Search

In winter, the Kuroshio Current intrudes onto the continental shelf in the East China Sea with a cyclonic branch occasionally veering northeast of the Taiwan Strait. The dynamic process and mechanism by which the southward Kuroshio branch enters the Taiwan Strait is investigated using a numerical model. Decomposition of the flow field and sea surface height (SSH) using the empirical orthogonal function (EOF) isolates the first mode of the southward flow and the east‐westward SSH slope, showing that the time series is closely related to wind strength. The southward Kuroshio branch is mainly controlled by geostrophic and Ekman effects. When northeasterly winds are weak, the elevated SSH on the eastern side, caused by the Kuroshio, generates a northward geostrophic current. This northward current overwhelms the Ekman current and triggers the Taiwan Warm Current. Conversely, when northeasterly winds are strong, an elevated SSH on the western side generates a southward geostrophic current. A theoretical equation is devised, which accounts for Ekman layer depth related to wind strength and geostrophic adjustments by the Ekman effect. This equation explains why the Kuroshio branch flows into the northern Taiwan Strait and why there is a counter‐wind current (i.e., Taiwan Warm Current) during winter. Furthermore, we propose a novel perspective that suggests the alongshore current and cross‐shore sea level in the Taiwan Strait are determined by the wind‐driven current and geostrophic adjustment, which are generated by the competition or cooperation between the wind and the alongshore pressure gradient caused by the Kuroshio. [ABSTRACT FROM AUTHOR]

Regional weather variability and extremes over Europe are strongly linked to variations in the North Atlantic jet stream, especially during the winter season. Projections of the evolution of the North Atlantic jet are essential for estimating the regional impacts of climate change. Therefore, separating forced trends in the North Atlantic jet from its natural variability is an extremely relevant task. Here, a deep learning based method, the Latent Linear Adjustment Autoencoder (LLAE), is used for this purpose on an ensemble of fully‐coupled climate simulations. The LLAE is based on a variational autoencoder and an additional linear component. The model uses detrended temperature and geopotential to predict the component of the zonal wind associated with natural variability. The residual between this prediction and the original wind field is interpreted as the forced component of the jet. The method is first tested for the geostrophic wind for which the forced trend can be obtained analytically from the difference between geostrophic wind computed from detrended and full geopotential. Despite the large variability of the total trends, the LLAE is shown to be effective in extracting the forced component of the trend for each individual ensemble member in both geostrophic and full wind fields. The LLAE‐derived forced trend shows an increase in the upper‐level zonal wind speed along a southwest–northeast oriented band over the ocean and a jet extension toward Europe. These are common characteristics over different periods and show some similarities to the upper‐level zonal wind speed trend obtained from the ERA5 reanalysis. Plain Language Summary: The North Atlantic jet stream, which is a narrow and strong air current flowing from west to east, has a substantial influence on daily weather variability and temperature and precipitation extremes in Europe. It is therefore essential to investigate how the North Atlantic jet is being modified under climate change. Trends in the North Atlantic jet are the combination of an external forcing produced by the increase in greenhouse gases and natural variability across multiple time scales. In this study, we use a data driven model called Latent Linear Adjustment Autoencoder (LLAE) to separate the jet trends related to the effect of the increase of greenhouse gases from the natural variability. The output of a set of climate simulations, namely zonal wind, temperature and geopotential, is used to adapt the adjustable parameters of the LLAE. After this step, the difference between the wind provided by the LLAE and the original field contains the effect of the external forcing. The method is successful in removing most effects associated with natural variability and reveals an intensification increase of the North Atlantic jet along a southwest–northeast oriented band and an extension over Europe. Key Points: A deep learning‐based method is able to extract forced trends in the winter North Atlantic jet stream from climate simulationsThe characteristics of the forced trends are consistent across different periodsForced trends show an intensification of the jet along a southwest–northeast oriented band and a jet extension over Europe [ABSTRACT FROM AUTHOR]

The governing equations for a viscous, incompressible fluid, in the thin-shell approximation, written in a coordinate frame which ensures validity at the North Pole, is simplified by invoking the f-plane approximation. Together with suitable stress conditions at the surface, describing the interaction of wind over ice and ice over water, a solution is developed for the Transpolar Drift current. This involves prescribing the near-surface geostrophic ocean flow and also, for ease of calculation, a simple version of the Ekman flow. Then, via the stress conditions at the surface, the near-surface wind consistent with these flows is obtained. An example, which exhibits the reduction in ice thickness as the flow moves over the North Pole towards the Fram Strait, and also describes the relative directions and speeds of the wind, ice and water, is presented in graphical form; this solution recovers what is observed. The considerable freedom available in this approach, allowing choices to be made for the geostrophic flow and the wind (both variable), and the ice profile at some initial position, is explained, opening the way to further, extensive investigations. [ABSTRACT FROM AUTHOR]

Southeastern South America (SESA) is a highly productive agricultural region and a hot spot for land‐atmosphere interactions. To evaluate the impact of dry soil moisture anomalies (SMAs) on SESA climate and the sensitivity of the regional climate response to the location of SMAs, we perform three experimental simulations using the Community Earth System Model (CESM) with prescribed dry SMAs over (a) SESA, (b) western SESA, and (c) eastern SESA. The dry SESA and eastern SESA simulations show widespread negative precipitation anomalies. In contrast, the dry western SESA simulation shows positive precipitation anomalies over northeastern Argentina, which are associated with the enhanced southward moisture flux co‐located with the South American low‐level jet exit region. A composite analysis of extremely dry cases over western SESA using reanalysis data agrees with the findings from our CESM experiment. These findings have potential implications for subseasonal forecasting in this region. Plain Language Summary: Large‐scale soil moisture anomalies evolve slowly and can provide an opportunity for better weather forecasting at timescales longer than 2 weeks. Therefore, it is critical to understand the causal physical mechanism and evaluate whether the regional climate response is sensitive to the location of soil moisture anomalies, especially in a productive agricultural region like southeastern South America (SESA). Using a numerical climate model, we simulate the impacts of dry soil over (a) SESA, (b) western SESA, and (c) eastern SESA. The simulations show that dry soil over western SESA can alter regional atmospheric circulation in the proximity of the existing corridor of poleward moisture transport, hence enhancing rainfall over northeastern Argentina. Conversely, dry soil over eastern SESA or the entire SESA region results in less precipitation because enhanced northerly transport is not co‐located with the low‐level wind corridor. Analysis of a dataset that incorporates observations supports our findings from numerical simulations. Key Points: The impact of dry soil moisture anomalies (SMAs) on southeastern South America (SESA) regional climate is sensitive to the location of SMAsThis study provides a causal mechanism linking soil moisture to precipitation via atmospheric circulationWhen western SESA has dry soil, it generates anomalous geostrophic wind, which is co‐located with the low‐level jet exit region [ABSTRACT FROM AUTHOR]

Global long-term stable 3D wind fields are a valuable information for climate analyses of atmospheric dynamics. Their monitoring remains a challenging task, given shortcomings of available observations. One promising option for progress is the use of radio occultation (RO) satellite data, which enable to derive wind fields based on the geostrophic and gradient wind approximations. In this study we focus on three main goals, explored through European Re-Analysis ERA5 and RO datasets, using monthly-mean January and July data over 2007–2020 with a 2.5° × 2.5° resolution. First, by comparing ERA5-derived geostrophic and gradient wind speeds to the ERA5 original wind speed, we examine the regions of validity for both these approximations. Second, to assess the potential added value of RO-derived geostrophic and gradient winds, we test how well they agree with the corresponding ERA5-derived winds. Third, we evaluate the potential of the RO wind fields relative to the ERA5 original wind fields. With this three-step analysis we decompose the total wind speed bias into a bias resulting from the approximation and the systematic difference between the RO and ERA5 datasets. We find that the geostrophic approximation is a valid method to be used to estimate tropospheric winds, while the gradient wind approximation works better in the stratosphere. Both approximations generally work well in the corresponding altitude regions, within 2 m s-1 accuracy almost globally (latitudes 5°–82.5°), with some exceptions in the winter hemisphere: monsoonal area at the lower altitudes, northern polar regions at higher altitudes, and larger mountain regions throughout all investigated altitude levels. RO- and ERA5-derived geostrophic and gradient winds mostly showed very good agreement, generally within 2 m s-1. However, when studying the decadal trend, temporal change in the systematic differences higher than 0.5 m s-1 per decade was found. This points to a potential effect of observing system changes in ERA5 around the year 2016. The overall high accuracy of the monthly-mean wind fields, backed by the long-term stability of the underlying RO data, highlights the added value and potential benefit of RO-derived winds for climate monitoring and analyses. [ABSTRACT FROM AUTHOR]

We introduce an analytical model that describes the vertical structure of Ekman boundary layer flows coupled to the Monin-Obukhov Similarity Theory (MOST) surface layer representation, which is valid for conventionally neutral (CNBL) and stable (SBL) atmospheric conditions. The model is based on a self-similar profile of horizontal stress for both CNBL and SBL flows that merges the classic 3/2 power law profile with a MOST-consistent stress profile in the surface layer. The velocity profiles are then obtained from the Ekman momentum balance equation. The same stress model is used to derive a new self-consistent Geostrophic Drag Law (GDL). We determine the ABL height (h) using an equilibrium boundary layer height model and parameterize the surface heat flux for quasi-steady SBL flows as a function of a prescribed surface temperature cooling rate. The ABL height and GDL equations can then be solved together to obtain the friction velocity (u ∗) and the cross-isobaric angle ( α 0 ) as a function of known input parameters such as the Geostrophic wind speed and surface roughness (z 0) . We show that the model predictions agree well with simulation data from the literature and newly generated Large Eddy Simulations (LES). These results indicate that the proposed model provides an efficient and relatively accurate self-consistent approach for predicting the mean wind velocity distribution in CNBL and SBL flows. [ABSTRACT FROM AUTHOR]

Extratropical storms are one of the major coastal hazards along the coastline of the German Bight, the southeastern part of the North Sea, and a major driver of coastal protection efforts. However, the predictability of these regional extreme events on a seasonal scale is still limited. We therefore improve the seasonal prediction skill of the Max Planck Institute Earth System Model (MPI-ESM) large-ensemble decadal hindcast system for German Bight storm activity (GBSA) in winter. We define GBSA as the 95th percentiles of three-hourly geostrophic wind speeds in winter, which we derive from mean sea-level pressure (MSLP) data. The hindcast system consists of an ensemble of 64 members, which are initialized annually in November and cover the winters of 1960/61–2017/18. We consider both deterministic and probabilistic predictions of GBSA, for both of which the full ensemble produces poor predictions in the first winter. To improve the skill, we observe the state of two physical predictors of GBSA, namely 70 hPa temperature anomalies in September, as well as 500 hPa geopotential height anomalies in November, in areas where these two predictors are correlated with winter GBSA. We translate the state of these predictors into a first guess of GBSA and remove ensemble members with a GBSA prediction too far away from this first guess. The resulting subselected ensemble exhibits a significantly improved skill in both deterministic and probabilistic predictions of winter GBSA. We also show how this skill increase is associated with better predictability of large-scale atmospheric patterns. [ABSTRACT FROM AUTHOR]

We present a new explicit quasi-Lagrangian integration scheme with the three-dimensional cubic spline function transform (transform = fitting + interpolation, referred to as the "spline format") on a spherical quasi-uniform longitude–latitude grid. It is a consistent longitude–latitude grid, and to verify the feasibility, accuracy, convergence, and stability of the spline format interpolation scheme for the upstream point on the longitude–latitude grid, which may map a quasi-uniform longitude–latitude grid, a set of ideal, exact test schemes is adopted, which are recognized and proven to be effective internationally. The equilibrium flow test, cross-polar flow test, and Rossby–Haurwitz wave test are used to illustrate the spline scheme uniformity to the linear scheme and to overcome the over-dense grid in the polar region and the non-singularity of the poles. The cross-polar flow test demonstrates that the geostrophic wind crosses the polar area correctly, including the South Pole and North Pole. A non-hydrostatic, fully compressible dynamic core is used to complete the density flow test, demonstrating the existence of a time-varying reference atmosphere and that the spline format can simulate highly nonlinear fine-scale transient flows. It can be compared for the two results of the density flow test between the solution with the spline format and the benchmark reference solution with the linear format. Based on the findings, the non-hydrostatic dynamic core with the spline format is recommended for adoption. When simulated for the flow over an ideal mountain, through the "topographic gravity wave test", the bicubic surface terrain and terrain-following height coordinates, time-split integration, and vector discrete decomposition can be derived successfully. These may serve as the foundations for a global, unified spline-format numerical model in the future. [ABSTRACT FROM AUTHOR]

It is known that the southwest vortex (SWV) is an important weather system that may induce severe weather. The southward deviation of an SWV track forecasted by the Global Assimilation and Prediction System of the China Meteorological Administration (CMA-GFS) is systematically diagnosed in this study. The southward shift of the SWV is directly attributed to the deviation of the steering flow caused by the weak forecast of the upper-level trough. According to the diagnosis of potential tendency, the underestimation of the initial vorticity advection forecasted by CMA-GFS dominates the weak development of the upper-level trough. The underestimation of the vorticity advection is eventually sourced to the weak geostrophic wind caused by the weak initial meridional and zonal gradients of the midlevel height in front of the trough. The assimilation process on the initial field of the CMA-GFS acts a negative effect on forecasting this SWV track. It weakens the π field at midmodel level, resulting in the weak midlevel height gradient in front of the trough. A verified numerical experiment initialized by a more reasonable field is carried out and the southward shift of the SWV is obviously modified. This study suggests that a reasonable analysis field is crucial for the accurate forecast of the SWV track. Significance Statement: The important impact of initial field deviation in key regions on the forecast in the late period is highlighted. A systematic diagnosis process for identifying and addressing forecast issues on SWV track is proposed. This research provides a comprehensive approach for diagnosing the forecast deviation associated with SWV track. [ABSTRACT FROM AUTHOR]

The development and breakdown of Kelvin–Helmholtz (KH) waves (billows) in the stable atmospheric boundary layer (SABL) and their impact on vertical transport of momentum and scalars have been examined utilizing large eddy simulations. These simulations are initialized with a vertically uniform geostrophic wind and a constant potential temperature lapse rate. An Ekman type of boundary layer develops, and an inflection point forms in the SABL, which triggers the KH instability (KHI). KHI develops with the kinetic energy (KE) in the KH billows growing exponentially with time. The subsequent onset of secondary shear instability along S-shaped braids leads to the turbulent breakdown of the KH billow cores and braids. The frictional ground surface tends to slow down the growth of KE near the surface, reduce the KH billow core depth, and likely suppress other types of secondary instability. KH billows induce substantial down-gradient transport of momentum and sensible heat, which can be further enhanced by the onset of secondary shear instability. Although the KHI-induced strong transport only lasts for around 10–20 min, it reduces vertical shear and stratification in the SABL, enhances surface winds, and results in a 2–3-fold increase in the SABL depth. [ABSTRACT FROM AUTHOR]

In this study, we examine eight major boreal Sudden Stratospheric Warming (SSW) events between 2007 and 2019 to understand the vertical coupling between the troposphere and stratosphere, as well as the relationship between SSWs and blocking events using Global Navigation Satellite System (GNSS) radio occultation (RO) observations. Our study covers the main aspects of SSW events, including the vertical structure of planetary wave propagation, static stability, geometry of the polar vortex, and the occurrence of blocking events. To analyze wave activity and atmospheric circulation, we compute the quasi-geostrophic Eliassen–Palm (EP) flux and geostrophic winds. The results show that the GNSS RO represent the primary dynamic features in agreement with theory and previous studies and provide a detailed view of their vertical structure. We observe a clear positive peak of upward EP flux in the stratosphere prior to all SSW events. In seven out of eight events, this peak is preceded by a clear peak in the troposphere. Within the observed timeframe, we identify two types of downward dynamic interactions and the emergence of blocking events. During the 2007 and 2008 "reflecting" events, we observe a displacement of the polar vortex, along with a downward propagation of wave activity from the stratosphere to the troposphere during vortex recovery, coinciding with the formation of blocking in the North Pacific region. Conversely, in the other six SSW "absorbing" events from 2009 to 2019, characterized by vortex split, we observe wave absorption and the subsequent formation of blocking in the Euro-Atlantic region. The analysis of the static stability demonstrates an enhancement of the polar tropopause inversion layer as the result of SSWs, which was stronger for the absorbing events. Overall, our study provides a purely observational view of the synoptic and dynamic evolution of the major SSWs, their link to blocking, and the impact on the polar tropopause. [ABSTRACT FROM AUTHOR]

Low-level jets (LLJs), in which the wind speed attains a local maximum at low altitudes, have been found to occur in the U.S. mid-Atlantic offshore, a region of active wind energy deployment as of 2023. In contrast to widely studied regions such as the U.S. southern Great Plains and the California coastline, the mechanisms that underlie LLJs in the U.S. mid-Atlantic are poorly understood. This work analyzes floating lidar data from buoys deployed in the New York Bight to understand the characteristics and causes of coastal LLJs in the region. Application of the Hilbert–Huang transform, a frequency analysis technique, to LLJ case studies reveals that mid-Atlantic jets frequently occur during times of adjustment in synoptic-scale motions, such as large-scale temperature and pressure gradients or frontal passages, and that they do not coincide with motions at the native inertial oscillation frequency. Subsequent analysis with theoretical models of inertial oscillation and thermal winds further reveals that these jets can form in the stationary geostrophic wind profile from horizontal temperature gradients alone—in contrast to canonical LLJs, which arise from low-level inertial motions. Here, inertial oscillation can further modulate the intensity and altitude of the wind speed maximum. Statistical evidence indicates that these oscillations arise from stable stratification and the associated frictional decoupling due to warmer air flowing over a cold sea surface during the springtime land–sea breeze. These results improve our conceptual understanding of mid-Atlantic jets and may be used to better predict low-level wind speed maxima. Significance Statement: The purpose of this work is to identify and characterize the atmospheric mechanisms that result in an occasional low-level maximum in the wind speed off the U.S. mid-Atlantic coastline. Our findings show that these low-level jets form due to horizontal temperature gradients arising from fronts and synoptic systems, as well as from the land–sea breeze that forces warmer air over the cold ocean surface. This work aids predictability of such jets, improves our understanding of this coastal environment, and has implications for future deployment of offshore wind energy in this region. [ABSTRACT FROM AUTHOR]

Extratropical storms are one of the major coastal hazards along the coastline of the German Bight, the southeastern part of the North Sea, and a major driver of coastal protection efforts. However, the predictability of these regional extreme events on a seasonal scale is still limited. We therefore improve the seasonal prediction skill of the Max-Planck-Institute Earth System Model (MPI-ESM) large-ensemble decadal hindcast system for German Bight storm activity (GBSA) in winter. We define GBSA as the 95th percentiles of three-hourly geostrophic wind speeds in winter, which we derive from mean sea-level pressure (MSLP) data. The hindcast system consists of an ensemble of 64 members, which are initialized annually in November and cover the winters of 1960/61–2017/18. We consider both deterministic and probabilistic predictions of GBSA, for both of which the full ensemble produces poor predictions in the first winter. To improve the skill, we observe the state of two physical predictors of GBSA, namely 70 hPa temperature anomalies in September, as well as 500 hPa geopotential height anomalies in November, in areas where these two predictors are correlated with winter GBSA. We translate the state of these predictors into a first guess of GBSA and remove ensemble members with a GBSA prediction too far away from this first guess. The resulting subselected ensemble exhibits a significantly improved skill in both deterministic and probabilistic predictions of winter GBSA. We also show how this skill increase is associated with better predictability of large-scale atmospheric patterns. [ABSTRACT FROM AUTHOR]

The Arctic is undergoing rapid changes due to global warming, including the expansion of the marginal ice zone (MIZ), a zone of mixed ice and open water surfaces. To predict the atmospheric interaction with these surfaces, a critical process in climate models, this paper examines a simplified theoretical framework to non-dimensionalize the dynamics of the atmospheric boundary layer (ABL) over a mixed ice-water surface (MIZ–ABL). A heterogeneity Richardson number, Ri h , is proposed to account for the difference in temperature between the ice and water surface in relation to the synoptic pressure gradient forcing. With the wind angle relative to the ice-water interface, α , this framework hypothesizes that these two dimensionless numbers, regardless of individual dimensional variables (surface temperature and geostrophic wind speed) are sufficient to predict the MIZ–ABL dynamics. To test this framework, large-eddy simulations were employed over half-ice and half-water surfaces, with varying surface temperatures and geostrophic wind velocities. While the surface heat fluxes over ice, water, and the aggregate surface seem to be captured reasonably well by α and Ri h , the mean wind and turbulent kinetic energy (TKE) profiles were not, suggesting that not only the difference in stability between the two surface, but also the individual stabilities over each surface influence the dynamics. The wind angle had a significant impact on the results, both in terms of heat fluxes at the surface, turbulent and dispersive fluxes in the MIZ–ABL, and the structure of the secondary circulations. When wind blows perpendicular to the water-ice interface, internal boundary layers are favoured except at the highest Ri h simulated. For cases with wind parallel to the interface, large rolls parallel to the shore emerge. The paper raises at least as many questions as it answers, highlighting the need for further studies of the MIZ–ABL. [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) are filaments of enhanced horizontal moisture transport in the atmosphere. Due to their prominent role in the meridional moisture transport and regional weather extremes, ARs have been studied extensively in recent years. Yet, the representations of ARs and their associated precipitation on a global scale remains largely unknown. In this study, we developed an AR detection algorithm specifically for satellite observations using moisture and the geostrophic winds derived from 3D geopotential height field from the combined retrievals of the Atmospheric Infrared Sounder and the Advanced Microwave Sounding Unit on NASA Aqua satellite. This algorithm enables us to develop the first global AR catalog based solely on satellite observations. The satellite‐based AR catalog is then combined with the satellite‐based precipitation (Integrated Muti‐SatellitE Retrievals for GPM) to evaluate the representations of ARs and AR‐induced precipitation in reanalysis products. Our results show that the spreads in AR frequency and AR length distribution are generally small across data sets, while the spread in AR width is relatively larger. Reanalysis products are found to consistently underestimate both mean and extreme AR‐related precipitation. However, all reanalyses tend to precipitate too often under AR conditions, especially over low latitude regions. This finding is consistent with the "drizzling" bias which has plagued generations of climate models. Overall, the findings of this study can help to improve the representations of ARs and associated precipitation in reanalyses and climate models. Plain Language Summary: Atmospheric rivers (ARs) are filaments of enhanced horizontal moisture transport in the atmosphere. These weather systems are responsible for most of the poleward atmospheric moisture transport over midlatitudes and can cause extreme precipitation around the world. For a long time, researchers relied heavily on reanalysis products to study ARs. Albeit incorporating information from observations, reanalyses are produced by numerical models and thus should not be treated as real observations. In this study, for the first time, we developed a near global AR detection algorithm specially for satellite observations. Unlike previous AR detection algorithms designed for satellite observations, which were applicable to mean moisture content and on regional scales, our algorithm utilizes both the moisture field and wind information from satellite observations. Our algorithm can thus better characterize the transport nature of the detected ARs. Using both the developed algorithm and NASA satellite observations, we developed the first satellite‐based near global AR catalog. This satellite‐based AR catalog together with NASA satellite precipitation product was then used to evaluate how well major reanalyses represent ARs and their associated precipitation. We found that reanalyses generally perform well in representing the AR occurrence frequency and length, but show relatively larger uncertainty in representing the AR width. In reanalyses, ARs produce precipitation that is both too weak and too frequent. Our findings can help to improve the representation of ARs and associated precipitation in reanalyses and climate models. As the quality of satellite observations continues to improve, the methodology presented here can be applied to other satellite observations to develop higher resolution or higher frequency AR statistics. Key Points: Developed a novel method to detect atmospheric rivers (ARs) using moisture and wind data from satellite observationsReanalyses and satellite observations show high agreement with each other in AR frequency distributionsUnder AR conditions, reanalyses tend to precipitate too often and too lightly [ABSTRACT FROM AUTHOR]

Particularly challenging classes of heterogeneous surfaces are ones where strong secondary circulations are generated, potentially dominating the flow dynamics. In this study, we focus on land-sea breeze (LSB) circulations resulting from surface thermal contrasts, in the presence of increasing synoptic pressure forcing. The relative importance and orientation of the thermal and synoptic forcings are measured through two dimensionless parameters: a heterogeneity Richardson number (measuring the relative strength of geostrophic wind and convection induced by buoyancy), and the angle a between the shore and geostrophic wind. Large-eddy simulations reveal the emergence of various regimes where the dynamics are asymmetric with respect to a. Along-shore cases result in deep LSBs similar to the scenario with no synoptic background, irrespective of the geostrophic wind strength. Across-shore simulations exhibit a circulation cell that decreases in height with increasing synoptic forcing. However, at the highest synoptic winds simulated, the circulation cell is advected away with sea-to-land winds, while a shallow circulation persists for land-to-sea cases. Scaling analysis that relates the internal parameters Qshore (net shore volumetric flux) and qshore (net shore advected kinematic heat flux) to the external input parameters results in a succinct model of the shore fluxes that also helps explain the physical implications of the identified LSBs. Finally, the vertical profiles of the shore-normal velocity and shore-advected heat flux are used, with the aid of k-means clustering, to independently classify the LSBs into four regimes (canonical, sea-driven, land-driven, and advected), corroborating our visual categorization. [ABSTRACT FROM AUTHOR]

The South China coast has a high incidence of warm‐sector heavy rainfall (WSHR) events. The ageostrophic winds in the boundary layer in most of these events associated with the southwesterly boundary layer jets (BLJs) mainly exhibit strong convergence at rainfall area. In this paper, we analyze two case studies of WSHR in May 2013 and May 2015, which occurred in similar synoptic environments but varied in intensity, extent, and duration of rainfall, where the ageostrophic winds are the confrontational confluence and asymptotic confluence pattern, respectively. ERA‐5 reanalysis data and the diagnostic equation of ageostrophic wind are used to examine the factors affecting the ageostrophic winds in the northern land region and the southern offshore region of the rainfall. The results suggest that land‐sea contrast leads to the convergence of ageostrophic winds in the rainfall area. Boundary layer friction dominates the northeasterly ageostrophic winds on land. The diurnal variation of BLJs dominates the ageostrophic winds and their diurnal variation at sea. It can contribute southwesterly or southeasterly ageostrophic winds, so the phase difference between the land and sea forms confrontational or asymptotic confluence, respectively. BLJs with different intensities, extents, and diurnal variations can lead to different ageostrophic wind patterns and their confluence modes. The land‐sea thermal contrast can directly affect ageostrophic winds, and it can also affect the diurnal variation of BLJs, thus affecting the ageostrophic winds and their confluence mode. It is further verified that the BLJs and thermal forcing are important in WSHR processes in South China. Plain Language Summary: Extremely heavy rainfall frequently occurs in coastal Southern China, which causes great damage and is difficult to accurately forecast. Most of this kind of rainfall is associated with southwesterly wind in the lower atmosphere and strong convergence of the lower airflow. Our purpose is to better understand the factors that lead to strong convergence, which is an important factor for heavy rainfall. Since the geostrophic wind component has no divergence, the convergence is mostly contributed by the ageostrophic wind component, which doesn't conform to the geostrophic balance. We find that different intensities, ranges, and diurnal changes of the southwesterly wind in the lower atmosphere can lead to different distributions of ageostrophic winds in the South China Sea. The thermal difference between land and sea can influence the diurnal change of the southwesterly wind in the lower atmosphere, and it can also affect the ageostrophic winds and their convergence. The results will potentially provide a better understanding of heavy rainfall events in Southern China and may lead to better forecasts and issuing of warnings. Key Points: Land‐sea contrast leads to the strong convergence in the coastal warm‐sector extreme heavy rainfall area in South ChinaMarine boundary layer jets (BLJs) and their diurnal variation can influence the ageostrophic winds and then affect the convergenceThe land‐sea thermal contrast can affect the ageostrophic winds and the convergence by changing the diurnal variation of BLJs [ABSTRACT FROM AUTHOR]

A set of CMIP6 models is evaluated for the turning of the wind over the planetary boundary layer (PBL) and the corresponding cross-isobaric mass flux. The bulk Richardson number method is used to calculate the height of the PBL to allow for comparisons with a climatology of observed wind-turning angles documented by Lindvall and Svensson based on more than 800 stations in the Integrated Global Radiosonde Archive. Wind-turning angles are found to be underestimated in all models, with the GFDL CM4 model having the closest distribution to the observations. Large, negative cross-isobaric mass fluxes (flow toward higher pressure) are found over high-terrain areas and the North Atlantic storm-track region in all models and the ERA-Interim reanalysis. Bulk Richardson number–derived PBLs are particularly shallow in the Norwegian Earth System Model, medium atmosphere-medium ocean resolution (NorESM2-MM), likely caused by a change in the turbulence and cloud scheme as compared to the CESM2 model that uses the same atmospheric model, leading to small wind-turning angles and cross-isobaric mass fluxes. Using the 850-hPa level as the upper boundary broadens the wind-turning angle distribution and increases the amount of cross-isobaric mass flux for all models. This makes the models closer to the observations, although substantial differences are still present. The assumption of a constant geostrophic wind throughout the PBL possibly affects the calculated mass fluxes. [ABSTRACT FROM AUTHOR]

The current work focuses on establishing the parameters that influence the wind's behavior over the Aegean and Ionian Seas and estimating the wind potential in the region based on long-term historic climate data. Combining a downscaling technique performed with the well-founded WRF-ARW computational algorithm and a number of simultaneous meteorological station time series, an attempt is made to investigate how regional changes may affect low-altitude wind speed distribution at hub height (100 m a.s.l.). The provided time-series coastal data span the entire region of interest from north to south. WRF-ARW v.3.9 is utilized to associate the geostrophic wind distribution obtained from long-term Copernicus ERA5 wind data with the localized wind potential over lower altitudes. Evaluation and correlation of the observational data to the predicted wind climate are performed, and the statistical differences that arise are investigated. High-accuracy wind resource potential maps are thus obtained in the region. Also, a few distinctive flow patterns are identified, such as wind speed cut-off regions and very high wind speed distributions, which are presented in specific southern regions of the Aegean Sea. [ABSTRACT FROM AUTHOR]

Globally available and highly vertical resolved wind fields are crucial for the analysis of atmospheric dynamics for the benefit of climate studies. Most observation techniques have problems to fulfill both requirements. Especially in the tropics and in the southern hemisphere more wind data availability is required. In this study we investigate the potential of radio occultation (RO) data for climate-oriented wind field monitoring in the tropics, with a specific focus on the equatorial area between ±5° latitude. In this region, the geostrophic balance breaks down, due to the Coriolis force term approaching zero. One further aim is to understand how the individual wind components of the geostrophic balance and equatorial balance approximations bridge across the equator and where each component breaks down. We analyze the equatorial balance equation within this latitude band. In a wider range over the tropics, we derive the RO wind fields also using the geostrophic approximation and we compared the RO winds with ERA5 data. From analyzing first the zonal and meridional wind component, we find that the meridional wind component is more volatile in its derivation, however the total wind speed benefits from a computation of both wind components. Investigating next the bias between the RO and ERA5 computed winds, we find that the systematic data bias is smaller than the bias resulting from the approximation itself. As a final aspect we inspected the monthly-mean RO wind data over the full example year 2009. The bias in the upper troposphere and lower stratosphere is mainly smaller than ±2ms-1, which is in line with the wind field requirements of the World Meteorological Organization. This is encouraging for the use of RO wind fields in climate monitoring over the entire globe including the equatorial region. [ABSTRACT FROM AUTHOR]

Increasing freshwater fluxes from the Greenland ice sheet and the Arctic to the Subpolar North Atlantic could cause a freshening of deep convection regions and affect the overturning circulation. However, freshwater pathways from the Greenland shelf to interior seas and deep convection regions are not fully understood. We investigate exchanges of liquid freshwater between the east Greenland shelf and neighboring seas using drifter data from five deployments carried out at different latitudes along the east Greenland shelf in 2019, 2020, and 2021, as well as satellite data and an atmospheric reanalysis. We compute Ekman transport from winds and geostrophic velocity from satellite altimetry at the shelfbreak and identify the Blosseville Basin and Cape Farewell as areas favorable to cross‐shelf exchanges. We further investigate exchange processes in these regions using drifter data. In the Blosseville Basin, drifters are brought off‐shelf toward the Iceland Sea and into the interior of the Basin. As they are advected downstream, they re‐enter the shelf and are driven toward the coast. At Cape Farewell, the wind appears to be the main driver, although on one occasion we found evidence of an eddy turning drifters away from the shelf. The drifters brought off‐shelf at Cape Farewell mostly continue around Eirik Ridge, where they re‐enter the West Greenland Current. Overall, the identified export over the east Greenland shelf is limited, small scale, and intermittent, thus unlikely to flux large amount of liquid freshwater into the interior, though exchange processes could enhance mixing in the near‐shelf region. Plain Language Summary: Climate change is expected to lead to a faster melt of the Greenland ice sheet. This may have a large effect on the ocean circulation due to its location near areas where strong winter cooling by the atmosphere turns warm ocean surface waters into cold, dense deep waters. As light fresh meltwater from Greenland enters these regions, winter cooling may not be strong enough anymore to make these waters dense enough to take part in the deep ocean circulation. In this study, we investigate tracks from drifting ocean instruments deployed near Greenland, as well as satellite data, to study where, and how much, freshwater could enter these deep water formation areas. We find two locations along the east Greenland shelf, at 60° and 68°N, where strong winds and ocean eddies can steer freshwater offshore, These processes are intermittent and only lead to small amounts of freshwater brought to areas where deep waters are formed. Key Points: We investigate cross‐shelf exchanges east of Greenland with a new data set of drifters deployed at the shelfbreak in 2019, 2020, and 2021Drifters, altimetry, and an atmospheric reanalysis, indicate enhanced cross‐shelf exchanges at the Blosseville Basin and Cape FarewellWinds, eddies, and local topography drive small scale and intermittent export east of Greenland, and can enhance mixing near the shelf [ABSTRACT FROM AUTHOR]

Global long-term stable wind fields are valuable information for climate analyses of atmospheric dynamics. Their monitoring remains a challenging task, given shortcomings of available observations. One promising option for progress is the use of radio occultation (RO) satellite data, based on which the winds are estimated using the geostrophic approximation. Hence, in this study we focus on two goals, explored through European Re-Analysis ERA5 and RO datasets, using monthlymean January and July data over 2007-2020 with a 2.5° × 2.5° resolution. First, we compare actual and geostrophic ERA5 wind speeds to evaluate the validity of the geostrophic approximation. Second, we test how well ERA5 and RO geostrophic winds agree. We find the geostrophic approximation to work well within 2 m s-1 accuracy almost globally (5°-85° latitude), especially over the summer hemisphere; larger differences (more than 5 m s-1) may occur in the winter stratosphere. We noticed the effect of large mountain ranges on the wind flow as a wave-like pattern, also in the difference between RO and ERA5 geostrophic winds, pointing to effects of different geopotential height estimations. Generally, RO and ERA5 geostrophic winds showed very good agreement. In the long-term, systematic differences in decadal trends of higher than 0.5 m s-1 per decade were found at subtropical latitudes, mainly related to observing system changes in the year 2016 that influenced ERA5. Together with the validity of the geostrophic approximation, this indicates that the long-term stability of RO-derived wind field monitoring can provide added value to reanalysis winds, for the benefit of climate monitoring and analyses. [ABSTRACT FROM AUTHOR]

Large-eddy simulations (LESs) of inversion-capped neutral atmospheric boundary layers (ABLs) are augmented to earlier small-domain LESs of a sparse forest and field observation to evaluate the budgets of all non-negligible resolved-scale Reynolds stress components. The focus is on the atmospheric surface layer comprised of the roughness sublayer (RSL) in and above horizontally homogeneous forests and the inertial sublayer (ISL) above the RSL over flat terrain. The greater LES domain and ABL depths result in greater depths of both the RSL and the ISL. A key result is that in the upper portions of the canopy and above, pressure redistribution is a major sink of normal stress in the horizontal direction with mean shear production as a major source, whereas in the horizontal direction absent of mean shear production and in the vertical direction, pressure redistributions are major sources of normal stresses. In the lower portions of the canopy where mean shear production and turbulent transport are much reduced, pressure redistributions are major sources of horizontal velocity variances but a major sink of vertical velocity variance. Pressure transport is a greater source of vertical velocity variance than turbulent transport from the ground level to just under the treetops where it transitions to a major sink up to about 1.5 times the canopy height. This greater significance of pressure transport over turbulent transport increases with increased vegetation area index (VAI). The impact of increased value of geostrophic wind speed is negligible compared to that of increased value of VAI on enhancing normalized budget terms in the vicinity of treetops. [ABSTRACT FROM AUTHOR]

The observed surface wind speed (SWS) over China has declined in the past four decades, and recently, the trend has reversed, which is known as SWS stilling and recovery. The observed SWS is vulnerable to changes in nonclimatic factors, i.e., inhomogeneity. Unfortunately, most of the existing studies on the long-term trend of SWS were based on raw datasets without homogenization. In this study, by means of geostrophic wind speed and penalized maximal t test, we conduct a systematic homogeneity test and exploration of the homogenization impact for SWS at over 2000 stations in China from 1970 to 2017. The results show that the inhomogeneity in the observed SWS over China is detectable at 59% of national weather stations. The breakpoint years are mainly concentrated in the late 1970s, mid-1990s, and early 2000s. Overall, 18% of breakpoints are caused by station relocations, and the remaining breakpoints are likely related to anemometer replacement and measurement environment changes that occurred during the mid-1990s and early 2000s. After homogenization, the decreasing trend in SWS during 1970–2017 decreased from −0.15 to −0.05 m s−1 decade−1. The homogenized SWS recovery period advanced from the early twenty-first century to the early 1990s, which is consistent with the SWS variations, excluding the impact of urbanization around weather stations. The phase change in the Western Hemisphere warm pool (WHWP) might be one of the causes of homogenized SWS recovery. [ABSTRACT FROM AUTHOR]

Residual current analysis of multiple stations' periodic observational data for sea currents, and multiple voyages, multiple seasons from 2018 to 2022, revealed the existence of a strong southwest current zone, 20–30 m underwater and within the coastal current area of Yangjiang, western Guangdong, in the northwest region of the South China Sea. The velocity of the residual currents in the strong–current zone was 38.2–100.3 cm/s. Observational data for wind, sea currents, salinity and tide from multiple coastal stations in the spring (from March 2019 to May 2019) and summer (from June 2019 to August 2019) of 2019 demonstrated the existence of abrupt strong currents in the coastal current sea area of Yangjiang, western Guangdong. Analyses of continuous sequence hydro meteorological data for the Yangjiang area indicated that wind stress was the main factor determining the direction of sea currents; increases in the near-shore water level produce a westward geostrophic current and a wind current in the shallow sea area; their joint effect was the key factor determining the velocity of the coastal current in this sea area. In spring and summer, when Pearl River runoff into the sea reaches a peak, and under the action of the northeast wind, the water level on the west coast of Guangdong rose. This created a barotropic pressure–gradient force from the shore to the outer sea, generating a southwestward geostrophic current in the same direction as the wind-driven current. The joint action of the wind-driven and geostrophic currents then generated a sudden southwestward coastal current. [ABSTRACT FROM AUTHOR]

An extremely heavy rainfall event lasting from 17 to 22 July 2021 occurred in Henan Province of China, with accumulated precipitation of more than 1000 mm over a 6-day period that exceeded its mean annual precipitation. The present study examines the roles of persistent low-level jets (LLJs) in maintaining the precipitation using surface station observations and reanalysis datasets. The LLJs triggered strong ascending motions and carried moisture mainly from the outflow of Typhoon In-fa (2021). The varying directions of the LLJs well corresponded to the meridional shifts of the rainfall. The precipitation rate reached a maximum during 20–21 July as the LLJs strengthened and expanded vertically into double LLJs, including synoptic-weather-system-related LLJs (SLLJs) at 850–700 hPa and boundary-layer jets (BLJs) at ∼950 hPa. The coupling of the SLLJ and BLJ provided strong mid- and low-level convergence on 20 July, whereas the SLLJ produced mid-level divergence at its entrance that coupled with low-level convergence at the terminus of the BLJ on 21 July. The formation mechanisms of the two types of LLJs are further examined. The SLLJs and the low-pressure vortex (or inverted trough) varied synchronously as a whole and were affected by the southwestward movement of the WPSH in the rainiest period. The persistent large total pressure gradient force at low levels also maintained the strength of low-level geostrophic winds, thus sustaining the BLJs on the synoptic scale. The results based on a Du-Rotunno 1D model show that the Blackadar and Holton mechanisms jointly governed the BLJ dynamics on the diurnal scale. [ABSTRACT FROM AUTHOR]

We evaluate the prediction skill of the Max Planck Institute Earth System Model (MPI-ESM) decadal hindcast system for German Bight storm activity (GBSA) on a multiannual to decadal scale. We define GBSA every year via the most extreme 3-hourly geostrophic wind speeds, which are derived from mean sea-level pressure (MSLP) data. Our 64-member ensemble of annually initialized hindcast simulations spans the time period 1960–2018. For this period, we compare deterministically and probabilistically predicted winter MSLP anomalies and annual GBSA with a lead time of up to 10 years against observations. The model produces poor deterministic predictions of GBSA and winter MSLP anomalies for individual years but fair predictions for longer averaging periods. A similar but smaller skill difference between short and long averaging periods also emerges for probabilistic predictions of high storm activity. At long averaging periods (longer than 5 years), the model is more skillful than persistence- and climatology-based predictions. For short aggregation periods (4 years and less), probabilistic predictions are more skillful than persistence but insignificantly differ from climatological predictions. We therefore conclude that, for the German Bight, probabilistic decadal predictions (based on a large ensemble) of high storm activity are skillful for averaging periods longer than 5 years. Notably, a differentiation between low, moderate, and high storm activity is necessary to expose this skill. [ABSTRACT FROM AUTHOR]

Copyright of Plateau Meteorology is the property of Plateau Meteorology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The wind energy industry relies on computationally efficient engineering-type models to design wind farms. Typically these models do not account for the effect of atmospheric stratification in either the boundary layer or the free atmosphere. This study proposes a new analytical model for fully developed wind-turbine arrays in conventionally neutral atmospheric boundary layers frequently encountered in nature. The model captures the effect of the free-atmosphere stratification, Coriolis force, wind farm layout and turbine operating condition on the wind farm performance. The model is developed based on the physical insight derived from large-eddy simulations. We demonstrate that the geostrophic drag law (GDL) for flow over flat terrain can be extended to flow over fully developed wind farm arrays. The presence of a vast wind farm significantly increases the wind farm friction velocity compared with flow over flat terrain, which is modelled by updated coefficients in the GDL. The developed model reliably captures the vertical wind speed profile inside the wind farm. Furthermore, the power production trends observed in simulations are reliably reproduced. The wind farm performance, normalized by the geostrophic wind speed, decreases as the free-atmosphere thermal stability increases or the Coriolis force decreases. In addition, we find that the optimal wind farm performance is obtained at a lower thrust coefficient than the Betz limit, which indicates that optimal operating conditions for turbines in a wind farm are different than for a single turbine. [ABSTRACT FROM AUTHOR]

A one-dimensional Atmospheric Boundary Layer (ABL1D) model is coupled with the NEMO ocean model and implemented over the Iberian–Biscay–Ireland (IBI) area at 1/36° resolution to investigate the damping effect of the current and the thermal feedback on the kinetic energy (KE) at the mesoscale. This type of coupling between an ocean model and an ABL1D is a newly proposed approach as an alternative of intermediate complexity between bulk forcing and full coupling with an atmosphere model. In ABL1D, the prognostic tracers are nudged toward large-scale variables and the wind is guided by a low-frequency geostrophic wind provided from the ERA-Interim reanalyses. First, the ABL1D is successfully validated against satellite observations regarding the wind, and the dynamic coupling coefficient (linking the near surface wind and wind-stress to the of the surface currents) are consistent with the literature, over the period 2016–2017. Our results show that the thermal feedback has a negligible impact on kinetic energy (KE) and does not influence the strength of the current feedback in the region. Given the ABL1D physics, this further indicates that the changes in the vertical wind structure caused by CFB are primarily governed by local mechanical mechanisms associated with surface wind-stress condition, rather than by thermodynamic or non-local processes within the planetary boundary layer. The induced KE reduction by the current feedback amounts to 14% at the surface and propagates down to 2000 m, indicating that it can modify the vertical distribution of KE throughout the water column. KE reductions in the surface boundary layer (0 – 300 m) and in the interior (300 – 2000 m) are attributed to a reduction of the surface wind work by 4%, and of the pressure work by 7%, respectively. The Ekman pumping anomalies induced by the current feedback tend to attenuate eddy activity and horizontal pressure gradients at depth, illustrating the potential of the current feedback to induce a geostrophic adjustment on the water column. These results illustrate the relevance of the proposed ABL1D coupling approach for reproducing the wind-current coupling (a.k.a. current feedback effect) which cannot be taken into account straightforwardly with simple bulk forcing. • A one-dimensional Atmospheric Boundary Layer (ABL1D) model is coupled with the NEMO ocean model over the Iberian–Biscay–Ireland area. • Our model is validated against satellite data and used to study current and thermal feedback effects on kinetic energy (KE). • The SST-wind coupling has a negligible impact on KE and on the current feedback (CFB) strength over the study area. • The wind structure is influenced by CFB mainly through local mechanical processes, not thermodynamic or non-local ones. • The CFB reduces surface KE by 14% and extends down to 2000m, altering KE distribution throughout the water column. • Our results highlights the relevance of ABL1D coupling to capture wind-current interaction compared to bulk forcing methods. [ABSTRACT FROM AUTHOR]

The aim of this study was to climatologically investigate forced and pressure‐driven channelling flow in the Kitakami Basin, Japan, through the use of observational surface wind and sea‐level pressure data collected over 23 years, which corresponded to 15,584 channelling flow events. Our results indicated that forced channelling tends to occur when the direction of the surface geostrophic wind is parallel to the valley axis. Contrastingly, pressure‐driven channelling tends to occur when the component perpendicular to the valley axis of the surface geostrophic wind is larger than that parallel to the valley axis of the surface geostrophic wind. The channelling flow type was determined by the wind direction of the surface geostrophic wind, which was more pronounced when the mountain Froude number of the flow across the valley was less than 1. This indicates that the type of channelling flow depends on the surface geostrophic wind speed. Moreover, neither channelling flow type was dominant when the component parallel to the valley axis of the surface geostrophic wind was slightly larger than the orthogonal component. This suggests that the flow type cannot be explained using the surface geostrophic wind alone, irrespective of whether forced channelling or pressure‐driven channelling is dominant. [ABSTRACT FROM AUTHOR]

Irregular shapes of atmospheric rivers (ARs) hamper easy AR composite analyses and understandings of AR's moisture transport mechanisms. We develop a method to composite AR‐related variables from a reanalysis data set. By averaging a large number of samples, the three‐dimensional structure and some evolutionary features of a typical North Pacific AR are revealed. An AR is typically located along and in advance of the surface cold front of an extratropical cyclone. A mesoscale secondary circulation is observed in the cross sections of the AR corridor, where both geostrophic and ageostrophic winds make indispensable contributions to the moisture fluxes. Geostrophic moisture advection across the cold front within the Equatorward half of the AR is created by baroclinicity of the system and serves as the primary moisture source for the AR. Moisture fluxes from the warm sector of the cyclone are primarily due to boundary layer ageostrophic winds and are more important within the poleward half of the AR, particularly during the genesis stage. Faster AR movement compared with low‐level winds enables the AR to collect downwind moisture. In an AR‐relative view, this is represented as an easterly flow, consistent with the "feeder‐airstream" concept, and emphasizes the role of local moisture recycling. Moisture transport within the Equatorward half is mostly due to geostrophic advection of the propagating AR‐cyclone couple in an Earth‐relative view. Driven by intensifying geostrophic winds, ARs tend to reach peak moisture transport intensity about 2 days after genesis. Then, reduced moisture level and influxes from lateral boundaries prevent further intensification. Plain Language Summary: Atmospheric rivers (ARs) are "river‐like" plumes of airborne horizontal moisture transport that play a crucial role in atmospheric water cycle. Unlike terrestrial rivers, ARs are highly transient and with constantly changing shapes, making it difficult to summarize their structural and dynamical features using a composite analysis. We attack this problem by straightening the irregular ARs using a two‐dimensional interpolation, thus allowing us to create composites from a large number of AR samples. The results reveal the typical structure of a North Pacific AR, its relationship with an accompanying cyclone, the manner in which AR collects moisture from its lateral boundaries, and the changes in different contributing factors following its lifetime evolution. Thanks to its faster propagation speed compared with low‐level winds, an AR collects downwind atmospheric moisture on its poleward‐facing half, a mechanism that is particularly important during its early development stage. The large‐scale circulation associated with the accompanying cyclone accounts for most of the vapor plumes toward the Equatorward‐facing half of the AR. Key Points: Develops a new method to composite atmospheric rivers with irregular shapes and orientationsReveals the meso‐scale secondary circulation across an atmospheric river and highlights the relationship with frontogenesis processesDepicts the 3D structure of moisture transport within an atmospheric river and different geostrophic and ageostrophic contributions [ABSTRACT FROM AUTHOR]

This study presents the results of large-eddy simulation (LES) of the evening transition in the atmospheric boundary layer in the case of free convection and in the presence of geostrophic wind. The turbulent kinetic energy (TKE) balance and its components are analyzed. It is shown that within the transition, periods of fast and slow decay can be distinguished. The differences in TKE anisotropy between these two periods are demonstrated. During the fast decay period, the majority of the energy within the vertical component is consumed due to inertial movement of the thermals after the cease of convection. This is followed by the TKE redistribution into large-scale horizontal components, which leads to the formation of quasi-horizontal turbulence, where the TKE dissipation is significantly slower in comparison to the isotropic state. It is shown that one-dimensional boundary-layer model, in which turbulent fluxes are parameterized by means of a two-equation closure, is not able to reproduce evening transition dynamics observed in LES. In particular, the use of the gradient approximation in the one-dimensional model leads to the preservation of the convective distribution of the heat flux along the vertical during the transition period and additional TKE generation in the boundary layer due to the action of buoyancy forces. The use of the phenomenological equation for the dissipation rate leads to decreased TKE decay rate during the fast decay period and increased TKE decay rate during the slow decay period. Possible approaches toward modification of the Reynolds-averaged Navier–Stokes (RANS) closures in order to correctly reproduce transition periods of the atmospheric boundary layer are discussed. [ABSTRACT FROM AUTHOR]

Topography-induced turbulence poses a potential hazard for aviation safety, in particular during the final approach and landing. In this context, it is essential to assure that the impact of topography-induced turbulence on the flight paths during take-off and landing is minimized already during the design and planning phase. As an example of the siting and planning of a potential new airport in complex terrain, this study investigates the distribution of terrain-induced boundary layer turbulence in the vicinity of the current Lofoten airport at Leknes (LKN). For that purpose, large-eddy simulations (LES) have been performed with the PAralellized Large-eddy Simulation Model (PALM) on a 40 × 45 × 4 km 3 computational domain around LKN. An initial parametric sensitivity study resulted in a grid spacing of 50 m and an overall simulation time of 12 h for our individual model runs. A suite of 32 model simulations for 16 different wind directions and two geostrophic wind speeds of 10 ms − 1 and 20 ms − 1 , was then performed and analysed. A turbulence risk analysis along idealized flight trajectories shows that the high-risk conditions are substantially determined by the wind conditions and their interaction with the topography. With respect to wind speed, the results indicate that for a geostrophic flow below 10 ms − 1 , the risk of aviation critical, terrain-induced boundary layer turbulence (BLT), is rather low in the vicinity of LKN. At 20 ms − 1 the situation has completely changed, as for 14 out of 16 investigated wind directions the 9 m 2 s − 2 aviation critical threshold of turbulent kinetic energy per unit air mass (TKE) is exceeded. In the northwesterly wind scenarios, the largest areas with critical turbulence in the vicinity of LKN are observed. [ABSTRACT FROM AUTHOR]

Multiscale processes from synoptic disturbances to diurnal cycles during the record-breaking heavy rainfall in summer 2020 were examined in this study. The heavy rainfall consisted of eight episodes, each lasting about 5 days, and were associated with two types of synoptic disturbances. The type-1 episodes featured a northwestward extending western Pacific subtropical high (WPSH), while the type-2 episodes had approaching midlatitude troughs with southward retreat in the WPSH. Each heavy rainfall episode had 2–3 occurrences of nocturnal low-level jets (NLLJs), in close association with intense rainfall in the early morning. The NLLJs formed partly due to the geostrophic wind by increased pressure gradients under both types of synoptic disturbances. The NLLJs were also driven by the ageostrophic wind that veered to maximum southerlies at late night due to the boundary layer inertial oscillation. The diurnal amplitudes of low-level southerlies increased remarkably after the onset of type-1 episodes, in which the extending WPSH provided strong daytime heating from solar radiation. By contrast, the wind diurnal amplitudes were less changed after the onset of type-2 episodes. The NLLJs strengthened the mesoscale low-level ascent, net moisture flux convergence, and convective instability in elevated warm moist air, which led to the upscale growth of MCSs at the northern terminus of the LLJ after midnight. The MCSs-induced mei-yu rainband was reestablished in Central China during the type-1 episodes with the increased diurnal variations. The findings highlight that the regional diurnal cycles of low-level winds in response to synoptic disturbances can strongly regulate mesoscale convective activities in a downscaling manner, and thus produce heavy rainfall. [ABSTRACT FROM AUTHOR]

Conventionally neutral atmospheric boundary layers (ABLs) are frequently encountered in nature, and their flow dynamics affect the transfer of momentum, heat, and humidity in the atmosphere. Therefore, insight into the flow structure in conventionally neutral ABLs is necessary to further improve models for long-term weather and climate forecast, while it provides further insight for atmospheric applications like the wind industry. The structure of conventionally neutral ABLs is complicated due to the coexistence of shear- and buoyancy-generated turbulence, and therefore analytical descriptions have been limited to the mean wind speed. Here we introduce an innovative model based on the Ekman equations and the basis function of the universal potential temperature flux profile that allows one to describe the vertical profiles of the horizontal components of wind and shear stress and hence capture features like the wind veer profile. Our formulation in terms of departure from the geostrophic wind allows us to describe the profiles as a function of one control parameter, although the description of wind speed profile still needs two. We find excellent agreement between analytical predictions, high-fidelity simulations, and field measurement campaigns. These findings advance the fundamental understanding of the ABL structures and atmospheric turbulence. [ABSTRACT FROM AUTHOR]

An analytical model is presented for the generation of a Blackadar-like nocturnal low-level jet in a broad baroclinic zone. The flow is forced from below (flat ground) by a surface buoyancy gradient and from above (free atmosphere) by a constant pressure gradient force. Diurnally varying mixing coefficients are specified to increase abruptly at sunrise and decrease abruptly at sunset. With attention restricted to a surface buoyancy that varies linearly with a horizontal coordinate, the Boussinesq-approximated equations of motion, thermal energy, and mass conservation reduce to a system of one-dimensional equations that can be solved analytically. Sensitivity tests with southerly jets suggest that (i) stronger jets are associated with larger decreases of the eddy viscosity at sunset (as in Blackadar theory); (ii) the nighttime surface buoyancy gradient has little impact on jet strength; and (iii) for pure baroclinic forcing (no free-atmosphere geostrophic wind), the nighttime eddy diffusivity has little impact on jet strength, but the daytime eddy diffusivity is very important and has a larger impact than the daytime eddy viscosity. The model was applied to a jet that developed in fair weather conditions over the Great Plains from southern Texas to northern South Dakota on 1 May 2020. The ECMWF Reanalysis v5 (ERA5) for the afternoon prior to jet formation showed that a broad north–south-oriented baroclinic zone covered much of the region. The peak model-predicted winds were in good agreement with ERA5 winds and lidar data from the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) central facility in north-central Oklahoma. [ABSTRACT FROM AUTHOR]

Extremely persistent heavy rainfall (EPHR) events frequently occur in southern China. The mechanisms underlying their generation and development are not well understood, and this leads to difficulties in accurately forecasting these events. In this paper, hourly precipitation grid data and ERA‐5 reanalysis data are used to examine the relationships between precipitation, ageostrophic winds, and low‐level jets (LLJ). The results suggest that ageostrophic winds in the boundary layer mainly exhibit confrontational and asymptotic convergence, with the former being accompanied by double LLJ, which transport more vapor and provide favorable uplift conditions, thus producing stronger precipitation and wider rainbands. The different cooperated relationships between LLJ and ageostrophic winds play an important role in the development of EPHR events, and greater account needs to be taken of these relationships when forecasting. Plain Language Summary: Coastal extremely persistent heavy rainfall (EPHR), long‐duration heavy rainfall in local coastal area, occurs frequently in Southern China, and this kind of rainfall is usually accompanied by southwesterly in the lower atmosphere, which we call as the southwest low‐level jet. Generally, the occurrence and development of rainfall is accompanied by the convergence of the lower air flow. Since the divergence of the geostrophic wind component of the wind field is zero, the convergence situation is characterized by the ageostrophic wind component, which doesn't satisfy the geostrophic balance. In this type of coastal EPHR accompanied by the Southwest Jet, the ageostrophic wind is mainly manifested in two different convergence situations. One is that the ageostrophic wind presents several opposite branches of directions presenting a confrontational pattern around the rainfall area, and the other is an asymptotic convergence in which two similar wind directions, such as northeasterly and southeasterly, gradually converge to the rain area. The former has stronger lower‐level uplift, greater precipitation and wider rain bands, while the latter is relatively weak. This is an interesting phenomenon, of which the specific mechanisms need to be further studied. Key Points: About 74% of extremely persistent heavy rainfall cases along the coast of southern China are accompanied by southwest jets in the marine boundary layerAgeostrophic winds mainly exhibit confrontational and asymptotic convergence, and the former is accompanied by double low‐level jetsCases of ageostrophic winds with confrontational convergence have stronger precipitation and wider rainband [ABSTRACT FROM AUTHOR]

In very stable boundary layers (SBLs), turbulence is expected to be weak, anisotropic, and non‐stationary. Moreover, viscous effects likely become relevant under very weak‐wind conditions. This study investigates the stratification regimes based on the relative strength of the forces that control the flow (i.e., buoyancy‐ or viscosity‐dominated flow) and the degree of turbulence anisotropy in large‐eddy simulations of the very SBL with weak winds. The simulations explored here correspond to an Ekman‐layer‐type boundary layer with geostrophic winds equal to ug=1$$ {u}_{\mathrm{g}}=1 $$ and 2 m·$$ \cdotp $$s−1$$ {}^{-1} $$, and surface cooling rates of 1 and 3 K·$$ \cdotp $$hr−1$$ {}^{-1} $$, at high latitude. According to the buoyancy Reynolds number Reb$$ R{e}_{\mathrm{b}} $$ and the horizontal Froude number Frh$$ F{r}_{\mathrm{h}} $$, these SBLs are in strongly stable conditions (i.e., Reb≫1$$ R{e}_{\mathrm{b}}\gg 1 $$ and Frh≪1$$ F{r}_{\mathrm{h}}\ll 1 $$). Moreover, the vertical profiles of Reb$$ R{e}_{\mathrm{b}} $$ indicate that the turbulent flows are in an energetic state near the surface but gradually transition to a viscosity‐affected stratified state in the upper part of the SBL as viscous effects become important. For the most stable simulation, the results show that even the small scales are somewhat affected by buoyancy. The invariant analysis of the Reynolds stress anisotropy tensor shows that the flow is in an anisotropic state and is governed by the streamwise component of the turbulent flux u′u′‾$$ \overline{u^{\prime }{u}^{\prime }} $$ (one‐component turbulence). Additionally, based on the two‐point spatial correlation function, the shape of the large coherent turbulent structures is nearly isotropic in the vertical direction and anisotropic in the horizontal direction, and their size increases with height. The evaluation of the turbulent kinetic energy budget shows that the most stable simulation is in a non‐stationary state, which impacts the turbulence mixing efficiency measured by the flux Richardson number. [ABSTRACT FROM AUTHOR]

The large‐eddy simulation of very stable boundary layers (SBLs) remains challenging due to the large anisotropy of turbulent motions resulting from buoyancy suppression effects, and to the general reduction of its energetic scales. Here, we develop a novel large‐eddy simulation model setup to enable the modeling of the very SBL, avoiding the use of the Monin–Obukhov similarity theory at its lower boundary condition, as it breaks down in these conditions in agreement with previous studies. The new approach requires very fine grids in the vertical direction, in the order of a few centimeters. A series of Ekman‐layer‐type boundary layers under weak‐wind conditions (ug=1$$ {u}_{\mathrm{g}}=1 $$ and 2 m·$$ \cdotp $$s−1$$ {}^{-1} $$), and strong surface cooling rates (1 and 3 K·$$ \cdotp $$hr−1$$ {}^{-1} $$) at high latitude is studied using isotropic grids of 0.05 and 0.10 m resolution. The low‐order statistics show some sensitivity to the grid resolution; for example, both the SBL height and the low‐level jet position decrease with increasing grid resolution. In general, some differences are spotted regarding the shape of the wind speed and temperature vertical profiles, compared with weakly to moderately stratified conditions, suggesting that the structure and characteristics of the SBL evolve with increasing stratification. Moreover, the wind turning is significant from the surface, where the surface wind angle relative to the geostrophic wind is about 50°$$ {}^{\circ} $$. Lastly, two regimes within very stable conditions can be qualitatively identified primarily based on the gradient Richardson number (Ri$$ Ri $$) profiles: In the less stable regime, Ri$$ Ri $$ increases almost linearly with height and a quasi‐steady state is achieved; however, in the most stable regime, Ri$$ Ri $$ sharply increases near the surface and remains constant throughout the SBL while increasing with time up to Ri≈0.4$$ Ri\approx 0.4 $$. Thus, a quasi‐steady state is not achieved; and although turbulence does not collapse, it remains weak. [ABSTRACT FROM AUTHOR]

Rapid rise of salinity is observed in the Black Sea in recent years, with the largest positive trend (0.07 psμ per 10 years) detected in the pycnocline. We use long-term hydrological measurements for 1985–2019 to show that salinity of pycnocline has intense seasonal and interannual variability modulated by the mechanical and convective mixing. In the warm period of a year, shear turbulence driven by strong winds and intense geostrophic currents causes the penetration of warm waters into the lower density layers. This is accompanied by the rise in their salinity, the source of which is the deep saline waters situated below the halocline. This process is most intense in the areas of downwelling and intensifies in the autumn period, when thermal stratification is relatively weak. Another important reason is the entrainment of salty Mediterranean waters in the upper part of the Black Sea halocline, which is modulated by the deepening of the seasonal thermocline near the Bosphorus strait and mechanical mixing. The increase of salinity is compensated during cold winters, when convective mixing transports fresher water influenced by river discharge into lower density layers of the basin and causes a decrease in pycnocline salinity. This process is most intense in the center of the cyclonic gyres, where pycnocline is located closer to the surface and winter temperature reaches minimal values. Due to the long-term warming of the Black Sea, the process of freshening of deep layers now is observed only in rare cold years. At the same time, an intensification of wind speed, vorticity, and geostrophic circulation processes promote the blurring of the halocline and the rise of the salinity of the Black Sea upper layers. Such rise begins after 2007 in the upper part of Black Sea halocline (depth 50–100 m) and is traced down to 250 m by 2020. • Global warming suppresses the freshening of pycnocline during winter convection. • Shear turbulence in warm period causes the entrainment of salt from deep layers. • Injections of Mediterranean waters in pycnocline intensify in warm conditions. • Intensification of currents and wind, and rise of winter temperature is observed. • All this processes cause rapid rise of pycnocline salinity in the Black Sea. [ABSTRACT FROM AUTHOR]

This study formulates three-dimensional (3D) residual flow, treating both stationary and transient waves. The zonal and meridional momentum equations contain four terms: the geostrophic wind tendency, Coriolis force for the residual horizontal flow, product of the geostrophic wind and potential vorticity other than the constant planetary vorticity, and friction. The thermodynamic equation contains three terms: the potential temperature tendency, advection of the basic potential temperature by the residual vertical flow, and diabatic heating. The zonal mean of the 3D residual flow equals the time mean of the residual flow of the transformed Eulerian-mean equations. The new residual flow is the sum of that derived by Plumb for transient waves and the quadratic terms of the time-mean fields, which correspond approximately to the Stokes correction due to stationary waves. The 3D residual flow and momentum equations are symmetric in the zonal and meridional directions, in contrast with those formulated by Kinoshita et al., which treat the time-mean zonal-mean zonal wind as the basic wind. The newly derived formulas are applied to the climatology of the 3D structure of the deep branch of the Brewer–Dobson circulation. In the Northern Hemisphere in December–February, the residual flows are directed inward toward the polar vortex strongly over east Siberia, where the downward flow is maximized, and weakly over the Atlantic; meanwhile, they are directed outward from the vortex over North America and Europe. A longitudinal dependence of the poleward flow is also observed in the Southern Hemisphere in June–August. [ABSTRACT FROM AUTHOR]

Changes in the meridional position of the sea surface temperature (SST) anomalies (SSTAs) associated with the interannual component (PC1-I) of the principal component 1 (PC1) of the first leading mode of the North Pacific SST (referred to here as PC1-I-related SSTAs) are investigated using reanalysis products and climate model output. It is found that the PC1-I-related SSTAs (or PC1-I anomalies) significantly shift southward at a rate of 1.04° latitude per decade and have moved southward by 4.4° since the 1960s. Our further analysis indicates that the southward shift of the PC1-I-related SSTAs is due to changes in ENSO teleconnections. Compared to the 1950–75 period (PRE era), the meridional width of the ENSO-induced tropical positive geopotential height (GH) anomaly is narrower during the 1991–2016 period (POST era), inducing a southward shift of the subtropical westerly anomaly over the North Pacific through geostrophic wind relations. This southward shift of the westerly anomaly favors the southward shift of the ENSO-induced negative GH anomaly (cyclonic circulation anomaly) over the North Pacific by positive vorticity forcing of the zonal wind shear. The southward-shifting GH anomaly associated with ENSO further forces the PC1-I anomaly to shift southward. Furthermore, the contraction of the ENSO-induced tropical positive GH anomaly is related to the contraction of the meridional width of ENSO. The modeling results support that the decrease in the ENSO meridional width favors the contraction of the ENSO-induced tropical positive GH anomaly and the southward shift of ENSO teleconnections over the North Pacific, contributing to the southward shift of the PC1-I anomaly. [ABSTRACT FROM AUTHOR]

Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (∼ 54 ∘ N) and northern Norway (∼ 69 ∘ N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower-thermosphere summer length (MLT-SL) using SMR and PRR winds and (2) the mesosphere summer length (M-SL) using the PRR and MLS. Under both definitions, the summer begins around April and ends around middle September. The largest year-to-year variability is found in the summer beginning in both definitions, particularly at high latitudes, possibly due to the influence of the polar vortex. At high latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity as well as large-scale atmospheric influences (e.g., quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at middle latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO. [ABSTRACT FROM AUTHOR]

We analyze the interannual variability and trends of the eddy heat transport (EHT) in the surface mixed layer of the Atlantic Ocean from covariances of sea surface temperature (SST) and geostrophic velocities from satellite observations between 1993 and 2018. The EHT is largest along the path of the Gulf Stream in the Northern Hemisphere and the vicinity of the Agulhas Retroflection and Argentine basin in the Southern Hemisphere. On average, meridional EHT in the mixed layer leads to a divergence of heat away from the subtropics in both hemispheres toward the equator and higher latitudes. Depending on latitude, the divergence of the EHT accounts for around 1%–5% of the atmospheric net surface heat flux, but reaches as much as 20% for certain regions. The EHT can be linked to eddy kinetic energy (EKE) and meridional SST gradients using mixing length hypothesis, but the variability of EKE and SST gradients are not enough to capture local EHT variability. For different latitudes, the EHT shows different behavior over time, with a strong increase in northward EHT in the Gulf Stream region, and a decadal oscillation of poleward EHT in the tropical and subtropical regions. This oscillation on larger scales is highly correlated with spatially averaged EKE and the large‐scale SST gradients. A comparison with climate indices indicates a relation to Atlantic climate variability, especially the meridional modes of the Tropical Atlantic Variability and the North Atlantic Oscillation (NAO). Plain Language Summary: Mesoscale eddies are swirling currents in the ocean that can transport immense amounts of heat, which is especially important in the Atlantic Ocean because of its role in the global climate system. This study uses satellite observations of ocean currents and temperatures to calculate the eddy heat transport (EHT) in the surface mixed layer and analyze how the transport has changed over time. On average, eddies tend to transport excess heat away from regions where heat is put into the ocean from the atmosphere and provide heat to regions where it is lost to the atmosphere. Over time, EHT has strongly increased in the Gulf Stream region, likely due to changes in the Gulf Stream dynamics. On the other hand, in tropical and subtropical regions the EHT oscillates with a 10‐year period, generally in concert with the changes in the large‐scale meridional temperature gradients and the average eddy kinetic energy over the region. This is likely related to the decadal climate variability of the Atlantic Ocean. Key Points: Eddy heat transport (EHT) variability in the Atlantic Ocean is estimated from satellite altimetry and sea surface temperatureEHT is divergent in tropical and subtropical regions and convergent in subpolar Atlantic and around the equatorEHT decadal oscillation in the tropics/subtropics is revealed, linked to meridional modes of Atlantic climate variability [ABSTRACT FROM AUTHOR]

Turbulent flow in a weakly convective marine atmospheric boundary layer (MABL) driven by geostrophic winds Vg = 10 m s−1 and heterogeneous sea surface temperature (SST) is examined using fine-mesh large-eddy simulation (LES). The imposed SST heterogeneity is a single-sided warm or cold front with jumps Δθ = (2, −1.5) K varying over a horizontal x distance of 1 km characteristic of an upper-ocean mesoscale or submesoscale front. The geostrophic winds are oriented parallel to the SST isotherms (i.e., the winds are alongfront). Previously, Sullivan et al. examined a similar flow configuration but with geostrophic winds oriented perpendicular to the imposed SST isotherms (i.e., the winds were across-front). Results with alongfront and across-front winds differ in important ways. With alongfront winds, the ageostrophic surface wind is weak, about 5 times smaller than the geostrophic wind, and horizontal pressure gradients couple the SST front and the atmosphere in the momentum budget. With across-front winds, horizontal pressure gradients are weak and mean horizontal advection primarily balances vertical flux divergence. Alongfront winds generate persistent secondary circulations (SC) that modify the surface fluxes as well as turbulent fluxes in the MABL interior depending on the sign of Δθ. Warm and cold filaments develop opposing pairs of SC with a central upwelling or downwelling region between the cells. Cold filaments reduce the entrainment near the boundary layer top that can potentially impact cloud initiation. The surface-wind–SST-isotherm orientation is an important component of atmosphere–ocean coupling. The results also show frontogenetic tendencies in the MABL. [ABSTRACT FROM AUTHOR]