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It is typically interpreted that more moisture in the atmosphere leads to more intense rains. This notion may be supported, for example, by taking a scatterplot between rain and column precipitable water. The present paper suggests, however, that the main consequence of intense rains with more moisture in the atmosphere is that there is a higher chance of occurrence rather than an increase in the expected magnitude. This tendency equally applies to any rains above 1 mm (6 h)−1 to a lesser extent. The result is derived from an analysis of 33 local rain gauge station data and a shared sounding over Friuli Venezia Giulia, northeast Italy. Significance Statement: Moisture is the source of clouds. Clouds, in turn, are source of rain. So we may expect that more moisture in the atmosphere causes more intense rains. We may further speculate that with more moisture in the atmosphere as a consequence of the global warning, we must face more catastrophic rain events and floods. However, this paper, by analyzing data over Friuli Venezia Giulia, northeast Italy, suggests otherwise: more moisture indeed increases frequencies of intense rains, but not their magnitudes as much. [ABSTRACT FROM AUTHOR]

The intensity of interannual variation of spring precipitation over southern China during 1979–2014 and possible reasons for it are investigated in this paper. There is a significant interdecadal change in the intensity of interannual variation of spring precipitation over southern China around 1995/96. The intensity of interannual variation of spring rainfall over South China is stronger during 1979–95 than that during 1996–2014. The possible reason may be the larger amplitude of the sea surface temperature anomaly (SSTA) in the western Pacific Ocean (WP) before 1995/96. The cooler (warmer) SSTA in WP may trigger an abnormal local anticyclone (cyclone) at lower levels. The anomalous southwesterly (northeasterly) flow at the northwestern flank of the WP anticyclone (cyclone) covers South China, transporting more (less) moisture to South China. Meanwhile, the anomalous winds converge (diverge) in South China at lower levels and diverge (converge) at upper levels, which causes the anomalous ascent (descent) to enhance (reduce) the precipitation over there. However, during 1996–2014, the intensity of interannual variation of spring rainfall over the middle and lower reaches of the Yangtze River valley becomes much stronger than that during 1979–95, which is related to the intensified interannual variation of the atmospheric circulation in the middle and high latitudes over Eurasia. The weak (strong) Siberian high and East Asian trough may reduce (enhance) the northerly wind from the middle and high latitudes. As a result, the middle and lower reaches of the Yangtze River valley are subjected to the anomalous southerly wind, favoring more (less) precipitation over there. [ABSTRACT FROM AUTHOR]

This paper reports a consistent seesaw relationship between interdecadal precipitation variability over North China and the Southwest United States, which can be found in observations and simulations with several models. Idealized model simulations suggest the seesaw could be mainly driven by the interdecadal Pacific oscillation (IPO), through a large-scale circulation anomaly occupying the entire northern North Pacific, while the Atlantic multidecadal oscillation (AMO) contributes oppositely and less. Modulation of precipitation by the IPO tends to be intensified when the AMO is in the opposite phase, but weakened when the AMO is in the same phase. The warm IPO phase is associated with an anomalous cyclone over the northern North Pacific; consequently, anomalous southwesterly winds bring more moisture and rainfall to the Southwest United States, while northwesterly wind anomalies prevail over North China with negative rainfall anomalies. The east–west seesaw of rainfall anomalies reverses sign when the circulation anomaly becomes anticyclonic during the cold IPO phase. The IPO-related tropical SST anomalies affect the meridional temperature gradient over the North Pacific and adjacent regions and the mean meridional circulation. In the northern North Pacific, the atmospheric response to IPO forcing imposes an equivalent barotropic structure throughout the troposphere. An important implication from this study is the potential predictability of drought-related water stresses over these arid and semiarid regions, with the progress of our understanding and prediction of the IPO and AMO. [ABSTRACT FROM AUTHOR]

This paper scrutinizes the zonal oscillation of the western North Pacific subtropical high (WNPSH) via diagnosing its two extreme phases, which are defined by the top 10% strongest (positive phase) and the weakest (negative phase) WNPSH index (WNPSHI) days during summers in 1979–2016. Key findings include the following: a tripole pattern consisting of intensified (weakened) precipitation over the Maritime Continent and the East Asian summer monsoon regions, and suppressed (strengthened) precipitation over the western North Pacific summer monsoon region during positive (negative) WNPSH phases; a westward movement of WNPSH-induced precipitation anomalies that subsequently affects eastern China, Japan, and the Korean Peninsula at different time lags; an OLR–vorticity pattern explained by atmospheric responses to thermal sources is suggested to drive the oscillation; and the competitive interaction of local air–sea feedbacks, especially during the positive phase. In addition, moderate-to-strong positive correlations between the WNPSHI and the Niño-3.4 index are found on 1–2-, 2–3-, and 3–6-yr time scales; both exhibit decadal shifts to a higher-frequency mode, suggesting the intensification of both the zonal WNPSH oscillation and the ENSO under the changing climate and their close interdecadal association. A nonlinear quasi-biennial WNPSH–ENSO relationship is identified: the positive (negative) WNPSH phase sometimes occurs during 1) a decaying El Niño (La Niña) in the preceding summer/autumn, and/or 2) a developing La Niña (El Niño) in the current summer/autumn. A full ENSO transition from moderate-to-strong El Niño to La Niña is often seen during the positive phase, offering potential in predicting ENSO events and extreme WNPSH phases and thereby the summer monsoon rainfall in East Asia. [ABSTRACT FROM AUTHOR]

The IPCC Fifth Assessment Report highlighted large uncertainty in European precipitation changes in the coming century. This paper investigates the sources of intermodel differences using CMIP5 model European precipitation data. The contribution of atmospheric circulation to differences in precipitation trends is investigated by applying cluster analysis to daily mean sea level pressure (MSLP) data. The resulting classification is used to reconstruct monthly precipitation time series, thereby isolating the component of precipitation variability directly related to atmospheric circulation. Reconstructed observed precipitation and reconstructions of simulated historical and projection data are well correlated with the original precipitation series, showing that circulation variability accounts for a substantial fraction of European precipitation variability. Removing the reconstructed precipitation from the original precipitation leaves a residual component related to noncirculation effects (and any small remaining circulation effects). Intermodel spread in residual future European precipitation trends is substantially reduced compared to the spread of the original precipitation trends. Uncertainty in future atmospheric circulation accounts for more than half of the intermodel variance in twenty-first-century precipitation trends for winter months for both northern and southern Europe. Furthermore, a substantial part of this variance is related to different forced dynamical responses in different models and is therefore potentially reducible. These results highlight the importance of understanding future changes in atmospheric dynamics in achieving more robust projections of regional climate change. Finally, the possible dynamical mechanisms that may drive the future differences in regional circulation and precipitation are illustrated by examining simulated teleconnections with tropical precipitation. [ABSTRACT FROM AUTHOR]

The transport of atmospheric water vapor plays a crucial role in the production of precipitation and the variation of precipitation isotopic composition ( d18Op). This study investigates three precipitation events with extremely depleted heavy isotopes in the summer of Adelaide, Australia. Using fundamental water vapor diagnosis and moisture calculation method, this research analyzes the impact of rainout degrees along moisture transport paths, atmospheric circulation patterns, water vapor sources, and moisture transport on the extreme depletion of precipitation isotopes in the study area. The purpose of this study is to reveal the direct cause of extremely depleted δ18Op at the hourly time scale. The results show the diversity and complexity of δ18Op variation in summer precipitation events in Adelaide. The rainout caused by local and upstream large precipitation may be the main reason for extremely low values of δ18Op. The phenomenon of subcloud secondary evaporation, which is driven by the interaction between relatively low humidity and high temperature at near-surface levels, plays a pivotal role in the entire precipitation process. This mechanism is particularly pronounced during the onset or cessation of precipitation events, thereby resulting in the observed enrichment of δ18Op values. The oxygen stable isotopic composition of water vapor (δ18Oa) would usually become higher, when the air mass mixes with new moisture with relatively high δ18Oa, suppressing the influence of the previous rainout. [ABSTRACT FROM AUTHOR]

The East Asian summer climate displays a marked change after the late 1990s. This is principally due to a weakening of the Pacific–Japan (PJ) teleconnection pattern that was a dominant driver of precipitation variability over East Asia. Nevertheless, western Japan has frequently experienced heavy rainfall events over the past several years. Atmospheric reanalysis and observational datasets are used to investigate summer precipitation variability over East Asia in a view of interdecadal changes from 1979 to 2020. East Asian summer precipitation has increased the most in Japan, especially over and around the Southwest Islands in southern Japan, where cumulus convection has been more dominant for the characteristic of precipitation variability including mei-yu–baiu rainfall than frontal structure since the mid-2000s. Atmospheric analysis in vertical structures and convective instability indicates that moist instability is neutralized by cumulus convection over the southern East China Sea and off the Pacific coast of Japan, where recent warming in sea surface temperature (SST) along the Kuroshio exceeds the SST threshold for convection. This decadal shift in precipitation variability has a close relationship with the second precipitation mode over East Asia, which has taken the place of the PJ pattern as a leading driver of precipitation variability over western Japan in the past decade. The enhanced cumulus convection is enabled to act as a forcing mechanism for the Rossby wave train from East Asia toward North America along the westerly jet or for the eastward extension of the Silk Road pattern, which possibly favors a heatwave in the Pacific Northwest areas of North America. [ABSTRACT FROM AUTHOR]

The tropical rain belt varies between unimodal and bimodal meridional precipitation distributions, both regionally and on seasonal to geological time scales. Here we show that this variation is largely driven by equatorial precipitation inhibition, and quantify it using an equatorial modality index (EMI) that varies continuously between 1 and 2 for purely unimodal and bimodal distributions. We show that tropical modality is a fundamental characteristic of tropical climate, which we define as annual-mean EMI. We examine large-scale aspects of tropical modality across 73 climate models from phases 5 and 6 of the Coupled Model Intercomparison Project, 45 paleo simulations (;300 million years ago to present), and observations. We find increased tropical modality to be strongly related to increased width of the tropical rain belt, wider and weaker meridional overturning circulation, colder equatorial cold tongues, and more severe double intertropical convergence zone bias in modern climate models. Tropical sectors (or global zonal means) with low tropical modality are characterized by monsoonal seasonal variations (i.e., seasonal migrations of rainbands following the sun). In sectors with high tropical modality we identify three important seasonal modes: (i) migration of the precipitation distribution toward the warmer hemisphere, (ii) variation in the latitudinal separation between hemispheric rainbands, and (iii) seesaw variation in the intensity of the hemispheric rainbands. In high tropical modality sectors, due to contrasting shifts of the migration and separation modes, counter to general wisdom, seasonal migrations of tropical rainbands cannot be generally assumed to follow the sun. [ABSTRACT FROM AUTHOR]

The summer North Atlantic Oscillation (SNAO), an important climate signal in regulating the interannual variability of Tibetan Plateau (TP) summer rainfall, is closely related to a meridional precipitation dipole pattern between the southeastern and northeastern TP. In this study, based on diagnoses of observations and multiple realizations of the CESM2 historical simulation, we find that there are fundamental differences between the formation processes dominating the SNAO-related summer rainfall anomaly in the southeastern and northeastern TP. An atmospheric moisture budget analysis reveals that the anomalous vertical (horizontal) moisture advection makes the largest contribution to the southeastern (northeastern) TP summer rainfall anomaly. During the negative phase of SNAO, the increased precipitation in the southeastern TP is related to the anomalous ascending flows, which are driven by two processes according to the moist static energy budget. The first is the southward shift of the subtropical westerly jet stream, which produces positive anomalous zonal advection of the climatological moist enthalpy in the upper-middle troposphere over the southeastern TP. The second is related to the enhanced transport of anomalous warm moist air to the south of TP, which produces positive anomalous meridional advection of anomalous moist enthalpy into the lower troposphere over the southeastern TP under the control of climatological monsoonal meridional circulations. This positive moist enthalpy advection enhances the atmospheric moist static energy and facilitate enhanced local convection. For the northeastern TP, the decreased precipitation is dominated by negative anomalous horizontal moisture advections due to the SNAO-induced equivalent-barotropic anomalous cyclone near the eastern edge of the TP. [ABSTRACT FROM AUTHOR]

Under the intensification of global warming, the characteristics of the Three Rivers source region (TRSR; i.e., headwaters of the Yellow River, the Yangtze River, and the Lancang River) in China were diagnosed in the summer season from 1979 to 2015 using observations and reanalysis data. The diagnoses indicate that summer precipitation decreased from 1979 to 2002 [by 9.01 mm day−1 (10 yr)−1; p < 0.05 by Student's t test] and increased significantly after 2002 [by 5.52 mm day−1 (10 yr)−1]. This abrupt change year (2002) was further confirmed by the cumulative anomaly method, the moving t-test method, and the Yamamoto method. By compositing the thermodynamics before and after the abrupt change year (2002), the results reveal that increased water vapor and more substantial lower-level convergence were present over the TRSR during 2003–15. This marked interdecadal variability in the TRSR summer precipitation responded to the interdecadal position and intensity of the large-scale forcing East Asian westerly jet (EAWJ), which is significantly modulated by the low-frequency variability associated with Southern Oscillation index. The connection between the interannual TRSR precipitation and the location and intensity of EAWJ was also explored. The position index of the EAWJ is negatively (with correlation coefficient R of −0.446; p < 0.05 by Student's t test) correlated with the precipitation over the TRSR, implying that southward and northward years of EAWJ are respectively associated with intensifying and weakening the TRSR summer precipitation, whereas the intensity of EAWJ is insignificantly correlated with the TRSR summer precipitation. [ABSTRACT FROM AUTHOR]

Spring rainfall is important for agriculture and economics in North China (NC). Thus, there is an imperative need for accurate seasonal prediction of the spring precipitation. This study implements a novel rainfall framework to improve understanding of NC spring rainfall. The framework is built based on a three-cluster normal mixture model. Distribution parameters are sampled using Bayesian inference and a Markov chain Monte Carlo algorithm. The probability behaviors of light, moderate, and heavy rainfall events can be reflected by the three rainfall clusters, respectively. Analysis of 61-yr data indicates that moderate rainfall makes the largest contribution (67%) to the total rainfall amount. The moderate rainfall intensity is mainly influenced by the sea surface temperature anomaly (SSTA) in the previous season over the equatorial eastern Pacific, and rainfall frequency is influenced by geopotential height anomaly in the mid- to high latitudes in spring. It is also found that more extreme precipitation events can be observed in the spring following an eastern Pacific El Niño event in the previous autumn and winter. Based on these relationships, we develop a multiple linear regression model. Hindcasts for spring precipitation using the model indicates that its anomaly correlation is 0.48, significant at the 99% confidence level. The result suggests that the newly developed model can well predict spring rainfall amount in NC. [ABSTRACT FROM AUTHOR]

Precipitation estimates are highly uncertain in complex regions such as High Mountain Asia (HMA), where ground measurements are very difficult to obtain and atmospheric dynamics poorly understood. Though gridded products derived from satellite-based observations and/or reanalysis can provide temporally and spatially distributed estimates of precipitation, there are significant inconsistencies in these products. As such, to date, there is little agreement in the community on the best and most accurate gridded precipitation product in HMA, which is likely area dependent because of HMA's strong heterogeneities and complex orography. Targeting these gaps, this article presents the development of a consensus ensemble precipitation product using three gridded precipitation datasets [the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG), the Climate Hazards Group Infrared Precipitation with Station data (CHIRPS), and the ECMWF reanalysis ERA5] with a localized probability matched mean (LPM) approach. We evaluate the performance of the LPM estimate along with a simple ensemble mean (EM) estimate to overcome the differences and disparities of the three selected constituent products on long-term averages and trends in HMA. Our analysis demonstrates that LPM reduces the high biases embedded in the ensemble members and provides more realistic spatial patterns compared to EM. LPM is also a good alternative for merging data products with different spatiotemporal resolutions. By filtering disparities among the individual ensemble members, LPM overcomes the problem of a certain product performing well only in a particular area and provides a consensus estimate with plausible temporal trends. [ABSTRACT FROM AUTHOR]

Rainfall in southeastern Australia (SEA) decreased substantially in the austral autumn (March–May) of the 1990s and 2000s. The observed autumn rainfall reduction has been linked to the climate change–induced poleward shift of the subtropical dry zone across SEA and natural multidecadal variations. However, the underlying physical processes responsible for the SEA drought are still not fully understood. This study highlights the role of sea surface temperature (SST) warming in the subtropical South Pacific (SSP) in the autumn rainfall reduction in SEA since the early 1990s. The warmer SSP SST enhances rainfall to the northwest in the southern South Pacific convergence zone (SPCZ); the latter triggers a divergent overturning circulation with the subsidence branch over the eastern coast of Australia. As such, the subsidence increases the surface pressure over Australia, intensifies the subtropical ridge, and reduces the rainfall in SEA. This mechanism is further confirmed by the result of a sensitivity experiment using an atmospheric general circulation model. Moreover, this study further indicates that global warming and natural multidecadal variability contribute approximately 44% and 56%, respectively, of the SST warming in the SSP since the early 1990s. [ABSTRACT FROM AUTHOR]

We propose a new approach to explain multiday rainfall accumulation over a French Alpine watershed using large-scale atmospheric predictors based on analogy. The classical analogy framework associates a rainfall cumulative distribution function (CDF) with a given atmospheric situation from the precipitation accumulations yielded by the closest situations. The analogy may apply to single-day or multiday sequences of pressure fields. The proposed approach represents a paradigm shift in analogy. It relies on the similarity of the local topology mapping the pressure field sequences, somehow forgetting the pressure fields per se. This topology is summarized by the way the sequences of pressure fields resemble their neighbors (dimensional predictors) and how fast they evolve in time (dynamical predictors). Although some information—and hence predictability—is expected to be lost when compared with classical analogy, this approach provides new insight on the atmospheric features generating rainfall CDFs. We apply both approaches to geopotential heights over western Europe in view of assessing 3-day rainfall accumulations over the Isère River catchment at Grenoble, France. Results show that dimensional predictors are the most skillful features for predicting 3-day rainfall—bringing alone 60% of the predictability of the classical analogy approach—whereas the dynamical predictors are less explicative. These results open new directions of research that the classical analogy approach cannot handle. They show, for instance, that both dry sequences and strong rainfall sequences are associated with singular 500-hPa geopotential shapes acting as local attractors—a way of explaining the change in rainfall CDFs in a changing climate. [ABSTRACT FROM AUTHOR]

Moisture transport over the northeastern Atlantic Ocean is an important process governing precipitation distribution and variability over western Europe. To assess its long-term variability, the vertically integrated horizontal water vapor transport (IVT) from a long-term climate simulation spanning the period 850–2100 CE was used. Results show a steady increase in moisture transport toward western Europe since the late-nineteenth century that is projected to expand during the twenty-first century under the RCP8.5 scenario. The projected IVT for 2070–99 significantly exceeds the range given by interannual–interdecadal variability of the last millennium. Changes in IVT are in line with significant increases in tropospheric moisture content, driven by the concurrent rise in surface temperatures associated with the anthropogenic climate trend. On regional scales, recent and projected precipitation changes over the British Isles follow the global positive IVT trend, whereas a robust precipitation decrease over Iberia is identified in the twenty-first century, particularly during autumn. This indicates a possible extension of stable and dry summer conditions and a decoupling between moisture availability and dynamical forcing. The investigation of circulation features reveals a mean poleward shift of moisture corridors and associated atmospheric rivers. In particular, in Iberia, a significant increase in the frequency of dry weather types is observed, accompanied by a decrease in the frequency of wet types. An opposite response is observed over the British Isles. These changes imply a stronger meridional north–south dipole in terms of pressure and precipitation distributions, enhancing the transport toward central Europe rather than to Iberia. [ABSTRACT FROM AUTHOR]

Skillful forecasts of the Indian summer monsoon rainfall (ISMR) at long lead times (4–5 months in advance) pose great challenges due to strong internal variability of the monsoon system and nonstationarity of climatic drivers. Here, we use an advanced causal discovery algorithm coupled with a response-guided detection step to detect low-frequency, remote processes that provide sources of predictability for the ISMR. The algorithm identifies causal precursors without any a priori assumptions, apart from the selected variables and lead times. Using these causal precursors, a statistical hindcast model is formulated to predict seasonal ISMR that yields valuable skill with correlation coefficient (CC) ~0.8 at a 4-month lead time. The causal precursors identified are generally in agreement with statistical predictors conventionally used by the India Meteorological Department (IMD); however, our methodology provides precursors that are automatically updated, providing emerging new patterns. Analyzing ENSO-positive and ENSO-negative years separately helps to identify the different mechanisms at play during different years and may help to understand the strong nonstationarity of ISMR precursors over time. We construct operational forecasts for both shorter (2-month) and longer (4-month) lead times and show significant skill over the 1981–2004 period (CC ~0.4) for both lead times, comparable with that of IMD predictions (CC ~0.3). Our method is objective and automatized and can be trained for specific regions and time scales that are of interest to stakeholders, providing the potential to improve seasonal ISMR forecasts. [ABSTRACT FROM AUTHOR]

Previous studies have noted that a strong El Niño event occurring in the preceding winter will result in westward stretching of the western North Pacific subtropical high (WPSH) in the following summer, causing anomalously high precipitation in the Yangtze–Huaihe River basin and anomalously low precipitation in southern China. The winters preceding the summers of 1998 and 2016 featured strong El Niño events, which, along with the El Niño event of 1982, represented the strongest El Niño events since 1950. Under these similar El Niño event backgrounds, the July precipitation anomaly in 2016 was similar to that in 1998, but the August precipitation anomalies in the two years featured opposite distributions. According to the atmospheric circulation analysis, we found that an anomalous ascending motion appeared over the Indian Ocean, while an anomalous descending motion appeared over the Pacific Ocean in August 1998. In addition, the WPSH stretched westward over southern China. However, the atmospheric circulation distribution in August 2016 was the opposite of that in 1998, and the WPSH was divided into eastern and western parts by the anomalous western Pacific cyclone. Further analysis showed that the number of tropical cyclones and typhoons over the western Pacific Ocean increased significantly in August 2016, and their activities were concentrated in the South China Sea (SCS)–southern China region and the western Pacific Ocean, resulting in the division of the WPSH. Therefore, the numbers, tracks, and strengths of tropical cyclones and typhoons were responsible for the differences in the anomalous precipitation distributions over the East Asia–Pacific Ocean region between August 2016 and August 1998. [ABSTRACT FROM AUTHOR]

Among various forms of atmospheric response to ocean mesoscale eddies, the rainfall response is the most difficult to quantify and is subject to considerable uncertainty. Here the robustness of the rainfall response is examined by comparing three different satellite-derived rainfall datasets: the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA), NOAA Climate Prediction Center (CPC) morphing technique (CMORPH) global precipitation, and the newly available Integrated Multisatellite Retrievals for Global Precipitation Measurement (IMERG) that is based on the latest remote sensing technology with finer spatial and temporal resolution. Results show that all datasets exhibit a similar rainfall response to ocean eddies, but the amplitude of the rainfall response is much stronger in IMERG than in the other two, despite the fact that IMERG provides the weakest time-mean rainfall estimate. In situ validation against the NOAA’s Ocean Climate Stations Project (OCS) Kuroshio Extension Observatory (KEO) buoy rainfall measurement shows that IMERG is more accurate in estimating both the mean value of rainfall and its intensity distribution than the other two products, at least in the Kuroshio Extension region. Further analysis reveals that 1) eddy-induced precipitation response is significantly stronger in winter than in summer, and 2) warm-eddy-induced rainfall response is considerably stronger than cold-eddy-induced response, and these asymmetries in rainfall response are more robust in IMERG than in the other two datasets. Documenting and analyzing these asymmetric rainfall responses is important for understanding the potential role of ocean eddies in forcing the large-scale atmospheric circulation and climate. [ABSTRACT FROM AUTHOR]

Properties of the rain estimation differences between Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) 2A25, TRMM Microwave Imager (TMI) 2A12, and TRMM Multisatellite Precipitation Analysis (TMPA) 3B42 are investigated with a focus on distinguishing between nonextreme and extreme rains over the Maritime Continent from 1998 to 2014. Statistical analyses of collocated TMI 1B11 85-GHz polarization-corrected brightness temperatures, PR 2A23 storm-top heights, and PR 2A25 vertical rain profiles are conducted to identify possible sources of the differences. The results indicate that a large estimation difference exists between PR and TMI for the general rain rate (extreme and nonextreme events). The PR-TMI rain-rate differences are larger over land and coast than over ocean. When extreme rain is isolated, a higher frequency of occurrence is identified by PR over ocean, followed by TMI and TMPA. Over land, TMI yields higher rain frequencies than PR with an intermediate range of rain rates (between 15 and 25 mm h−1), but it gives way to PR for the highest extremes. The turnover at the highest rain rates arises because the heaviest rain depicted by PR does not necessarily accompany the strongest ice-scattering signals, which TMI relies on for estimating precipitation over land and coast. [ABSTRACT FROM AUTHOR]

This study provides an improved understanding of the diurnal cycle of warm season (June-September) rainfall over West Africa, including its underlying physical processes. Rainfall from the Tropical Rainfall Measuring Mission and atmospheric dynamics fields from reanalyses are used to evaluate the 1998-2013 climatology and a case study for 2006. In both the climatology and the 2006 case study, most regions of West Africa are shown to have a single diurnal peak of rainfall either in the afternoon or at night. Averaging over West Africa produces a diurnal cycle with two peaks, but this type of diurnal cycle is quite atypical on smaller space scales. Rainfall systems are usually generated in the afternoon and propagate westward, lasting into the night. Afternoon rainfall peaks are associated with an unstable lower troposphere. They occur either over topography or in regions undisturbed by nocturnal systems, allowing locally generated instability to dominate. Nocturnal rainfall peaks are associated with the westward propagation of rainfall systems and not generally with local instability. Nocturnal rainfall peaks occur most frequently about 3°-10° of longitude downstream of regions with afternoon rainfall peaks. The diurnal cycle of rainfall is closely associated with the timing of extreme rainfall events. [ABSTRACT FROM AUTHOR]

Convection-permitting simulations at 3-km resolution using a regional climate model are analyzed to improve the understanding of the diurnal cycle of rainfall over West Africa and its underlying physical processes. The warm season of 2006 is used for the model simulations. The model produces an accurate representation of the observed seasonal mean rainfall and lower-troposphere circulation and captures the observed westward propagation of rainfall systems. Most of West Africa has a single diurnal peak of rainfall in the simulations, either in the afternoon or at night, in agreement with observations. However, the number of simulated rainfall systems is greater than observed in association with an overestimation of the initiation of afternoon rainfall over topography. The longevity of the simulated propagating systems is about 30% shorter than is observed, and their propagation speed is nearly 20% faster. The model captures the observed afternoon rainfall peaks associated with elevated topography (e.g., the Jos Plateau). Nocturnal rainfall peaks downstream of the topographic afternoon rainfall are also well simulated. However, these nocturnal rainfall peaks are too widespread, and the model fails to reproduce the observed afternoon rainfall peaks over regions removed from topographic influence. This deficiency is related to a planetary boundary layer that is deeper than observed, elevating unstable profiles and inhibiting afternoon convection. This study concludes that increasing model resolution to convection-permitting space scales significantly improves the diurnal cycle of rainfall compared with the models that parameterize convection, but this is not sufficient to fully resolve the issue, perhaps because other parameterizations remain. [ABSTRACT FROM AUTHOR]

Spring persistent rainfall (SPR) over southern China has great impact on its society and economics. A remarkable feature of the SPR is high frequency. However, SPR frequency obviously decreases over the period of 1997-2011. In this study, the possible causes have been investigated from the perspective of the individual and concurrent effects of the East Asian subtropical jet (EASJ) and East Asian polar front jet (EAPJ). A close relationship is detected between SPR frequency and EASJ intensity (but not EAPJ intensity). Associated with strong EASJ, abundant water vapor is transported to southern China by the southwesterly flow, which may trigger the SPR. Additionally, frequencies of both strong EASJ and weak EAPJ events are positively correlated with SPR frequency. Further investigation of the concurrent effect indicates a significant positive correlation between the frequencies of SPR and the strong EASJ-weak EAPJ configuration. Associated with this configuration, southwesterly flow strengthens in the lower troposphere, while northerly wind weakens in the upper troposphere. This provides a dynamic and moist condition, as enhanced ascending motion and intensified convergence of abundant water vapor over southern China, which favors the SPR. All analyses suggest that the EASJ may play a dominant role in the SPR occurrence and that the EAPJ may play a modulation role. Finally, a possible mechanism maintaining the strong EASJ-weak EAPJ configuration is proposed. Significant cooling over the northeastern Tibetan Plateau may induce a cyclone anomaly in the upper troposphere, which could result in an accelerating EASJ and a decelerating EAPJ. [ABSTRACT FROM AUTHOR]

Based on daily rainfall observations and Japanese 25-year Reanalysis Project data during ~1981-2010, a three-dimensional circulation structure that formed before heavy summer rainfall in central north China (CNC) is revealed in this study. Composite analyses of circulation in advance of 225 heavy rain days show that the circulation structure is characterized by a remarkable upper-tropospheric warm anomaly (UTWA), which covers most of northern China with a center at ~300 hPa. Under hydrostatic and geostrophic equilibriums, the UTWA contributes to the generation of an anticyclonic (cyclonic) anomaly above (below). The anticyclonic anomaly strengthens (weakens) westerly winds to the north (south) of the warm center and pushes the high-level westerly jet to the north. The cyclonic anomaly deepens the trough upstream of CNC and intensifies lower southwesterly winds to the mideast of the warm center. As a result, the northerly stretched high-level jet produces upper divergence in its right-front side and the intensified southwesterly winds induce lower moisture convergence in its left-front side, causing heavy rainfall in CNC. Correlation analyses further confirm the close connections between UTWA and circulation in the upper and lower troposphere. The correlation coefficients between UTWA and the upper geopotential height, upper westerly jet, and lower southerly flow reach 0.95, 0.70, and 0.39, implying that the two critical factors leading to intense rainfall in CNC, the high-level jet and the low-level southerly flow, are closely connected with the UTWA. Consequently, in the future analyses and forecasts of heavy rainfall over northern China, more attention should be paid to the temperature in the upper troposphere. [ABSTRACT FROM AUTHOR]

The climatic features of the distinctive cold season precipitation over the Yungui Plateau of China and the corresponding circulation background are investigated. From daily rainfall data observed with a high-density station network, it is found that the highest rainfall frequency in southern China during November-February appears over the Yungui Plateau. The rainfall intensity in this region is fairly low, and there is no remarkable rainfall-amount maximum. In comparison with the rainfall in southeastern China, the precipitation over the Yungui Plateau is more concentrated in weak events, with 85.9% of rainfall days having daily precipitation amounts of less than 3 mm. By regressing the circulation field on the rainfall frequency index, a favorable climatic background for high rainfall frequency is explored. In high-rainfall-frequency years, the surface wind exhibits southwesterly wind anomalies west of 104°E and cold air penetrates from the north on the eastern side. These two branches converge on the eastern edge of the Hengduan Mountains. In the lower troposphere, southwesterly winds prevail and anomalous water vapor fluxes converge over the Yungui Plateau. In the middle and higher troposphere, the westerly zonal wind strengthens and leads to an anomalous divergence. These dynamic and moist conditions contribute to the formation of clouds and precipitation. The northward- and eastward-facing slopes of the Yungui Plateau uplift the shallow, cold air carried by the northerly and easterly winds, and the terrain effects trigger the precipitation process. The low temperature and small specific humidity over the Yungui Plateau modulate the rainfall intensity to a low level. [ABSTRACT FROM AUTHOR]

C-band polarimetric radar measurements spanning two wet seasons are used to study the effects of the large-scale environment on the statistical properties of stratiform and convective rainfall around Darwin, Australia. The rainfall physical properties presented herein are the reflectivity fields, daily rainfall accumulations and raining area, rain rates, and drop size distribution (DSD) parameters (median volume diameter and 'normalized' intercept parameter). Each of these properties is then analyzed according to five different atmospheric regimes and further separated into stratiform or convective rain categories following a DSD-based approach. The regimes, objectively identified by radiosonde thermodynamic and wind measurements, represent typical wet-season atmospheric conditions: the active monsoon regime, the 'break' periods, the 'buildup' regime, the trade wind regime, and a mixture of inactive/break periods. The large-scale context is found to strongly modulate rainfall and cloud microphysical properties. For example, during the active monsoon regime, the daily rain accumulation is higher than in the other regimes, while this regime is associated with the lowest rain rates. Precipitation in this active monsoon regime is found to be widespread and mainly composed of small particles in high concentration compared to the other regimes. Vertical profiles of reflectivity and DSD parameters suggest that warm rain processes are dominant during this regime. In contrast, rainfall properties in the drier regimes (trade wind/buildup regimes) are mostly of continental origin, with rain rates higher than in the moister regimes. In these drier regimes, precipitation is mainly formed of large raindrops in relatively low concentration due to a larger contribution of the ice microphysical processes on the rainfall formation. [ABSTRACT FROM AUTHOR]

The Okinawa baiu (summer rainy season) starts in early May and ends in late June, preceding the baiu in mainland Japan by approximately 1 month. This study investigates the time evolution of the large-scale circulation associated with the Okinawa baiu using 10-yr (1997-2006) climatologies of precipitation and meteorological fields, with particular focus on temperature advection at 500 hPa.The onset of the Okinawa baiu occurs in early May and is followed by an initial peak in precipitation during mid-May. The baiu rainband then moves southeastward, leading to a short break in baiu precipitation during late May. The rainband returns to Okinawa in early June, and a second peak in precipitation occurs during mid-June. The baiu rainband withdraws northward in late June. The mid-May precipitation peak is associated with warm advection at 500 hPa, mainly due to the meridional temperature gradient and the prevailing southerly winds. This warm advection coincides with upward motion near Okinawa; however, the warm advection is insufficient to explain the peak precipitation amount. Enhancement of precipitation by a transient disturbance probably contributes to the peak amount. The break period during late May coincides with the peak of South China Sea monsoon. Warm advection at 500 hPa strengthens again in June because of the strong zonal thermal contrast between the warm Tibetan Plateau and cold Pacific. This warm advection is able to adequately explain both the upward motion and precipitation. These results indicate that the large-scale meteorological characteristics are different during the first and second peaks. [ABSTRACT FROM AUTHOR]

The mechanisms associated with the intraseasonal variability of precipitation over South America during the spring season are investigated with emphasis on the influence of a quasi-stationary anomalous circulation over the southeastern South Pacific Ocean (SEP). A spectral analysis performed to the bandpass-filtered time series of daily precipitation anomalies for the La Plata Basin (LPB) and the South Atlantic convergence zone (SACZ) regions revealed several statistically relevant peaks corresponding to periods of roughly 23 days and 14–16 days—with the lower (higher) frequency peaks more prevalent for the SACZ (LPB). The large-scale circulation patterns preconditioning precipitation variability over both regions were explored by means of a regression analysis performed on the daily 500-hPa geopotential anomaly field provided by the NCEP–NCAR reanalysis dataset. The most prominent feature of the regression fields is the presence of a quasi-stationary anomalous anticyclonic (cyclonic) circulation over the southeastern South Pacific Ocean associated with positive rainfall anomalies over the LPB (SACZ) and, emanating from that high (low), an external Rossby wave propagating northeastward toward the South American continent. The synoptic-scale activity, quantified in terms of a frontal activity index, showed a strong influence on precipitation over the LPB and to a lesser extent over the SACZ. Moreover, the frontal activity is actually modulated by the anomalous high circulation over the SEP region. The behavior of this anomalous circulation may be supported by a positive feedback mechanism that can enhance the response of the high anomaly itself, which in turns reinforces the Rossby wave train propagating toward the South American continent. [ABSTRACT FROM AUTHOR]

This study uses the NASA Seasonal-to-Interannual Prediction Project (NSIPP-1) AGCM to investigate the physical mechanisms by which the leading patterns of annual mean SST variability impact U.S. precipitation. The focus is on a cold Pacific pattern and a warm Atlantic pattern that exert significant drought conditions over the U.S. continent. The precipitation response to the cold Pacific is characterized by persistent deficits over the Great Plains that peak in summer with a secondary peak in spring, and weakly pluvial conditions in summer over the Southeast (SE). The precipitation response to the warm Atlantic is dominated by persistent deficits over the Great Plains with the maximum deficit occurring in late summer. The precipitation response to the warm Atlantic is overall similar to the response to the cold Pacific with, however, considerably weaker amplitude. An analysis of the atmospheric moisture budget combined with a stationary wave model diagnosis of the associated atmospheric circulation anomalies is conducted to investigate mechanisms of the precipitation responses. A key result is that, while the cold Pacific and warm Atlantic are two spatially distinct SST patterns, they nevertheless produce similar diabatic heating anomalies over the Gulf of Mexico during the warm season. In the case of the Atlantic forcing, the heating anomalies are a direct response to the SST anomalies, whereas in the case of Pacific forcing they are a secondary response to circulation anomalies forced from the tropical Pacific. The diabatic heating anomalies in both cases force an anomalous low-level cyclonic flow over the Gulf of Mexico that leads to reduced moisture transport into the central United States and increased moisture transport into the eastern United States. The precipitation deficits over the Great Plains in both cases are greatly amplified by the strong soil moisture feedback in the NSIPP-1 AGCM. In contrast, the response over the SE to the cold Pacific during spring is primarily associated with an upper-tropospheric high anomaly over the southern United States that is remotely forced by tropical Pacific diabatic heating anomalies, leading to greatly reduced stationary moisture flux convergences and anomalous subsidence in that region. Moderately reduced evaporation and weakened transient moisture flux convergences play secondary roles. It is only during spring that these three terms are all negative and constructively contribute to produce the maximum dry response in spring. The above findings based on the NSIPP-1 AGCM are generally consistent with observations, as well as with four other AGCMs included in the U.S. Climate Variability and Predictability (CLIVAR) project. [ABSTRACT FROM AUTHOR]