [15–25 kt (24 h)−1], and rapid [≥30 kt (24 h)−1], are analyzed. RH is also investigated within different shear environments and in shear-relative quadrants around the storm. While low shear and high RH are found to be most favorable for rapid intensification (RI), there is still a significant probability that RI will occur within less favorable environments. RI cases decrease in 850–200-hPa shear in the 24 h leading up to RI, whereas slight intensification cases increase, which is evident in both the standard shear and a shallower layer at 48 h prior to onset. The inner-core RH for RI increases prior to onset whereas it decreases in the environments. RH analysis by shear-relative quadrants demonstrates the importance of moistening in the upshear-right quadrant before onset of RI. Results indicate the potential value of multiple annuli and shear-relative analysis for moisture and a shallower, 925–400-hPa layer for shear in RI forecasting. Significance Statement: The purpose of this study was to investigate the wind and moisture around different areas of intensifying North Atlantic tropical cyclones between 1980 and 2021 using reanalysis data. Average wind and moisture evolve differently around the onset of different intensification rates as well as in different near and far regions from the storm center. These results provide additional indicators that forecasters may consider when examining how the environment around the storm and the situation in the inner core may influence its future intensity. [ABSTRACT FROM AUTHOR]

Derechos are extensive swaths of damaging winds produced by some long-lived, widespread mesoscale convective systems. Little research has been conducted concerning how derecho mechanisms might change in a future, warmer climate. In this study, the pseudo–global warming method is utilized to evaluate how the 10 August 2020 midwestern U.S. derecho, the costliest thunderstorm event in U.S. history to date, might differ if it instead occurred in a warmer climate at the end of this century. The 10 August derecho event is first simulated in its observed environment, and then resimulated in environments altered according to projections from different climate models using a high-emissions climate change scenario. Results suggest that near the end of this century, a similar derecho event may not necessarily have more intense winds but could possibly impact a geographical area 50% to 100% larger. The physical chain of events leading to this greater geographical impact result from the derecho winds beginning earlier in the storm lifetime, due to increased precipitation combined with decreased relative humidity right above the ground, and derecho winds extending northward due to a strengthening of the parent storm from increased instability there. All these factors enhance the area of evaporative cooling and thus the cold pool, which in turn extends the area covered by the rear-inflow jet within the storm, the likely main mechanism for most of the damaging winds at the ground in the historical event. More study of other cases is required to evaluate the generality of this result. [ABSTRACT FROM AUTHOR]

A cloud-resolving model is used to examine the intensification of tilted tropical cyclones from depression to hurricane strength over relatively cool and warm oceans under idealized conditions where environmental vertical wind shear has become minimal. Variation of the SST does not substantially change the time-averaged relationship between tilt and the radial length scale of the inner core, or between tilt and the azimuthal distribution of precipitation during the hurricane formation period (HFP). By contrast, for systems having similar structural parameters, the HFP lengthens superlinearly in association with a decline of the precipitation rate as the SST decreases from 30° to 26°C. In many simulations, hurricane formation progresses from a phase of slow or neutral intensification to fast spinup. The transition to fast spinup occurs after the magnitudes of tilt and convective asymmetry drop below certain SST-dependent levels following an alignment process explained in an earlier paper. For reasons examined herein, the alignment coincides with enhancements of lower–middle-tropospheric relative humidity and lower-tropospheric CAPE inward of the radius of maximum surface wind speed rm. Such moist-thermodynamic modifications appear to facilitate initiation of the faster mode of intensification, which involves contraction of rm and the characteristic radius of deep convection. The mean transitional values of the tilt magnitude and lower–middle-tropospheric relative humidity for SSTs of 28°–30°C are respectively higher and lower than their counterparts at 26°C. Greater magnitudes of the surface enthalpy flux and core deep-layer CAPE found at the higher SSTs plausibly compensate for less complete alignment and core humidification at the transition time. [ABSTRACT FROM AUTHOR]

El Niño–Southern Oscillation (ENSO) has a profound influence on the occurrence of extreme precipitation events at local and regional scales in the present-day climate, and thus it is important to understand how that influence may change under future global warming. We consider this question using the large-ensemble simulations of CESM2, which simulates ENSO well historically. CESM2 projects that the influence of ENSO on extreme precipitation will strengthen further under the SSP3–7.0 scenario in most regions whose extreme precipitation regimes are strongly affected by ENSO in the boreal cold season. Extreme precipitation in the boreal cold season that exceeds historical thresholds is projected to become more common throughout the ENSO cycle. The difference in the intensity of extreme precipitation events that occur under El Niño and La Niña conditions will increase, resulting in "more extreme and more variable hydroclimate extremes." We also consider the processes that affect the future intensity of extreme precipitation and how it varies with the ENSO cycle by partitioning changes into thermodynamic and dynamic components. The thermodynamic component, which reflects increases in atmospheric moisture content, results in a relatively uniform intensification of ENSO-driven extreme precipitation variation. In contrast, the dynamic component, which reflects changes in vertical motion, produces a strong regional difference in the response to forcing. In some regions, this component amplifies the thermodynamic-induced changes, while in others, it offsets them or even results in reduction in extreme precipitation variation. [ABSTRACT FROM AUTHOR]

Precipitation and the atmospheric moisture budget are analyzed for 40-yr periods of recent and future climate simulated by the 10 CMIP6 models from which the vertically integrated moisture flux data are available. This allows new assessments of this important atmospheric transport, which can typically only be approximated. Seasonal climatological fields are compared with those from the ERA5 reanalysis. Using the four-season average M skill score for the globe, precipitation from the 10 models is similar in skill to that from a further 25 models. The scores for six moisture variables, including flux and its convergence, demonstrate global skill for each model. The 10-model average fields have better skill than any individual model. Changes are calculated for 2040–79 under the SSP5-8.5 forcing scenario. The focus is on the 10-model average scaled as a projection of change for a global warming of 2°C. The changes in precipitation, temperature, and pressure in each season are largely consistent with those from the larger ensemble. The changes in flux, convergence, water column, and winds at three levels are presented. The role of convergence in balancing precipitation changes over many land regions is evident. Flux, convergence, and precipitation are shown in detail for selected cases, including central North America, the South American monsoon, southern Africa in summer, southern South America in winter, Europe in summer, and both polar regions. Typically, flux is enhanced, but the associated convergence may shift. The production of the vertically integrated flux vector as a standard output from climate models is supported. [ABSTRACT FROM AUTHOR]

In July 2021, Typhoon In-Fa (TIF) triggered a significant indirect heavy precipitation event (HPE) in central China and a direct HPE in eastern China. Both these events led to severe disasters. However, the synoptic-scale conditions and the impacts of these HPEs on future estimations of return periods remain poorly understood. Here, we find that the remote HPE that occurred ∼2200 km ahead of TIF over central China was a predecessor rain event (PRE). The PRE unfolded under the equatorward entrance of the upper-level westerly jet. This event, which encouraged divergent and adiabatic outflow in the upper level, subsequently intensified the strength of the upper-level westerly jet. In contrast, the direct HPE in eastern China was due primarily to the long duration and slow movement of TIF. The direct HPE occurred in areas situated less than 200 km from TIF's center and to the left of TIF's propagation trajectory. Anomaly analyses reveal favorable thermodynamic and dynamic conditions and abundant atmospheric moisture that sustained TIF's intensity. A saddle-shaped pressure field in the north of eastern China and peripheral weak steering flow impeded TIF's movement northward. Hydrologically, the inclusion of these two HPEs in the historical record leads to a decrease in the estimated return periods of similar HPEs. Our findings highlight the potential difficulties that HPEs could introduce for the design of hydraulic engineering infrastructure as well as for the disaster mitigation measures required to alleviate future risk, particularly in central China. [ABSTRACT FROM AUTHOR]

As global mean temperature rises, extreme drought events are expected to increasingly affect regions of the United States that are crucial for agriculture, forestry, and natural ecology. A pressing need is to understand and anticipate the conditions under which extreme drought causes catastrophic failure to vegetation in these areas. To better predict drought impacts on ecosystems, we first must understand how specific drivers, namely, atmospheric aridity and soil water stress, affect land surface processes during the evolution of flash drought events. In this study, we evaluated when vapor pressure deficit (VPD) and soil moisture thresholds corresponding to photosynthetic shutdown were crossed during flash drought events across different climate zones and land surface characteristics in the United States. First, the Dynamic Canopy Biophysical Properties (DCBP) model was used to estimate the thresholds that define reduced photosynthesis by assimilating vegetation phenology data from theModerate Resolution Imaging Spectroradiometer (MODIS) to a predictive phenology model. Next, we characterized and quantified flash drought onset, intensity, and duration using the standardized evaporative stress ratio (SESR) and NLDAS-2 reanalysis. Once periods of flash drought were identified, we investigated how VPD and soil moisture coevolved across regions and plant functional types. Results demonstrate that croplands and grasslands tend to be more sensitive to soil water limitations than trees across different regions of the United States. We found that whether VPD or soil moisture was the primary driver of plant water stress during drought was largely region specific. The results of this work will help to inform land managers of early warning signals relevant for specific ecosystems under threat of flash drought events. [ABSTRACT FROM AUTHOR]

El Niño–Southern Oscillation (ENSO) is the strongest interannual signature in the tropical air–sea system and can affect global atmospheric variables, but ENSO's climatic impact on land surface variables such as skin temperature, snow cover, and soil moisture that can serve as seasonal climate predictors remains relatively little known. Here we examine ENSO's impact on the three land surface variables in Northern Hemisphere extratropics by regression analysis, using a combination of ERA5 reanalysis, GLDAS, satellite-based observations, and CESM2 simulations. Our results show that during El Niño winters anomalous land surface warming occurs in Northern Hemisphere midlatitudes, accompanied by snow cover reduction, especially in northern North America and eastern Europe, while anomalous cooling occurs in southern North America and the Tibetan Plateau, accompanied by increased snow cover. Meanwhile, increased soil moisture is observed in southern North America, central Asia, and southeast China. Further analysis indicates that atmospheric moisture processes dominate the formation of the land surface anomalies, for which the changes in water vapor and precipitation induced by the ENSO-related large-scale atmospheric teleconnection are critical. The anomalous land surface warming in midlatitudes mainly results from the increased downward longwave radiation due to the increase of water vapor, whereas the physical pathway causing water vapor anomalies is different over North America and Eurasia. Controlled by ENSO-induced changes in atmospheric circulation and moisture transport, anomalous precipitation explains most of the snow cover and soil moisture anomalies, which play a certain role in shaping the skin temperature anomalies in some regions through snow–albedo feedback and evaporation, respectively. [ABSTRACT FROM AUTHOR]

Tropical rainfall variations are of direct societal relevance and drive climate variations worldwide via teleconnections. The convective rainfall tends to occur when sea surface temperature (SST) exceeds a threshold, SSTthr, usually taken to be constant in time and space. We analyze 40-yr monthly observations and find that SSTthr varies by up to 48C in space and with season. Based on local convective instability, we develop a quantitative theory that largely explains the SSTthr variations using the climatological state of the tropical atmosphere. Although it is often assumed that spatial variations of tropical upper-tropospheric temperature are small and can be neglected, it is shown that lower climatological values favor a lower SSTthr. Similarly, a small increase in climatological surface relative humidity also leads to a decrease in SSTthr, as does a lower climatological air–sea temperature difference. Consequently, efforts to understand and predict natural or forced variations in tropical rainfall must account for, in addition to SST, the temperatures aloft and the near-surface humidity and temperature and requires improved understanding of what controls their distribution in space and time. [ABSTRACT FROM AUTHOR]

Recently, we presented a classification of "primitive" relative humidity (RH) profiles into eight distinct clusters over Earth's oceans, based on about 18 years (2003–20) of observations from the AIRS on NASA's Aqua satellite. Here we investigate the seasonal variability and decadal trends, both in the vertical structure of these RH profiles, and in their associated area of occurrence. Since vertical structures (except in the marine boundary layer) of each RH class are generally robust across all seasons and change only weakly in a warming climate, seasonal or decadal changes to their occurrence areas shift patterns of global moisture distribution. Globally, the marine boundary layer exhibits nonlinear moistening effects after about 2010, the end of the warming hiatus. Annual time series of ocean areas dominated by RH classes have linear trends, which are positive only for the most moist and driest RH classes (in terms of the free troposphere) associated with deep convection and large-scale subsidence favoring conditions for low-level stratocumulus clouds, respectively. Based on estimated linear trends of RH-class occurrences and sea surface temperatures, we infer projected linear responses of RH in a warming climate. Ocean areas dominated by most moist and driest RH classes (in terms of the free atmosphere) are estimated to increase by about 1% and 2%, respectively (corresponding to about 2.5% K−1 and 4.5% K−1, respectively). The averaged global and tropical RH structure remain almost constant in a warming climate. While this is consistent with other studies, our results show how increases in most moist and dry areas compensate each other, indicating possible increases in the frequency or persistence of future extreme events. [ABSTRACT FROM AUTHOR]

Extensive expansion in irrigated agriculture has taken place over the last half century. Due to increased irrigation and resultant land-use–land-cover change, the central United States has seen a decrease in temperature and changes in precipitation during the second half of the twentieth century. To investigate the impacts of widespread commencement of irrigation at the beginning of the growing season and continued irrigation throughout the summer on local and regional weather, the Great Plains Irrigation Experiment (GRAINEX) was conducted in the spring and summer of 2018 in southeastern Nebraska. GRAINEX consisted of two 15-day intensive observation periods. Observational platforms from multiple agencies and universities were deployed to investigate the role of irrigation in surface moisture content, heat fluxes, diurnal boundary layer evolution, and local precipitation. This article provides an overview of the data collected and an analysis of the role of irrigation in land–atmosphere interactions on time scales from the seasonal to the diurnal. The analysis shows that a clear irrigation signal was apparent during the peak growing season in mid-July. This paper shows the strong impact of irrigation on surface fluxes, near-surface temperature and humidity, and boundary layer growth and decay. [ABSTRACT FROM AUTHOR]

Cascade reservoirs were designed for the Yalong River basin, which ranks as the third-largest hydropower base in China. Reservoir impoundment has certain impacts on local climate, but few researches focus on this field. This paper integrates spatiotemporal analysis techniques and geostatistical methods to identify the reservoir projected meteorological indices (MIs) variations as well as the impact scope under different circumstances. Results show that the reservoir projected variations of moisture indexes were much more significant than temperatures. Anthropogenic disturbance has led to a dramatic decrease of relative humidity in the past decade, far beyond its periodic amplitude range. Moreover, dry valleys faced more serious drought risk, but reservoir impoundment of this region alleviated the local drought risk. The daily minimum temperature, relative to the maximum temperature, was more sensitive to catchment changes with an earlier appearance of temperature zone variation. Since impounding, the MIs' internal relationships in nondry valleys varied significantly more than that in dry valleys, with a positive correlation of 0.7 between the relative humidity and precipitation weakening greatly. The severe warming hotspots are distributed in the upper reach and have a probability of 0.95-1 to exceed the 1.5°C IPCC control target. This study provides references for the disturbance of reservoir impoundment to local climate at multiple temporal--spatial scales under different circumstances and contributes to the MIs variation pattern identification and quantitative risk assessment. [ABSTRACT FROM AUTHOR]

Better understanding of how moisture, clouds, and precipitation covary under climate warming lacks a comprehensive observational view. This paper analyzes the tropical atmospheric water cycle's evolution with sea surface temperature (SST), using for the first time, the synergistic dataset of instantaneous observations of the relative humidity profile from the Megha-Tropiques satellite, clouds from the CALIPSO satellite, and near-surface precipitation from the CloudSat satellite, and quantifies their rates of change with SST warming. The dataset is partitioned into three vertical velocity regimes, with cloudy grid boxes categorized by phase (ice or liquid), opacity (opaque or thin), and the presence of near-surface precipitation. Opaque cloud cover is always larger in the presence of near-surface precipitation (high ice clouds especially). Low liquid water clouds in the descending regime dominate for SSTs < 299.25 K, where the free troposphere is dry (~20%), and opaque liquid water cloud cover decreases with SST warming (−8% K−1) and thin liquid water cloud cover stays constant (~20%). High ice clouds dominate the ascending regime in which, for 299.25 < SST < 301.75 K, humidity increases with SST in the lower free troposphere and peaks around 302 K. Over the warm SST range (>301.75 K), in the ascending regime, opaque high ice cloud cover decreases with SST (−13% K−1), while thin ice cloud cover increases (+6% K−1). Over the warm SST range, total cloudiness decreases with warming in all regimes. This paper characterizes fundamental relationships between aspects of the tropical atmospheric water cycle and SST. [ABSTRACT FROM AUTHOR]

Much of our conceptual understanding of midlatitude atmospheric motion comes from two-layer quasigeostrophic (QG) models. Traditionally, these QG models do not include moisture, which accounts for an estimated 30%–60% of the available energy of the atmosphere. The atmospheric moisture content is expected to increase under global warming, and therefore, a theory for how moisture modifies atmospheric dynamics is crucial. We use a two-layer moist QG model with convective adjustment as a basis for analyzing how latent heat release and large-scale moisture gradients impact the scalings of a midlatitude system at the synoptic scale. In this model, the degree of saturation can be tuned independently of other moist parameters by enforcing a high rate of evaporation from the surface. This allows for study of the effects of latent heat release at saturation, without the intrinsic nonlinearity of precipitation. At saturation, this system is equivalent to the dry QG model under a rescaling of both length and time. This predicts that the most unstable mode shifts to smaller scales, the growth rates increase, and the inverse cascade extends to larger scales. We verify these results numerically and use them to verify a framework for the complete energetics of a moist system. We examine the spectral features of the energy transfer terms. This analysis shows that precipitation generates energy at small scales, while dry dynamics drive a significant broadening to larger scales. Cascades of energy are still observed in all terms, albeit without a clearly defined inertial range. Significance Statement: The effect of moist processes, especially the impact of latent heating associated with condensation, on the size and strength of midlatitude storms is not well understood. Such insight is particularly needed in the context of global warming, as we expect moisture to play a more important role in a warmer world. In this study, we provide intuition into how including condensation can result in midlatitude storms that grow faster and have features on both larger and smaller scales than their dry counterparts. We provide a framework for quantifying these changes and verify it for the special case where it is raining everywhere. These findings can be extended to the more realistic situation where it is only raining locally. [ABSTRACT FROM AUTHOR]

Statistical downscaling (SD) of climate change projections is a key piece for impact and adaptation studies due to its low computational expense compared to dynamical downscaling, which allows exploration of uncertainties through the generation of large ensembles. SD has been extensively evaluated and applied in the extratropics, but few examples exist in tropical regions. In this study, several state-of-the-art methods belonging to different families have been evaluated for maximum/minimum daily temperature and daily accumulated precipitation (both from the ERA5 at 0.258) in two regions with very different climates: Spain (midlatitudes) and Central America (tropics). Some key assumptions of SD have been tested: the strength of the predictor–predictand links, the skill of different approaches, and the extrapolation capability of each method. It has been found that relevant predictors are different in both regions, as is the behavior of statistical methods. For temperature, most methods perform significantly better in Spain than in Central America, where transfer function (TF) methods present important extrapolation problems, probably due to the low variability of the training sample (present climate). In both regions, model output statistics (MOS) methods have achieved the best results for temperature. In Central America, TF methods have achieved better results than MOS methods in the evaluation in the present climate, but they do not preserve trends in the future. For precipitation, MOS methods and the extreme gradient boost machine learning method have achieved the best results in both regions. In addition, it has been found that, although the use of humidity indices as predictors improves results for the downscaling of precipitation, future trends given by statistical methods are very sensitive to the use of one or another index. Three indices have been compared: relative humidity, specific humidity, and dewpoint depression. The use of the specific humidity has been found to lead to trends given by the downscaled projections that deviate seriously from those given by raw global climate models in both regions. [ABSTRACT FROM AUTHOR]

The impact of assimilating dropsonde data from the 2020 Atmospheric River (AR) Reconnaissance (ARR) field campaign on operational numerical weather forecasts was assessed. Two experiments were executed for the period from 24 January to 18 March 2020 using the National Centers for Environmental Prediction (NCEP) Global Forecast System version 15 (GFSv15) with a four-dimensional hybrid ensemble-variational (4DEnVar) data assimilation system. The control run (CTRL) used all of the routinely assimilated data and included data from 628 ARR dropsondes, whereas the denial run (DENY) excluded the dropsonde data. Results from 17 intensive observing periods (IOPs) indicate a mixed impact for mean sea level pressure and geopotential height over the Pacific-North American (PNA) region in CTRL compared to DENY. The overall local impact over the U.S. WestCoast and Gulf of Alaska for the 17 IOPs is neutral (20.45%) for integrated vapor transport (IVT), but positive for wind and moisture profiles (0.5%-1.0%), with a spectrum of statistically significant positive and negative impacts for various IOPs. The positive dropsonde data impact on precipitation forecasts over U.S. West Coast domains appears driven, in part, by improved low-level moisture and wind fields at short-forecast lead times. Indeed, data gaps, especially for accurate and unbiased moisture profiles and wind fields, can be at least partially mitigated to improve U.S. West Coast precipitation forecasts. [ABSTRACT FROM AUTHOR]

A lack of routine environmental observations located near deepening cumulus congestus clouds limits verification of important theorized and simulated updraft–environment interaction processes occurring during deep convection initiation (CI). We analyze radiosonde profiles collected during several hundred CI events near a mountain range in central Argentina during the CACTI field campaign. Statistical analyses illustrate environmental conditions supporting radar-observed CI outcomes that span a spectrum of convective cell depths, widths, and durations, as well as events lacking precipitating convection. Tested environmental factors include a large variety of sounding-derived measurements of CAPE, CIN, moisture, terrain-relative winds, vertical shear, and lifted parcel properties, with supplemental model reanalysis of background larger-scale vertical motion. CAPE and CIN metrics do not consistently differentiate CI success from failure. Only a few environmental factors contain consistent monotonic relationships among the spectrum of cloud depths achieved during CI: (i) the depth and strength of background ascent, and (ii) the component of low-level flow oriented parallel to the ridgeline. These metrics suggest that the ability of the surrounding flow to lift parcels to their LFC and terrain-modified flow are consistently relevant processes for CI. Low- to midlevel relative humidity strongly discriminated between CI and non-CI events, likely reflecting entrainment-driven dilution processes. However, we could not confidently conclude that relative humidity similarly discriminated robust from marginal CI events. Circumstantial evidence was found linking cell width, an important cloud property governing the probability of CI, to LCL height, boundary layer depth, depth and magnitude of the CIN layer, and ambient wind shear. [ABSTRACT FROM AUTHOR]