Summertime Rainfall Research Articles

Role of local absorbing aerosols in modulating Indian summer monsoon rainfall

Absorbing aerosols and their impact on the Indian monsoon system is highly complex and demands more scientific understanding. Our study using a chemistry-coupled regional climate model (RegCM 4.5) with idealized experiments observed that natural and anthropogenic absorbing aerosols (i.e., dust and carbonaceous aerosols) reduce monsoon precipitation in a seasonal time scale. More than 1 mm day−1 decline in mean summertime rainfall was observed over parts of the central Indian region and Indo-Gangetic plane for dust aerosol. A substantial reduction in the land-sea pressure gradient and lower tropospheric moisture distribution were found to control the observed modulation in rainfall. Near-surface wind circulation responded distinctly to natural (dust) and anthropogenic (carbonaceous) aerosols. The dust forcing weakened the monsoon trough by creating an anomalous anticyclonic circulation. The Northern Arabian Sea acted as a moisture source for the carbonaceous aerosol forcing. Intraseasonal rainfall over central India appeared to have a sharp reduction for dust forcing during early June, with a moderate increase for carbonaceous aerosols. Such quantification is essential for understanding the impact of aerosol forcing on regional climate change and the water cycle and has implications for emissions management and mitigation policies.

Why Has the Summertime Central U.S. Warming Hole Not Disappeared?

Abstract A cooling trend in summer (May–August) daytime temperatures since the mid-twentieth century over the central United States contrasts with strong warming of the western and eastern United States. Prior studies based on data through 1999 suggested that this so-called warming hole arose mainly from internal climate variability and thus would likely disappear. Yet it has prevailed for two more decades, despite accelerating global warming, compelling reexamination of causes that in addition to natural variability could include anthropogenic aerosol–induced cooling, hydrologic cycle intensification by greenhouse gas increases, and land use change impacts. Here we present evidence for the critical importance of hydrologic cycle change resulting from ocean–atmosphere drivers. Observational analysis reveals that the warming hole’s persistence is consistent with unusually high summertime rainfall over the region during the first decades of the twenty-first century. Comparative analysis of large ensembles from four different climate models demonstrates that rainfall trends since the mid-twentieth century as large as observed can arise (although with low probability) via internal atmospheric variability alone, which induce warming-hole-like patterns over the central United States. In addition, atmosphere-only model experiments reveal that observed sea surface temperature changes since the mid-twentieth century have also favored central U.S cool/wet conditions during the early twenty-first century. We argue that this latter effect is symptomatic of external radiative forcing influences, which, via constraints on ocean warming patterns, have likewise contributed to persistence of the U.S. warming hole in roughly equal proportion to contributions by internal variability. These results have important ramifications for attribution of extreme events and predicting risks of record-breaking heat waves in the region. Significance Statement Our paper makes a significant contribution to analysis of a cooling trend in summer (May–August) daytime temperatures since the mid-twentieth century over the central United States, contrasting with strong warming over the remainder of the United States and having important ramifications for assigning cause to and predicting record-breaking heat waves in the region. Observations and model simulations reveal the critical importance of hydrologic cycle change resulting from ocean–atmosphere impacts. Precipitation has increased substantially over the region as a result of atmospheric circulation trends consisting of generally lower pressure and cooler air advection into the region. The persistence of this pattern of increased rainfall/lower temperatures is likely due to near-equal contributions of external forcing (climate change) and internal climate variability.

California margin temperatures modulate regional circulation and extreme summer precipitation in the desert Southwest

In August 2022, Death Valley, the driest place in North America, experienced record flooding from summertime rainfall associated with the North American monsoon (NAM). Given the socioeconomic cost of these type of events, there is a dire need to understand their drivers and future statistics. Existing theory predicts that increases in the intensity of precipitation is a robust response to anthropogenic warming. Paleoclimatic evidence suggests that northeast Pacific (NEP) sea surface temperature (SST) variability could further intensify summertime NAM rainfall over the desert southwest. Drawing on this paleoclimatic evidence, we use historical observations and reanalyzes to test the hypothesis that warm SSTs on the southern California margin are linked to more frequent extreme precipitation events in the NAM domain. We find that summers with above-average coastal SSTs are more favorable to moist convection in the northern edge of the NAM domain (southern California, Arizona, New Mexico, and the southern Great Basin). This is because warmer SSTs drive circulation changes that increase moisture flux into the desert southwest, driving more frequent precipitation extremes and increases in seasonal rainfall totals. These results, which are robust across observational products, establish a linkage between marine and terrestrial extremes, since summers with anomalously warm SSTs on the California margin have been linked to seasonal or multi-year NEP marine heatwaves. However, current generation earth system models (ESMs) struggle to reproduce the observed relationship between coastal SSTs and NAM precipitation. Across models, there is a strong negative relationship between the magnitude of an ESM’s warm SST bias on the California margin and its skill at reproducing the correlation with desert southwest rainfall. Given persistent NEP SST biases in ESMs, our results suggest that efforts to improve representation of climatological SSTs are crucial for accurately predicting future changes in hydroclimate extremes in the desert southwest.

Comparison of Bias Correction Methods for Summertime Daily Rainfall in South Korea Using Quantile Mapping and Machine Learning Model

This study compares the bias correction techniques of empirical quantile mapping (QM) and the Long Short-Term Memory (LSTM) machine learning model for summertime daily rainfall simulation focusing on precipitation-dependent bias and temporal variation. Numerical experiments using Weather Research and Forecasting (WRF) were conducted over South Korea with lateral boundary conditions of ERA5 reanalysis data. For the spatial distribution of mean summertime rainfall, the bias-uncorrected WRF simulation (WRF_RAW) showed dry bias for most of the region of South Korea. The WRF results corrected by QM and LSTM (WRF_QM and WRF_LSTM, respectively) were improved for the mean summer rainfall simulation with the root mean square error values of 0.17 and 0.69, respectively, which were smaller than those of the WRF_RAW (1.10). Although the WRF_QM performed better than the WRF_LSTM in terms of the summertime mean and monthly precipitation, the WRF_LSTM presented a closer interannual rainfall variation to the observation than the WRF_QM. The coefficient of determination for calendar-day mean rainfall was the highest in the following order: the WRF_LSTM (0.451), WRF_QM (0.230), and WRF_RAW (0.201). However, the WRF_LSTM had a limitation in reproducing extreme rainfall exceeding 50 mm/day due to the few cases of extreme precipitation in training data. Nevertheless, the WRF_LSTM better simulated the observed light-to-moderate precipitation (10–50 mm/day) than the others.

Radial Growth Responses of Four Southeastern USA Pine Species to Summertime Precipitation Event Types and Intense Rainfall Events

Previous dendroclimatic studies have examined the relationship between total precipitation amounts and tree radial growth in the southeastern USA, yet recent studies indicate that specific precipitation event types and rainfall intensities influence longleaf pine (Pinus palustris Mill.) radial growth unequally. It remains unknown if other pine species respond similarly regarding specific precipitation types and intensities as most dendroclimatic studies have focused on precipitation amounts on monthly-to-annual scales without examining either the event type or intensity nor focusing on daily data. Here, we examine summertime climate–radial growth relationships in the southeastern USA for four native pine species (longleaf, shortleaf, Virginia, pitch) during 1940–2020. We examine and compare each species’ response to precipitation event types and intense rainfall events (IREs) and address if the temporal sensitivity to these events is species specific. Distinct temporal sensitivities exist among species, and there is a consistent association between convective, stationary front, and quasi-stationary precipitation and radial growth. All species except Virginia pine have significant (p < 0.001) associations between IREs and radial growth, even though IREs account for ~49% of summertime rainfall. These results suggest precipitation-type sensitivity to radial growth may have dendroclimatic implications.

Cost-Effectiveness Analysis of a Sponge City Construction Based on the Life Cycle Cost Theory—A Case Study of the Yanshan South Road Area of Qian’an City, China

In semi-humid regions of China, annual precipitation is not evenly distributed. Heavy, summertime rainfall with a short duration frequently causes urban flooding, and annual rainfall less than evaporation results in urban water scarcity. In 2014, Hebei Province’s Qian’an city was ranked among the first group of sponge city pilot cities. This paper investigates the historically flooded section of Yanshan South Road and its surrounding area in Qian’an, focusing on the cost of resolving an urban water problem. Using the storm flood management model (SWMM) and the life cycle cost (LCC) method, the waterlogging reduction effect and life-cycle cost of various low impact development (LID) scenarios were evaluated. Six rainfall design scenarios were simulated and calculated so that the hydrological performance and cost-effectiveness could be comprehensively evaluated to establish the economic value and effectiveness of implementing LID facilities. This study found that the cost-effectiveness values of sunken green space (SG), SG + infiltration ponds (IP) (3:1), SG + IP (1:1), and SG + IP (1:3) scenarios for infiltration LID schemes were relatively high, up to 2.10. In the infiltration-storage LID scheme, the cost-effectiveness of the SG + reservoirs (RE) (1:1) was grater, which was 1.84. In semi-humid regions, the regulation and storage of rainwater, regarding its collection and use, can be widely applied to the construction of sponge cities.

Interdecadal Variability of Spring Eurasian Snowmelt and Its Impact on Eastern China Summer Precipitation

In this study, the interdecadal variability of Eurasian spring snowmelt and its relationship with Eastern China summer precipitation (ECSP) are investigated based on observations and reanalysis data. Results show that the second mode of the empirical orthogonal function (EOF) analysis of the spring snowmelt, featured as a west-east dipole pattern, displays two interdecadal changes near the late-1970s and in the mid-2000s. The increased spring snowmelt over the Western Siberia (WSI) and the opposite situation over the Eastern Europe (EEU) are significantly linked to a meridional quadrupole summertime rainfall pattern on interdecadal time scales, with excessive rainfall over the regions of southern China (SC) and the Huang-Huai River (HHR) and deficient rainfall over the middle and lower reaches of Yangtze River Valley (YRV) and Inner Mongolia-northeastern China (IMNC). Besides, the possible mechanisms are discussed from the perspective of the hydrological effect related to snowmelt-induced soil moisture. Increased spring snowmelt can produce more water inflowing into the soil, leading to anomalous soil moisture, which can persist into summer. Excessive (deficient) snowmelt-related soil moisture anomalies over the WSI (EEU) in summer will change local land thermal conditions and thus generate cyclonic (anticyclonic) circulations with a quasi-barotropic structure. Therefore, the eastward-propagating wave-activity flux (WAF) could be strengthened over the WSI and EEU in the mid- and upper-troposphere and further propagate downstream. Combining the local response of atmospheric circulations to summer surface heating induced by local snowmelt-related SM anomalies over Mongolia, an anomalous anticyclonic (cyclonic) circulation near the Lake Baikal (SC) and the related strong descending (ascending) motion over the IMNC (SC) are founded. Besides, strong positive (negative) upper-level divergence appears over the YRV (HHR), which is the right (left) side of the exit region of upper-level jet. The upper-level divergence could be attributed to the convergence (divergence) of meridional winds on the right side of the anticyclonic circulation, thus the uniform descending (ascending) motion also appears over the YRV (HHR) through the collocation of upper-level convergence (divergence) and the compensatory lower-level circulations. Consequently, a quadrupole pattern of secondary circulation over Eastern China is founded, thereby causing the anomalously distributed ECSP under favorable water vapor conditions.

Estimating unprecedented extremes in UK summer daily rainfall

The UNSEEN (unprecedented simulated extremes using ensemble) method involves using a large ensemble of climate model simulations to increase the sample size of rare events. Here we extend UNSEEN to focus on intense summertime daily rainfall, estimating plausible rainfall extremes in the current climate. To address modelling limitations simulations from two climate models were used; an initialised 25 km global model that uses parameterised convection, and a dynamically downscaled 2.2 km model that uses explicit convection. In terms of the statistical characteristics that govern very rare return periods, the models are not significantly different from the observations across much of the UK. Our analysis provides more precise estimates of 1000 year return levels for extreme daily rainfall, reducing sampling uncertainty by 70%–90% compared to using observations alone. This framework enables observed daily storm profiles to be adjusted to more statistically robust estimates of extreme rainfall. For a damaging storm in July 2007 which led to surface water flooding, we estimate physically plausible increases in the total daily rainfall of 50%–100%. For much of the UK the annual chance of record-breaking daily summertime rainfall is estimated to be around 1% per year in the present-day climate. Analysis of the dynamical states in our UNSEEN events indicates that heavy daily rainfall is associated with a southward displaced and meandering North Atlantic jet stream, increasing the advection of warm moist air from across Southern Europe and the Mediterranean, and intensifying extratropical storms. This work represents an advancement in the use of climate modelling for estimating present-day climate hazards and outlines a framework for applying UNSEEN at higher spatial and temporal resolutions.

Impact process and mechanism of summertime rainfall on thermal–moisture regime of active layer in permafrost regions of central Qinghai–Tibet Plateau

Rainfall can potentially change upper thermal–moisture boundary conditions and influence the hydrological and thermal state of the active layer in permafrost regions. Studying the relationship between rainfall and ground temperature represents an emerging issue in permafrost engineering and environment but the interactive mechanisms of rainfall and the active layer are not well understood. This study aims to analyze the effects and mechanisms of summertime rainfall on the thermal-moisture dynamics of the active layer by field observations and simulation. The observation data demonstrated that frequent light rainfall events had a minor impact on the active layer, whereas consecutive rainfall events and heavy rainfall events had significant effects on soil temperature and water content. Moreover, the soil temperatures were more susceptible to summertime rainfall events. These rapidly cooled the shallow ground and delayed the temperature rise. Summertime rainfall significantly increased the surface latent heat flux, but decreased the net radiation, sensible heat flux, and soil surface heat flux. Rainfall also enhanced the amount of downward liquid water and water vapor, but the impact of rainfall on the increase in the convective heat transfer of the liquid water was lower than the decreases in the heat conduction flux, latent heat flux by vapor diffusion, and heat flux by convection of vapor. Thus, the reduction in the total soil heat flux caused by rainfall directly leads to a cooling effect on the soil temperature and delays the increase in soil temperature. The cooling effect of rainfall events may mitigate the warming rate and maintain the active layer at a relatively low temperature. The results provide new insights into understanding the inner mechanisms of the effect of rainfall on the active layer and on the possible long–term change trends of permafrost under increasing precipitation in the central Qinghai–Tibet Plateau.

Sensitivity of selected summertime rainfall characteristics to pre-event atmospheric and near-surface conditions

Predicting the erosive potential of rain events and the overall risk of soil erosion (and other hydrologic phenomena triggered by or related to extreme rainfall) requires understanding of the background meteorology that affects rainfall characteristics. The sensitivity of total rainfall kinetic energy, 15-min peak intensities, and event total depth were evaluated with respect to pre-event atmospheric conditions in the northern part of the Pannonian Plain. Five static stability parameters: the Convective Available Potential Energy (CAPE), Convective Inhibition (CIN), Total Totals index (TT), K-index, and the composite CAPE-shear parameter were used as proxies describing the atmospheric static stability and the effect of wind shear on the development of convective precipitation. In addition to these stability parameters, near-surface air temperature and dew point temperature at 2-m above the ground were used as additional covariates in bivariate quantile regression. The primary objective of this paper was to determine the sensitivity of the rainfall characteristics to changes in the analyzed covariates and their ability to explain changes in the distribution of the considered rainfall characteristics. The analyses revealed a strong responsiveness of rainfall kinetic energy and 15-min peak rainfall intensities to dew point temperature with incremental changes following the super–CC (7–14%K−1) scaling regime when near-surface dew point temperature exceeds ~288 K. The findings have important implications for forecasting the erosive potential of rains and estimations of soil erosion in the future.

Trends in Diurnal Cycle of Summertime Rainfall Over Coastal South China in the Past 135 Years: Characteristics and Possible Causes

AbstractThe interdecadal changes in the diurnal cycle of summertime rainfall over coastal South China (CSC) were analyzed for the period of 1884–2018. The results revealed that rainfall intensity has increased with the decrease of occurrence frequency during both pre‐ and mid‐summer in the past 135 years. During the most recent 40 years, occurrence frequency and intensity of nocturnal rainfall, especially extreme rainfall, have substantially increased over CSC, whereas those of daytime extreme rainfall have decreased. Different increasing rates of near‐surface temperature over land and sea are the main contributors to the distinct trends of daytime and nocturnal extreme rainfall. During daytime, a higher increasing rate of near‐surface temperature over sea results in a smaller temperature gradient along the coast, leading to a less convective atmosphere. This negates the effect of increased air‐temperature over CSC in both pre‐ and mid‐summer. In contrast, during nighttime, a higher increasing rate of near‐surface temperature over sea results in a greater temperature gradient along the coast, leading to stronger low‐level offshore winds and more convective layers over coastal waters. During pre‐summer, the stronger nocturnal low‐level offshore winds meets the warm and moist monsoonal flows, substantially enhancing nocturnal extreme rainfall along the coast. In mid‐summer, more vigorous upward motions over coastal waters of CSC enhance convective activities during nighttime, which also lead to an intensification of nocturnal rainfall. The most significant increase and intensification of extreme rainfall is observed in western CSC during pre‐summer, where the land‐sea difference of near‐surface temperature is the greatest over CSC.

Future changes in monsoon duration and precipitation using CMIP6

Future change in summertime rainfall under a warmer climate will impact the lives of more than two-thirds of the world’s population. However, the future changes in the duration of the rainy season affected by regional characteristics are not yet entirely understood. We try to understand changes in the length of the rainy season as well as the amounts of the future summertime precipitation, and the related processes over regional monsoon domains using phase six of the Coupled Model Intercomparison Project archive. Projections reveal extensions of the rainy season over the most of monsoon domains, except over the American monsoon. Enhancing the precipitation in the future climate has various increasing rates depending on the subregional monsoon, and it is mainly affected by changes in thermodynamic factors. This study promotes awareness for the risk of unforeseen future situations by showing regional changes in precipitation according to future scenarios.

Identifying the Externally Forced Atlantic Multidecadal Variability Signal Through Florida Rainfall

AbstractThe North Atlantic experiences basin‐wide, multidecadal changes in sea surface temperature (SST), and this SST variability is linked with regional‐to‐continental scale impacts. These impacts often serve as motivation to study the underlying contributors to Atlantic Multidecadal Variability (AMV). However, these impacts can be more than motivation—they can be tools to study the AMV itself. Herein, we consider the positive correlation between Florida summertime rainfall and the AMV (Enfield et al., 2001, https://doi.org/10.1029/2000GL012745). First, we show that this relationship is apparent in updated observational data sets. Next, we demonstrate that large ensembles of climate models are capable of producing the observed relationship between the AMV and Florida summertime rainfall. Finally, using large ensembles from multiple climate models, we show that historical forcing makes models more likely to capture the observed relationship in summer precipitation. Our findings have implications for our understanding of the AMV and for precipitation projections in at‐risk South Florida.

Re-activation of landslide in sub-Arctic areas due to extreme rainfall and discharge events (the mouth of the Great Whale River, Nunavik, Canada)

Climate change is impacting surficial geomorphic processes, especially in sensitive areas such as the sub-Arctic. One of the most common examples involves landslides, which often develop in glacio-isostatically raised marine clays in northeastern Canada. One of these sites, an expansive area of complex landslide terrain located at the mouth of the Great Whale River in Nunavik, has already been studied due to its age and morphology. We present new data, based on the multidisciplinary research including geomorphic, dendrochronological, and hydroclimatological analyses, allowing us to determine how contemporary climate change has affected landslide reactivation during the last 80 years. Our research included collecting 60 cores from Picea glauca trees, growing on the marginal zone of a landslide deposit, as well as from a reference site. The tilted trees formed eccentric growth-ring patterns, which provided us with reliable dates on the landslide events. In addition to these dendrochronological data, we studied these landslides using repeated aerial photography, which showed changes in river channel constrictions in the period 1969–2019. Based on the eccentricity index of the tree ring data, we recognized disturbance events due to landslides. We compared these data with the hydroclimatological conditions and found clearly visible correlations between heavy rainfall and discharge (>95th percentile) of the Great Whale River. The increased landslide activity over the past several years can be linked to an increase in extreme summertime rainfall events. Increased landslide activity poses a real threat, through its input of large amounts of fine-grained sediment to the river, causing it to narrow.

A Tripole Pattern of Summertime Rainfall and the Teleconnections Linking Northern China to the Indian Subcontinent

Abstract Because of the interactive margin between the East Asian summer monsoon and westerly circulation, summer rainfall in northern China (NC) exhibits high variability. By employing reanalysis data and geostationary satellite data from the Fengyun-2G (FY-2G) satellite and using the linear baroclinic model (LBM) and Hybrid Single-Particle Lagrangian Integrated Trajectory model, this study suggests a tripole pattern in summer rainfall over NC and the Indian subcontinent (IS) that is related to the Indian summer monsoon. The distributions of atmospheric circulation indicate three teleconnections: one is from the IS via the Indo-China Peninsula (ICP) and NC, enhancing the Pacific–Japan (PJ) pattern; another is from the IS via west-central Asia and NC, arousing a Eurasian wave pattern; and the third is an IS–TP–NC pattern via the Tibetan Plateau (TP). Those teleconnections modulate vorticity and atmospheric stability over NC. In addition, along with the circulation distribution related to those teleconnections, two pathways of moisture transport related to the IS rainfall are suggested, except for moisture transport via the Bay of Bengal: one is from the Indo-Pacific to NC due to enhancing cyclones over the Indo-Pacific and a PJ-like pattern; and another is from the IS to NC via the TP within the midtroposphere, which modulates midtroposphere moisture fluxes and atmospheric stability over NC. Both teleconnections and moisture transport result in anomalous rainfall over NC. This study reveals a new mechanism and pathway of the Indian summer monsoon impacting NC rainfall, possibly explaining the reason behind the high variability in NC rainfall.

Summertime variability of the western North Pacific subtropical high and its synoptic influences on the East Asian weather

Variation of the western North Pacific subtropical high (WNPSH) is an important meteorological factor for determining summertime rainfall and temperature over East Asia. Here, three major modes of summertime WNPSH variability are identified and corresponding environmental changes are investigated using cyclostationary empirical orthogonal function analysis. The leading mode exhibits a clear reinforcement of WNPSH associated with global warming. The second and third modes are characterized by intra-seasonal variation of the WNPSH intensity related to sea surface temperature variability in the central and eastern equatorial Pacific. Although WNPSH variability is regarded as a local manifestation, it reflects much wider changes in the entire North Pacific. The three modes exert seasonally and geographically distinct impacts on the East Asian weather by setting anomalous atmospheric circulation and altering the direction of moisture and heat transport. As such, the leading WNPSH modes are an important indicator of summertime weathers in countries neighboring the western North Pacific. This study also shows that extreme weather events are likely to increase as global warming intensifies.

Variability of Summer Precipitation Events Associated with Tropical Cyclones over Mid-Lower Reaches of Yangtze River Basin: Role of the El Niño–Southern Oscillation

Based on tropical cyclone (TC) track data and gridded observational rainfall data of CN05.1 during the period of 1961 to 2014, we examine the contribution of TCs on three metrics of summertime rainfall regimes and identify the connection between TC-induced precipitation events and El Niño–Southern Oscillation (ENSO) in middle–lower reaches of Yangtze River Basin (MLYRB). At the regional scale, TCs are responsible for approximately 14.4%, 12.5%, and 6.9% of rainfall events for normal, 75th, and 95th percentile precipitation cases, respectively. There is no evidence of significant long-term trends of the three type events linked with TCs, while their interdecadal variability is remarkable. Fractionally, larger proportions of TC-induced events occur along southeast coastal regions of MLYRB for normal rainfall events, and they are recorded over southwest and central-east MLYRB for 95th percentile cases. Moreover, a larger contribution of 95th percentile precipitation events to summer total rainfall is found than that for 75th percentile cases, suggesting that TCs may exert stronger impacts on the upper tail of summertime precipitation distribution across MLYRB. The TC-induced normal rainfall events tend to occur more frequency over central-west MLYRB during negative phase of ENSO in summer. However, the higher likelihood of TC-induced rainfall for three defined metrics are found over the majority of areas over MLYRB during negative ENSO phase in spring. In preceding winter, La Niña episode plays a crucial role in controlling the frequency of both normal and 75th percentile precipitation events.

Regional Characteristics of Heavy Summertime Rainfall in and around Urban Tokyo from the Viewpoint of the Spatial Extent of Rainfall Area

本研究では，東京都と埼玉県を主な対象とし，15年間の夏季（6～9月）における，稠密な降水量観測網（290地点）の時間降水量データを用いて，降水域（≧5 mm/h）の局地性と広域性に着目して強雨（≧20 mm/h）発現の地域的な特性を解析した．その結果，関東山地東麓と都区部西部や北部で局地的強雨の頻度が高く，それによる降水量も多い．全強雨に占める局地的強雨の頻度割合は，都区部北部から埼玉県東部で大きく，総降水量への寄与も大きい．一方，関東山地や埼玉県西部，多摩地域では広域的な降水に伴う強雨の頻度や降水量が多い．また，南風時に都心の数十km風下側の埼玉県東部で夏季を通して局地的強雨の頻度割合が大きいことに関し，強雨発現と風系との関係を調べた．局地的強雨には基本的に風の収束が関与しており，いわゆるE-S型風系だけでなく，関東平野内陸からの北風に伴う収束や，相模湾からの南風と東京湾からの南東風との収束も重要と考えられた．

The Influence of CO2Forcing on North American Monsoon Moisture Surges

Widespread multiday convective bursts in the southwestern United States during the North American monsoon are often triggered by Gulf of California moisture surges (GoC surges). However, how GoC surges, and the amount and intensity of associated precipitation, will change in response to CO2-induced warming remains little known, not least because the most widely available climate models do not currently resolve the relevant mesoscale dynamics because of their coarse resolution (100 km or more). In this study, a 50-km-resolution global coupled model is used to address this question. It is found that the mean number of GoC surge events remains unchanged under CO2doubling, but intermediate-to-high intensity surge-related precipitation tends to become less frequent, thus reducing the mean summertime rainfall. Low-level moisture fluxes associated with GoC surges as well as their convergence over land to the east of the GoC intensify, but the increases in low-level moisture are not matched by the larger increments in the near-surface saturation specific humidity because of amplified land warming. This results in a more unsaturated low-level atmospheric environment that disfavors moist convection. These thermodynamic changes are accompanied by dynamic changes that are also detrimental to convective activity, with the midlevel monsoonal ridge projected to expand and move to the west of its present-day climatological maximum. Despite the overall reduction in precipitation, the frequency of very intense, localized daily surge-related precipitation in Arizona and surrounding areas is projected to increase with increased precipitable water.

Investigating observed northwest Australian rainfall trends in Coupled Model Intercomparison Project phase 5 detection and attribution experiments

Mean and extreme northwest Australian (NWA) summertime rainfall has increased significantly since 1950. While previous studies have explored a range of possible factors impacting NWA rainfall, the causes of this increase and possible future changes remain uncertain. This study explores the increasing NWA summertime rainfall trends in Coupled Model Intercomparison Project phase 5 (CMIP5) models. By using a suite of models that contributed realizations of the historical period with various forcings, we explore the impact to this region of greenhouse gases and aerosol emissions since 1950 on mean rainfall and three extreme rainfall indices. The observed NWA rainfall trend is better captured in models when all forcings are included compared to simulations with only greenhouse gas forcing or with only natural forcing, although the models have a large spread. We hypothesise that anthropogenic aerosols played a major role in the observed rainfall trends, and the associated increase in monsoonal flow, and hence historicalNat and historicalGHG simulations tend not to capture observed rainfall trend. Throughout the 21st century, CMIP5 models simulate a stronger increase in mean summer precipitation and extreme indices of NWA rainfall in representative concentration pathway (RCP) 8.5 simulations than in RCP2.6. The NWA region shows intensified extreme events with fewer heavy precipitation days, but the reliability of these projections in this region should be further tested with estimates of future anthropogenic aerosol changes.