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Large parts of southern Africa are influenced by extra-tropical weather systems for most of the year. During late summer (December–March), the circulation over the area becomes distinctly tropical. This paper introduces the Africáne, a synoptic scale tropical low-pressure system which has been shown to cause widespread and heavy rainfall over the southern sub-continent of Africa. The frequency of occurrence of Africánes, their contribution to rainfall and interannual variability are discussed in this paper. Africánes occur most frequently at the longitude of the Caprivi area with a second peak in frequency at around 32.5° E. They mostly occur over Namibia, Botswana and Zimbabwe and only infrequently infiltrate as far south as the borders of South Africa. However, when they do occur over South Africa, they cause widespread heavy rainfall and floods. Rainfall is mostly confined to the eastern flank of Africánes and between 20 and 35% of the annual rainfall over southern Africa in late summer can be attributed to these systems. There are two main synoptic regimes associated with Africánes: a westerly wave or tropical-temperature trough combines with the Africáne to pull rainfall southwards into South Africa. The second, is a mid-level subtropical high pressure, located south of the Africáne, which causes the rainfall to be confined to the north. The interannual variability of Africánes are closely linked to rainfall over southern Africa, such that an above normal number of Africánes in a season causes above normal rainfall over southern Africa. The number of Africánes that form per year is linked to El Niño–Southern Oscillation (ENSO). It is recommended that the predictability of Africánes on different time scales should be investigated. [ABSTRACT FROM AUTHOR]

Dynamical prediction of monsoon rainfall has been an important topic and a long-standing issue in both research and operational community. This paper provides a comprehensive evaluation of the subseasonal-to-seasonal (S2S) prediction skill of the East Asian summer monsoon (EASM) rainfall using the hindcast record from the Beijing Climate Center Climate System Model, BCC CSM1.1m, during the period 1983–2019. The model exhibits reasonable skills for predicting the EASM rainfall at all lead times with the skill dropping dramatically from the shortest lead time of about 2 weeks (LM0) to 1-month lead (LM1), and then fluctuating remarkably throughout 2-month to 12-month lead times. Over the EASM domain, the rapid decline of the S2S rainfall prediction skill from LM0 to LM1 is mainly caused by the inferior skills over Central China in July and over Northeast China in August. Composite analysis based on hindcast records suggest that these inferior skills are directly tied to the model's difficulties in capturing above-normal precipitation over eastern Central China and Northeast China in the respective months, which are further shown to be associated with anomalous weakening and meridional movement of the Northwestern Pacific subtropical high and the activity of large-scale teleconnection pattern hard to be predicted over northeastern Asia in summer, respectively. These findings inform the intrinsic limits of the S2S predictability of the EASM rainfall by a dynamical model, and strongly suggest that the level of confidence placed upon S2S forecasts should be stratified by large-scale circulation anomalies known to significantly affect the prediction skill, e.g., the subtropical high and high-latitude teleconnection patterns for summer monsoon rainfall prediction in this region. [ABSTRACT FROM AUTHOR]

Better understanding on the phases of Indian summer monsoon i.e., onset, progress and withdrawal helps in the decision making and proper management. In this paper, date of onset of monsoon (DOM) and date of withdrawal of monsoon (DWM) is estimated using the long-term daily rainfall data observed by the India Meteorological Department (IMD) and cumulative rainfall anomaly concept, as the onset/withdrawal represents a signature of rainfall transition. The results are promising with a better match between the estimated DOM and IMD announced DOM over Kerala. The length of monsoon season variability is also analysed. The concept of onset at grid point (OGP) in India is also evaluated and progress of monsoon is monitored by considering the latitudinal variation, persistent & strength of rainfall in the continental India. The zonal classification of the spread of monsoon in terms of the intensity and time (duration) is presented. Zone wise pentad analysis of onset clearly signifies the progress of monsoon and its dependency on various physical factors like SST, OLR, TTG, ITCZ, Energy budget and wind circulation. The impact of ENSO on the OGP is brought out, which clearly shows that DOM, DWM and progress is almost influenced by the largescale processes. The intensity–duration–frequency analysis quantified the actual ISM length, seasonal rainfall and rainy days at all locations in India. This study will help modellers in better understanding of onset and progress of monsoon dynamics in India and can be integrated with NWP for the accurate and advance prediction of DOM and the progress. [ABSTRACT FROM AUTHOR]

Quantifying the extent of change in climatic and hydrological variables in the past and the future is essential for climate change-resilient development, especially in the climate change sensitive region of Nepal. This paper analyzed future climatic trends and extremes, historical hydrological extremes and their linkage with historical precipitation extremes, and discussed the impacts of climate change on various sectors in the less-studied Small and Medium-sized Eastern River Basins (SMERB) of Nepal. Applying an ensemble of five Coupled Model Intercomparison Project phase 6 (CMIP6) global climate models (GCMs), we evaluated 10 precipitation and 13 temperature extreme indices using Climpact2 for the historical (1979–2020), near-future (2021–2045), mid-future (2046–2070) and far-future (2071–2100) under two Shared Socioeconomic Pathway (SSP), SSP245 and SSP585. Hydrological extreme indices were assessed using Indicators of Hydrologic Alteration tool. Projected climate revealed significant increase (9–73%) in annual and seasonal rainfall except winter; frequent intense rainfall extremes but prolonged dry spells; significant increasing minimum and maximum temperature trends (0.4–5 °C); rise in extreme heat events; increasing minimum and maximum discharge extreme trends in most hydrological stations; as well as strong association between maximum 1 day precipitation (Rx1day) and 1 day max flow for all stations. Growing dry periods but intense rainfall in few wet days, coupled with warming pattern all-over SMERB with frequent extreme events indicate high risk for future climate-related disasters. The harsher climate will potentially have damaging implications, especially in climate-induced disasters, food security, and water and sanitation infrastructure. [ABSTRACT FROM AUTHOR]

This paper investigates the process behind climate model biases, especially in rainfall, during the southwest (June to August; JJA) and northeast monsoon (December to February; DJF) seasons over the Philippines using the downscaled Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment-Southeast Asia (SEACLID/CORDEX-SEA). Biases in the surface and near-surface climate are assessed over land in eastern (EastPH) and western (WestPH) Philippines using observation and reanalysis datasets. Results show that models with well-represented sea surface temperature (SST) provided better added values after downscaling. Although the CMIP5-driven models tend to overestimate both surface energy and water partitioning, simulations with well-represented SST have better surface energy partitions. In contrast to the reference simulation driven by ERA-Interim reanalysis, the CMIP5-driven simulations tend to underestimate (overestimate) moisture flux convergence during the DJF (JJA) season, which might have contributed to the overall underestimation (overestimation) of rainfall in this season. Further investigation along a vertical transect shows the differences in governing mechanisms for each season. During DJF (JJA) season, the simulation with underestimated (overestimated) SST, near-surface air temperature, and mixing ratio results in underestimated (overestimated) rainfall over EastPH (WestPH). In addition, the CMIP5-driven models were found to overestimate low-level winds, which may have induced the deep convection regardless of season, suggesting the role of low-level winds in the convection scheme used in RegCM4 aside from topography and land cover. [ABSTRACT FROM AUTHOR]

Stratospheric Aerosol Geoengineering (SAG) is one of the solar geoengineering approaches that have been proposed to offset some of the impacts of anthropogenic climate change. Past studies have shown that SAG may have adverse impacts on the global hydrological cycle. Using a climate model, we quantify the sensitivity of the tropical monsoon precipitation to the meridional distribution of volcanic sulfate aerosols prescribed in the stratosphere in terms of the changes in aerosol optical depth (AOD). In our experiments, large changes in summer monsoon precipitation in the tropical monsoon regions are simulated, especially over the Indian region, in association with meridional shifts in the location of the intertropical convergence zone (ITCZ) caused by changes in interhemispheric AOD differences. Based on our simulations, we estimate a sensitivity of − 1.8° ± 0.0° meridional shift in global mean ITCZ and a 6.9 ± 0.4% reduction in northern hemisphere (NH) monsoon index (NHMI; summer monsoon precipitation over NH monsoon regions) per 0.1 interhemispheric AOD difference (NH minus southern hemisphere). We also quantify this sensitivity in terms of interhemispheric differences in effective radiative forcing and interhemispheric temperature differences: 3.5 ± 0.3% change in NHMI per unit (Wm−2) interhemispheric radiative forcing difference and 5.9 ± 0.4% change per unit (°C) interhemispheric temperature difference. Similar sensitivity estimates are also made for the Indian monsoon precipitation. The establishment of the relationship between interhemispheric AOD (or radiative forcing) differences and ITCZ shift as discussed in this paper will further facilitate and simplify our understanding of the effects of SAG on tropical monsoon rainfall. [ABSTRACT FROM AUTHOR]

In this paper, the influence of emerging low level jet (LLJ) core zone on the Indian summer monsoon rainfall (ISMR) examined during the period 1981–2020 using ERA5 reanalysis, Indian Monsoon Data Assimilation and Analysis (IMDAA) along with Indian Meteorology Department (IMD) rainfall. The analysis spans the entirety of India, encompassing various regions, including Central India (68°–87° E, 15° N–26° N), Northeast India (83°–98.5° E, 21°–30° N), Northwest India (69°–84° E, 22°–37° N), and the Southern Peninsula (74°–85° E, 7°–19° N). Our analysis found a significant correlation between the magnitude of LLJ core zone and ISMR across these regions, except for North-East India, in both data sets (0.5–0.8) at a 0.01 significance level. ERA5 and IMDAA, both data sets, reflect the spatial climatological aspects of LLJ at seasonal and sub-seasonal scales. However, compared to the ERA5 data, the amplitude of LLJ wind is lower in the IMDAA data over the evolving core zone. This study provides pivotal insights into the dynamics of the core LLJ zone and its relationship with ISMR, including its associations with varying Indian monsoon patterns and teleconnections with the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) phases. This study hold significant implications for various sectors reliant on accurate monsoon predictions and weather forecasting. By elucidating the evolving dynamics of the core zone of the LLJ and its connection with ISMR, the research provides a valuable framework for enhancing the precision of monsoon forecasting models. [ABSTRACT FROM AUTHOR]

In early summer 2020, eastern China was hit by record-breaking Meiyu rainfall. The anomalous western Pacific subtropical high (WPSH) played a crucial role in this event. This paper exposes the abnormal features of the WPSH and its related vertical circulations and reveals its dynamical causes in this period, from the perspective of the three-pattern decomposition of global atmospheric circulation (3P-DGAC). Analyzing the anomalous spatial patterns of the three stream functions R, H, and W of the horizontal, meridional, and zonal circulations defined by the 3P-DGAC, we discovered that the WPSH described by R at 850 hPa was intensified and westward from the horizontal circulation. For the vertical circulations, the local meridional circulation along East Asia expressed by H manifested an abnormal downdraft at 20° N, which was beneficial to strengthening the WPSH. The zonal circulation in the tropical Pacific represented by W was anomalously far to the west, which was related to the westward extension of the WPSH. Then, to uncover the causes of the anomalous WPSH from the dynamics, we used the vorticity equation based on the 3P-DGAC and found that the negative relative vorticity advection anomaly over the western Pacific played a key role in this event. Due to the positive sea surface temperature (SST) anomaly and enhanced convection over the Maritime Continent (MC), local vertical circulations at low latitudes were significantly activated. The intense local meridional circulation along East Asia carried the relative vorticity from the MC to the western Pacific and strengthened the WPSH. The ascending branch of the zonal circulation in the tropical Pacific moved westward, which may favor the westward extension of the WPSH. This study represents a promising step in revealing the three-dimensional (3D) structure and internal dynamics of anomalous WPSH. [ABSTRACT FROM AUTHOR]

A significant declining trend in rainfall over the Congo basin has been observed over the past three decades. Since the Madden–Julian oscillation (MJO) is a major forcing mechanism for tropical convection and rainfall, the interannual variability and trend in rainfall over the Congo may be partly attributable to variability or changes in the MJO. This study explores the long-term (1979–2018) relationship between the active MJO diagnosed by the real-time multivariate (RMM) MJO phase index data and observed rainfall and cloud data over the Congo during October–March. Since the MJO may significantly enhance rainfall during the wet phases or suppress rainfall during the dry phases, the crux of this paper includes how trends in MJO activity may impact the overall observed precipitation trend over the Congo. The relationship between MJO activity and rainfall over the Congo was documented using statistical techniques and composite analysis. A new, yet simple approach was developed to partition seasonal rainfall depending on the MJO phase (i.e., wet, dry, inactive, and other). Results show a significant correlation between the number of wet and dry MJO days, and rainfall enhancement and suppression over the Congo. While there exists considerable interannual variability in MJO activity and rainfall over the Congo, there is a significant increase in the number of dry MJO days (3.47 days decade−1) which tends to intensify the large-scale drying trend over the Congo during October–March. The increasing trend in the number of dry MJO days is likely enhancing the net drying trend by 13.6% over the Congo. [ABSTRACT FROM AUTHOR]

Using 30 years of hourly rain gauge records and reanalysis data, this paper reveals two types of heavy summer rainfall in central North China (CNC): the lasting mountain (LM) rainfall and the mountain-plain (MP) rainfall, and further explains their formation mechanisms. The evolutions of the rainfall are found to be closely connected to upper-tropospheric temperature anomalies. When upper-tropospheric warm (cold) anomalies dominate North China (the Japan Sea) and stretch down to lower layers, positive (negative) geopotential anomalies, which appear above the warm (cold) anomalies governed by hydrostatic equilibrium, extend from upper to lower troposphere. In the lower layers, evident low-level easterly anomalies form to the south (north) of the positive (negative) geopotential center, controlling the downstream of CNC and making convective systems difficult to progress eastward. Correspondingly, the low-level convergence and strong convection maintain in the west, and the rainfall persists over the mountains constituting the LM process. But when upper-tropospheric cold (warm) anomalies dominate the west (east) of 110°E and stretch down to lower layers, CNC is situated between an upstream trough and a downstream ridge. As the trough-ridge system progresses eastward, the low-level moisture tongue and strong convections in front of the trough move from the mountains to the plains, and the rainfall propagates downstream constituting the MP process. Results in this paper indicate that upper-tropospheric temperature has significant influence on the evolution of rainfall, and pay attention to its characteristics could provide helpful information in the analysis and forecasting of rainfall propagation. [ABSTRACT FROM AUTHOR]

The Congo Basin situated in equatorial Africa is home to the second largest rainforest on the globe and plays an important role in the Earth's climate system. For instance, the Congo is one of the most convective regions in the world and enhances the tropical large-scale circulation. Although tropical convection is crucial for rainfall and the sustenance of the rainforest, the most intense thunderstorms do not necessarily produce the largest amounts of rainfall over equatorial Africa. Aiming to better understand the decreasing rainfall trend over the Congo, trends in thunderstorm intensity from 1983 to 2018 are analyzed in this paper using the Gálvez-Davison index (GDI), i.e., a thermodynamic index used to measure thunderstorm potential. Consistent with previous studies, thunderstorm activity increased during all seasons over the Congo. The GDI suggests that the increasing trends in thunderstorm activity are attributable to an increase in hydrostatic instability. More specifically, the GDI shows that (1) an increase of cold troughs at 500 hPa, (2) an increase in the temperature gradient between 700 and 950 hPa, and (3) a decrease of the equivalent potential temperature ( θ e ) gradient with height collectively act to promote taller, more intense thunderstorms. This study concludes by proposing feedback mechanisms explaining the intensification of thunderstorms. The mechanisms highlight the cooling and moistening of the mid-troposphere, drying and warming at the surface and lower troposphere, and a decrease in vertical stability and convective inhibition. These factors may act to re-enforce the drying trend which has stressed the Congo rainforest over the past 40-years. [ABSTRACT FROM AUTHOR]

The pre-summer rainy season (PSRS; April–June) is the period of most frequent strong precipitation in South China (SC). Persistent strong precipitation events (PSPEs) can cause greater loss of life and property than short-lived strong precipitation. This paper investigates extended-range signals near the tropopause preceding two persistent positive climatological intraseasonal oscillations (PPCISOs) in precipitation corresponding to two climatological PSPEs in SC during the PSRS, using the CISO analysis of isentropic potential vorticity (IPV). It is found that precursor signals can be better seen in IPV CISOs than in unfiltered IPV. The first rainfall PPCISO is caused by an eastward propagation of high-IPV CISOs along the dynamical tropopause originating from the Arctic region and west of the Tibetan Plateau. The second is the most persistent (13 days). Its high-IPV CISOs originate over the Arctic region and the tropical monsoon region, respectively, 20 or more days prior to the rainfall CISO peak. The west of the Tibetan Plateau and the northern region of the East Asian westerly jet near the tropopause are two transit points where extratropical high-IPV CISOs strengthen and then change their propagation direction. The southward high-IPV CISOs and the northward positive IPV CISO caused by the monsoon begin to interact about 10 days prior to rainfall peak. Since then, the interaction leads a narrow meridional positive PV channel on the inclined 345 K isentropic surface. A specific CISO pattern, characterized by an elongated high-IPV CISO isolated by three negative IPV CISOs (the South Asian, the Okhotsk and the western Pacific subtropical highs), is responsible for the long duration of the second rainfall PPCISO. [ABSTRACT FROM AUTHOR]

Most of the annual rainfall over India occurs during the Southwest (June-September) and Northeast (October-December) monsoon periods. In March 2008, however, Southern peninsular India and Sri Lanka received the largest rainfall anomaly on record since 1979, with amplitude comparable to summer-monsoon interannual anomalies. This anomalous rainfall appeared to be modulated at intraseasonal timescale by the Madden Julian Oscillation, and was synchronous with a decaying La Niña event in the Pacific Ocean. Was this a coincidence or indicative of a teleconnection pattern? In this paper, we explore factors controlling rainfall over southern India and Sri Lanka between January and April, i.e. outside of the southwest and northeast monsoons. This period accounts for 20% of annual precipitation over Sri Lanka and 10% over the southern Indian states of Kerala and Tamil Nadu. Interannual variability is strong (about 40% of the January-April climatology). Intraseasonal rainfall anomalies over southern India and Sri Lanka are significantly associated with equatorial eastward propagation, characteristic of the Madden Julian Oscillation. At the interannual timescale, we find a clear connection with El Niño-Southern Oscillation (ENSO); with El Niños being associated with decreased rainfall (correlation of −0.46 significant at the 98% level). There is also a significant link with local SST anomalies over the Indian Ocean, and in particular with the inter-hemispheric sea surface temperature (SST) gradient over the Indian Ocean (with colder SST south of the equator being conducive to more rainfall, correlation of 0.55 significant at the 99% level). La Niñas/cold SSTs south of the equator tend to have a larger impact than El Niños. We discuss two possible mechanisms that could explain these statistical relationships: (1) subsidence over southern India remotely forced by Pacific SST anomalies; (2) impact of ENSO-forced regional Indian Ocean SST anomalies on convection. However, the length of the observational record does not allow distinguishing between these two mechanisms in a statistically significant manner. [ABSTRACT FROM AUTHOR]

This paper examines the projected changes in rainfall in Southeast Asia (SEA) in the twenty-first century based on the multi-model simulations of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment–Southeast Asia (SEACLID/CORDEX–SEA). A total of 11 General Circulation Models (GCMs) have been downscaled using 7 Regional Climate Models (RCMs) to a resolution of 25 km × 25 km over the SEA domain (89.5° E–146.5° E, 14.8° S–27.0° N) for two different representative concentration pathways (RCP) scenarios, RCP4.5 and RCP8.5. The 1976–2005 period is considered as the historical period for evaluating the changes in seasonal precipitation of December–January–February (DJF) and June–July–August (JJA) over future periods of the early (2011–2040), mid (2041–2070) and late twenty-first century (2071–2099). The ensemble mean shows a good reproduction of the SEA climatological mean spatial precipitation pattern with systematic wet biases, which originated largely from simulations using the RegCM4 model. Increases in mean rainfall (10–20%) are projected throughout the twenty-first century over Indochina and eastern Philippines during DJF while a drying tendency prevails over the Maritime Continent. For JJA, projections of both RCPs indicate reductions in mean rainfall (10–30%) over the Maritime Continent, particularly over the Indonesian region by mid and late twenty-first century. However, examination of individual member responses shows prominent inter-model variations, reflecting uncertainty in the projections. [ABSTRACT FROM AUTHOR]

This paper reveals a change in the relationship between the Yangtze River valley (YRV) summer rainfall (YRVSR) and the spring Southern Hemisphere annular mode (SAM) during the period of 1958–2012. It is found that the positive correlation between the YRVSR and the previous May SAM is insignificant during the period of 1958–1987 (P1), while it becomes significant during the period of 1988–2012 (P2). Such interdecadal change is relevant to the interdecadal change of the relationship between the May SAM and the Indian Ocean sea surface temperature anomaly (SSTA). During P2, the strong exchange of the air-sea heat flux related to the May SAM can excite a prominent meridional Indian Ocean tri-pole (IOT) SSTA pattern, whereas during P1, the May SAM associated air-sea heat exchange is weak and the IOT SSTA is not evident. Further observational and simulated evidences show that the May SAM associated IOT SSTA can persist through the following boreal summer and modulate the rainfall anomalies over the Maritime Continent (MC) by changing the zonal circulation over the tropical Indian Ocean-MC region. Subsequently, wave energies generated by the latent heat release of the MC rainfall (MCR) anomalies can propagate northward to the YRV along the great circle route. Responding to the positive (or negative) MCR heating forcing, anticyclonic (or cyclonic) circulation anomalies are excited over the western North Pacific, transporting more (or less) moisture to the YRV. Correspondingly, abnormal ascending (or sinking) motions are induced over the YRV, favoring sufficient (or insufficient) YRVSR. Thus, the IOT SSTA and MCR act as the "ocean" and "atmosphere" bridge role in connecting the May SAM and the YRVSR, respectively. Those models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) which well capture the May SAM-YRVSR relation can reproduce the IOT SSTA pattern and MCR anomalies associated with the May SAM. This further affirms that both the IOT SSTA and MCR are responsible for the strengthened relationship between the SAM and the YRVSR. [ABSTRACT FROM AUTHOR]

Anticyclones are one of the main synoptic features of the global midlatitudes, where they are associated with dry, settled conditions. Previous studies have identified southern Australia as a region where significant long-term changes have occurred in recent decades, including a decline in cool season rainfall and an increase in the frequency of anticyclones. This paper uses several reanalyses to assess the climatology, variability, trends and impacts of anticyclones in southern Australia and several subregions. A robust increase in the frequency of anticyclones in southern Australia is demonstrated between 1960–1979 and 1997–2014, particularly in southeast Australia and the Tasman Sea during the cool months of the year. At least 30% of the recent cool season rainfall decline in southeast Australia can be associated with an increased frequency of anticyclones in this region. However, there are no clear trends in anticyclone frequency near southwestern Western Australia, where long-term rainfall declines are due to changes in other weather systems. [ABSTRACT FROM AUTHOR]

Impacts of global warming and local scale urbanization on precipitation are evident from observations; hence both must be considered in future projections of urban precipitation. Dynamic regional models at a fine spatial resolution can capture the signature of urbanization on precipitation, however simulations for multiple decades are computationally expensive. In contrast, statistical regional models are computationally inexpensive but incapable of assessing the impacts of urbanization due to the stationary relationship between predictors and predictand. This paper aims to develop a unique modelling framework with a demonstration for Mumbai, India, where future urbanization is projected using a Markov Chain Cellular Automata approach, long term projections with climate change impacts are performed using statistical downscaling and urban impacts are simulated with a dynamic regional model for limited number of years covering different precipitation characteristics. The evaluation of the statistical downscaling methodology over historical time period reveals large underestimation of the extreme rainfall, which is improved effectively by applying another regression model, for extreme days. The limited runs of dynamic downscaling models with different stages of urbanization for Mumbai, India, reveal spatially non uniform changes in precipitation, occurring primarily at the higher quantiles. The statistical and dynamical outputs are further integrated using quantile transformation for precipitation projection in Mumbai during 2050s. The projections show dominant impacts of urbanization compared to those from large scale changing patterns. The uniqueness of this computationally efficient framework lies in an integration of global and local factors for precipitation projections through a conjugal statistical–dynamical approach. [ABSTRACT FROM AUTHOR]

Precipitation, in short periods of time, is a phenomenon associated with high levels of uncertainty and variability. Given its nature, traditional forecasting techniques are expensive and computationally demanding. This paper presents a soft computing technique to forecast the occurrence of rainfall in short ranges of time by artificial neural networks (ANNs) in accumulated periods from 3 to 7 days for each climatic season, mitigating the necessity of predicting its amount. With this premise it is intended to reduce the variance, rise the bias of data and lower the responsibility of the model acting as a filter for quantitative models by removing subsequent occurrences of zeros values of rainfall which leads to bias the and reduces its performance. The model were developed with time series from ten agriculturally relevant regions in Brazil, these places are the ones with the longest available weather time series and and more deficient in accurate climate predictions, it was available 60 years of daily mean air temperature and accumulated precipitation which were used to estimate the potential evapotranspiration and water balance; these were the variables used as inputs for the ANNs models. The mean accuracy of the model for all the accumulated periods were 78% on summer, 71% on winter 62% on spring and 56% on autumn, it was identified that the effect of continentality, the effect of altitude and the volume of normal precipitation, have an direct impact on the accuracy of the ANNs. The models have peak performance in well defined seasons, but looses its accuracy in transitional seasons and places under influence of macro-climatic and mesoclimatic effects, which indicates that this technique can be used to indicate the eminence of rainfall with some limitations. [ABSTRACT FROM AUTHOR]

In this paper some basic mechanisms for rainfall teleconnections to a localized tropical sea surface temperature anomaly are re-visited using idealized AGCM aqua-planet simulations. The dynamical response is generally in good agreement with the Gill-Matsuno theory. The mechanisms analyzed are (1) the stabilization of the tropical troposphere outside the heating region, (2) the Walker circulation modification and (3) Ekman pumping induced by the low-level circulation responses. It is demonstrated that all three mechanisms, and in particular (2) and (3), contribute to the remote rainfall teleconnections. However, mechanism (3) best coincides with the overall horizontal structure of rainfall responses. It is shown by using the models boundary layer parameterization that low-level vertical velocities are indeed caused by Ekman pumping and that this induces vertical velocities in the whole tropospheric column through convective feedbacks. Also the modification of the responses due to the presence of idealized warm pools is investigated. It is shown that warm pools modify the speed of the tropical waves, consistent with Doppler shifts and are thus able to modify the Walker circulation adjustments and remote rainfall responses. The sensitivity of the responses, and in particular the importance of the Ekman pumping mechanism, to large variations in the drag coefficient is also tested, and it is shown that the Ekman pumping mechanism is robust for a wide range of values. [ABSTRACT FROM AUTHOR]

Our early work (Wang and Wang in J Clim 26:1322-1338, ) separates El Niño Modoki events into El Niño Modoki I and II because they show different impacts on rainfall in southern China and typhoon landfall activity. The warm SST anomalies originate in the equatorial central Pacific and subtropical northeastern Pacific for El Niño Modoki I and II, respectively. El Niño Modoki I features a symmetric SST anomaly distribution about the equator with the maximum warming in the equatorial central Pacific, whereas El Niño Modoki II shows an asymmetric distribution with the warm SST anomalies extending from the northeastern Pacific to the equatorial central Pacific. The present paper investigates the influence of the various groups of El Niño events on the Indian Ocean Dipole (IOD). Similar to canonical El Niño, El Niño Modoki I is associated with a weakening of the Walker circulation in the Indo-Pacific region which decreases precipitation in the eastern tropical Indian Ocean and maritime continent and thus results in the surface easterly wind anomalies off Java-Sumatra. Under the Bjerknes feedback, the easterly wind anomalies induce cold SST anomalies off Java- Sumatra, and thus a positive IOD tends to occur in the Indian Ocean during canonical El Niño and El Niño Modoki I. However, El Niño Modoki II has an opposite impact on the Walker circulation, resulting in more precipitation and surface westerly wind anomalies off Java-Sumatra. Thus, El Niño Modoki II is favorable for the onset and development of a negative IOD on the frame of the Bjerknes feedback. [ABSTRACT FROM AUTHOR]

In the original published version of the paper, the figures reported in Sect. 4 relating the proportion of rainfall in southern Australia that is due to each of the weather types, were incorrectly stated for the entire Australian landmass, with the same error also affecting Figs. 6 and 8. [ABSTRACT FROM AUTHOR]

This paper introduces an objective definition of local onset and demise of the Indian summer monsoon (ISM) at the native grid of the Indian Meteorological Department’s rainfall analysis based on more than 100 years of rain gauge observations. The variability of the local onset/demise of the ISM is shown to be closely associated with the All India averaged rainfall onset/demise. This association is consistent with the corresponding evolution of the slow large-scale reversals of upper air and ocean variables that raise the hope of predictability of local onset and demise of the ISM. The local onset/demise of the ISM also show robust internannual variations associated with El Nino and the Southern Oscillation and Indian Ocean dipole mode. It is also shown that the early monsoon rains over northeast India has a predictive potential for the following seasonal anomalies of rainfall and seasonal length of the monsoon over rest of India. [ABSTRACT FROM AUTHOR]

This paper makes a regional evaluation of trend in yearly maxima of daily rainfall in southern France, both at point and spatial scales on a regular grid of 8 × 8 km2. In order to filter out the high variability of rainfall maxima, the current analysis is based on a non-stationary GEV modeling in which the location parameter is allowed to vary with time. Three non-stationary models are considered for each series of maxima by constraining the location parameter to vary either linearly, linearly after a given date or linearly up to a final date. Statistical criteria are used to compare these models and select the best starting or final point of putative trends. The analysis shows that, at regional scale, the best distribution of maxima involves a linear trend starting in year 1985 and that this trend is significant in half the region, including most of the mountain ranges and part of the Rhône valley. Increases in yearly maxima are considerable since they reach up more than 60 mm/day in 20 years, which is more than 40 % of the average maximum in this area. [ABSTRACT FROM AUTHOR]

El Niño Modoki events can be further classified into El Niño Modoki I and II in terms of their opposite impacts on southern China rainfall (Wang and Wang, J Clim 26:1322-1338, 2013) and the Indian Ocean dipole mode (Wang and Wang, Clim Dyn 42:991-1005, 2014). The present paper develops an index to identify the types of El Niño events. The El Niño Modoki II (MII) index is defined as the leading principle component of multivariate empirical orthogonal function analysis of the normalized El Niño Modoki index, Niño4 index and 850 hPa relative vorticity anomalies averaged near the Philippine Sea during autumn. The MII index exhibits dominant variations on interannual (2-3 and 4-5 years) and decadal (10-20 years) timescales. El Niño Modoki II events can be well identified by using the MII index value being larger than 1 standard deviation. Further analyses and numerical model experiments confirm that the MII index can portray the major oceanic and atmospheric features of El Niño Modoki II events. The constructed MII index along with previous ENSO indices can be used for classifying and identifying all types of El Niño events. Because of distinct impacts induced by different types of El Niño events, the implication of the present study is that climate prediction and future climate projection under global warming can be improved by using the MII index and other indices to identify the types of El Niño events. [ABSTRACT FROM AUTHOR]

In this paper, the regional climate model REMO is used to investigate the added value of downscaling low resolutions global climate models (GCMs) and the climate change projections over Central Africa. REMO was forced by two GCMs (EC-Earth and MPI-ESM), for the period from 1950 to 2100 under the Representative Concentration Pathway 8.5 scenario. The performance of the REMO simulations for current climate is compared first with REMO simulation driven by ERA-Interim reanalysis, then by the corresponding GCMs in order to determine whether REMO outputs are able to effectively lead to added value at local scale. We found that REMO is generally able to better represent some aspects of the rainfall inter-annual variability, the daily rainfall intensity distribution as well as the intra-seasonal variability of the Central African monsoon, though few biases are still evident. It is also found that the boundary conditions strongly influences the spatial distribution of seasonal 2-m temperature and rainfall. From the analysis of the climate change signal from the present period 1976-2005 to the future 2066-2095, we found that all models project a warming at the end of the twenty-first century although the details of the climate change differ between REMO and the driving GCMs, specifically in REMO where we observe a general decrease in rainfall. This rainfall decrease is associated with delayed onset and anticipated recession of the Central African monsoon and a shortening of the rainy season. Small-scales variability of the climate change signal for 2-m temperature are usually smaller than that of the large-scales climate change part. For rainfall however, small-scales induce change of about 70% compared to the present climate statistics. [ABSTRACT FROM AUTHOR]

Global Climate Models (GCMs) have been extensively used in many climate change impact studies. However, the coarser resolution of these GCM outputs is not adequate to assess the potential effects of climate change on local scale. Downscaling techniques have thus been proposed to resolve this problem either by dynamical or statistical approaches. The statistical downscaling (SD) methods are widely privileged because of their simplicity of implementation and use. However, many of them ignore the observed spatial dependence between different locations, which significantly affects the impact study results. An improved multi-site SD approach is thus presented in this paper to downscaling of daily precipitation at many sites concurrently. This approach is based on a combination of multiple regression models for rainfall occurrences and amounts and the Singular Value Decomposition technique, which models the stochastic components of these regression models to preserve accurately the space-time statistical properties of the daily precipitation. Furthermore, this method was able to describe adequately the intermittency property of the precipitation processes. The proposed approach has been assessed using 10 rain gauges located in the southwest region of Quebec and southeast region of Ontario in Canada, and climate predictors from the National Centers for Environmental Prediction/National Centre for Atmospheric Research re-analysis data set. The results have indicated the ability of the proposed approach to reproduce accurately multiple observed statistical properties of the precipitation occurrences and amounts, the at-site temporal persistence, the spatial dependence between sites and the temporal variability and spatial intermittency of the precipitation processes. [ABSTRACT FROM AUTHOR]

Recent studies using convection-permitting (CP) climate simulations have demonstrated a step-change in the representation of heavy rainfall and rainfall characteristics (frequency-intensity) compared to coarser resolution Global and Regional climate models. The goal of this study is to better understand what explains the weaker frequency of precipitation in the CP ensemble by assessing the triggering process of precipitation in the different ensembles of regional climate simulations available over Europe. We focus on the statistical relationship between tropospheric temperature, humidity and precipitation to understand how the frequency of precipitation over Europe and the Mediterranean is impacted by model resolution and the representation of convection (parameterized vs. explicit). We employ a multi-model data-set with three different resolutions (0.44°, 0.11° and 0.0275°) produced in the context of the MED-CORDEX, EURO-CORDEX and the CORDEX Flagship Pilot Study "Convective Phenomena over Europe and the Mediterranean" (FPSCONV). The multi-variate approach is applied to all model ensembles, and to several surface stations where the integrated water vapor (IWV) is derived from Global Positioning System (GPS) measurements. The results show that all model ensembles capture the temperature dependence of the critical value of IWV (IWVcv), above which an increase in precipitation frequency occurs, but the differences between the models in terms of the value of IWVcv, and the probability of its being exceeded, can be large at higher temperatures. The lower frequency of precipitation in convection-permitting simulations is not only explained by higher temperatures but also by a higher IWVcv necessary to trigger precipitation at similar temperatures, and a lower probability to exceed this critical value. The spread between models in simulating IWVcv and the probability of exceeding IWVcv is reduced over land in the ensemble of models with explicit convection, especially at high temperatures, when the convective fraction of total precipitation becomes more important and the influence of the representation of entrainment in models thus becomes more important. Over lowlands, both model resolution and convection representation affect precipitation triggering while over mountainous areas, resolution has the highest impact due to orography-induced triggering processes. Over the sea, since lifting is produced by large-scale convergence, the probability to exceed IWVcv does not depend on temperature, and the model resolution does not have a clear impact on the results. [ABSTRACT FROM AUTHOR]

Recent observational and modeling studies have demonstrated the substantial influence of the Tibetan Plateau (TP) spring land surface temperature (LST) and subsurface temperature (SUBT) on downstream summer droughts/floods events in East Asia, highlighting the potential application of LST/SUBT on sub-seasonal to seasonal prediction (S2S). In this study, we employ the National Centers for Environment Prediction—Global Forecast System/Simplified Simple Biosphere model version 2 (GFS/SSiB2) to investigate the potential role of the late spring warm LST anomaly over the TP on the extraordinary June 1998 flood in the south of the Yangtze River region. Numerical experiments indicate that the warmer (above normal) May LST over the TP may contribute to the extreme flood of 1998 over the south of the Yangtze River region, with the LST reproducing about 57% and 64% of observed above-normal rainfall anomaly over the south of the Yangtze River region and southeastern China, respectively. Further analyses reveal a possible effect of springtime TP's LST on summer southern and eastern Asian rainfall and identify some hot spots, suggesting that the TP's spring LST effect is not only limited to the Yangtze River region, but to a much larger scale. The imposed warm LST/SUBT over the TP triggers a strong wave activities propagating eastward along the upper-level westerly jet, associated with an increase of the atmospheric baroclinic instability as well as a strengthening and southeastward movement of the South Asian high, leading to intensified moisture convergence and convective instability favorable to the excessive rainfall in the downstream region of East Asia. The results of the 1998 case have also been compared with the results from year of 2003, which had a very cold spring LST anomaly over the TP and a severe downstream June 2003 drought (flood) in southern (northern) of the Yangtze River Basin area. Simulation results provide further evidence of the great importance of the TP spring land surface temperature anomaly in regulating summer extreme hydroclimatic events (e.g. droughts and floods) in South and East Asia. The present study suggests that consideration of LST/SUBT anomalies has a strong potential for more skillful S2S prediction of extreme hydroclimatic events such as floods, droughts and heatwaves over both Southern and Eastern Asia. [ABSTRACT FROM AUTHOR]

Spain is one of the European countries with most environmental problems related to water scarcity and droughts. Additionally, several studies suggest trends of increasing temperature and decreasing rainfall, mainly for the Iberian Peninsula, due to climate variability and change. While Regional Climate Models (RCM) are a valuable tool for understanding climate processes, the causes and plausible impacts on variables and meteorological extremes present a wide range of associated uncertainties. The multi-model ensemble approach allows the quantification and reduction of uncertainties in the predictions. The combination of models (RCM in this case), generally increases the reliability of the predictions, although there are different weighting methodologies. In this paper, a strategy is presented for the building of non-stationary PDF (probability density functions) ensembles with the aim of evaluating the spatial pattern of future risk of drought for an area. At the same time, the uncertainty associated with the metric used in the construction of the PDF ensembles is assessed. A comparative study of methodologies based on the application of the Reliability Ensemble Averaging (REA), assessing its factors using two performance measures, on the one hand the Perkins Score Methods, on the other hand the Kolmogorov-Smirnov test, is proposed. The evaluation of the sensitivity of the methodologies used in the construction of ensembles, as proposed in this paper, although without completely eliminating uncertainty, allows a better understanding of the sources and magnitude of the uncertainties involved. Despite the differences between the spatial distribution results from each metric (which can be in the order of 40 % in some areas), both approaches concluded about a plausible significant and widespread increase throughout continental Spain of the mean value of annual maximum dry spell lengths (AMDSL) between the years 1990 and 2050. Finally, the more parsimonious approach in the building of ensembles PDF, based on AMDSL in peninsular Spain, is identified. [ABSTRACT FROM AUTHOR]

Daily weather generators are used in many applications and risk analyses. The present paper explores the potential of neural network architectures to design daily weather generator models. Focusing this first paper on precipitation, we design a collection of neural networks (multi-layer perceptrons in the present case), which are trained so as to approximate the empirical cumulative distribution (CDF) function for the occurrence of wet and dry spells and for the precipitation amounts. This approach contributes to correct some of the biases of the usual two-step weather generator models. As compared to a rainfall occurrence Markov model, NNGEN-P represents fairly well the mean and standard deviation of the number of wet days per month, and it significantly improves the simulation of the longest dry and wet periods. Then, we compared NNGEN-P to three parametric distribution functions usually applied to fit rainfall cumulative distribution functions (Gamma, Weibull and double-exponential). A data set of 19 Argentine stations was used. Also, data corresponding to stations in the United States, in Europe and in the Tropics were included to confirm the results. One of the advantages of NNGEN-P is that it is non-parametric. Unlike other parametric function, which adapt to certain types of climate regimes, NNGEN-P is fully adaptive to the observed cumulative distribution functions, which, on some occasions, may present complex shapes. On-going works will soon produce an extended version of NNGEN to temperature and radiation. [ABSTRACT FROM AUTHOR]

Twenty years of IMERG precipitation estimates are used to evaluate the contributions of mesoscale convective system (MCS) rainfall to total rainfall in the Congo Basin. Studying these systems advances our basic understanding of Congo Basin rainfall on all time scales. The seasonality of MCS rainfall in the Congo Basin follows the seasonality of total rainfall with high rainfall in spring, summer, and fall and a winter dry season in each hemisphere. In the equinoctial seasons, MCS rainfall accounts for ≥ 80% of total rainfall within 5° of the equator with the highest rainfall rates occurring along the eastern and western boundaries of the basin. In boreal summer, MCS rainfall maxima occur near the Cameroon Highlands (9°–18°E) and in boreal winter, they occur along the eastern orography (22°–28°E). The 80% percent contribution is sustained in the continental interior (15°–25°E, 5°S–5°N) throughout the year. The diurnal cycle of MCS rainfall is similar to that of total rainfall. Diurnal cycles are unimodal in the equinoctial seasons but are regionally and seasonally inhomogeneous in the solstitial seasons. Regardless of modality, MCS rainfall is highest at 15Z (1600/1700 LT) and lowest at 10Z. MCS percent contribution changes little throughout the diurnal cycle but is highest (≥ 90%) at 04Z close to the continental interior. Larger MCSs contribute their greatest percentage of MCS rainfall (83–92%) between 04Z and 07Z, while more-intensely precipitating MCSs have no seasonally or regionally consistent diurnal cycle. Seasonal and diurnal MCS rainfall maxima are associated with unstable MSE profiles in the lower troposphere. Changes in moisture drive the seasonal cycle of MSE while changes in temperature drive its diurnal cycle. [ABSTRACT FROM AUTHOR]

This study examines the interlinkages between the South Atlantic Ocean Dipole (SAOD) and Central Africa (CA) rainfall variability from June to August, spanning from 1981 to 2018, using observational and reanalysis datasets. The results show that during positive SAOD events termed pSAOD, positive (negative) rainfall anomalies feature the southwestern CA termed zone I (the rest of CA landmass, termed zone II) more pronounced in reanalysis data. The reverse rainfall anomalies' pattern characterises negative SAOD events termed nSAOD. The relationship between CA rainfall variability and SAOD strengthens when the signal of El Niño-Southern Oscillation (ENSO) is removed. However, the impact of ENSO on the SAOD is contrasted between the northern and southern CA, so that ENSO compensates for the reduced rainfall associated with the SAOD north of 10°N. South of 10°N, ENSO attenuates wetness over the southwestern zone, and reinforces the dryness over the southeastern zone. During the pSAOD events, increased precipitation is associated with increased moisture convergence over zone I, with advection originating from neighbouring oceanic regions. At the same time, convergent moisture towards zone II is weak and recirculates towards southern Africa and does not contribute to moistening the region. However, during nSAOD, zone I instead experiences strong moisture divergence, whereas zone II is a sink of moisture convergence originating from zone I and from Sahelian and west African regions. In general, the atmospheric circulation seems better developed throughout the tropospheric column during nSAOD events than during pSAOD events and would suggest strong modulating impacts of regional-and local-scale processes in the rainfall variability. [ABSTRACT FROM AUTHOR]

This study evaluates the reliability of merged satellite precipitation products based on single machine learning (SML) and double machine learning (DML) algorithms to estimate meteorological drought events over Kenya from 2000 to 2019. This study selected four SML algorithms, including Random Forest (RF), Support Vector Machine (SVM), K-nearest neighbors (KNN), and the gradient boosting machine (GBM) algorithm, to merge satellite precipitation products. In contrast, the DML algorithm is developed based on the classification RF approach and combined with these regression machine learning models (i.e., RF-RF, RF-SVM, RF-KNN, and RF-GBM). The four gridded precipitation products, including Climate Hazards Group Infra-Red Precipitation with Station (CHIRPS), Integrated Multi-Satellite Retrievals for GPM (IMERG-v06), Precipitation Estimation from Remotely Sensed Information using Artificial Neural Network-Climate Data Record (PERSIANN-CDR) and ERA-5 reanalysis datasets utilized in merging approaches. In total, we compared twelve precipitation products, including four gridded precipitation products (GPPs), four SML, and four DML, against Climate Research Unit (CRU) based rain-gauge (RG) observation. The Standardized precipitation indices (SPI) were estimated from these twelve precipitation datasets and compared their performance of CRU observation across Kenya. In addition, a total of seven statistical metrics were utilized for the performance assessment of all precipitation estimates. The results revealed that the DML merged products achieved higher accuracy (CC = 0.75–0.9) compared to SML products or other GPPs products against observational data. Also, DML merged products show higher performance (CC = 0.8–0.86) for two rainy seasons (long rain and short rain) than other GPPs, and SML merged products, except RF. Moreover, our findings indicate that the DML merged precipitation products identified major meteorological drought events with the lowest RMSE error than other precipitation products based on multiple SPI timescales (SPI-3, SPI-6, and SPI-12). The DML algorithm significantly improves SML models for precipitation estimates to monitor meteorological drought events over Kenya. [ABSTRACT FROM AUTHOR]

Sub-daily rainfall extremes are of significant societal interest, with implications for flash flooding and the design of urban stormwater systems. It is increasingly recognised that extreme subdaily rainfall will intensify as a result of global temperature increases, with regional climate models (RCMs) representing one of the principal lines of evidence on the likely magnitude and spatiotemporal characteristics of these changes. To evaluate the ability of RCMs to simulate subdaily extremes, it is common to compare the simulated statistical characteristics of the extreme rainfall events with those from observational records. While such analyses are important, they provide insufficient insight into whether the RCM reproduces the correct underlying physical processes; in other words, whether the model 'gets the right answers for the right reasons'. This paper develops a range of metrics to assess the performance of RCMs in capturing the physical mechanisms that produce extreme rainfall. These metrics include the diurnal and seasonal cycles, relationship between rainfall intensity and temperature, temporal scaling, and the spatial structure of extreme rainfall events. We evaluate a high resolution RCM-the Weather Research Forecasting model-over the Greater Sydney region, using three alternative parametrization schemes. The model shows consistency with the observations for most of the proposed metrics. Where differences exist, these are dependent on both the rainfall duration and model parameterization strategy. The use of physically meaningful performance metrics not only enhances the confidence in model simulations, but also provides better diagnostic power to assist with future model improvement. [ABSTRACT FROM AUTHOR]

Interactions between midlatitude disturbances and the monsoonal circulation are significant for the Indian summer monsoon (ISM) rainfall. This paper presents examples of monsoon-midlatitude linkage through anticyclonic Rossby wave breaking (RWB) over West Asia during June, July and August of the years 1998-2010. RWB events over West Asia are identified by the inversion of the potential vorticity air mass at three different isentropic levels (340, 350, and 360 K) using daily NCEP-NCAR reanalysis. It is observed that RWB took place over West Asia before/during breaks in the ISM. Further, these events occur on the anticyclonic shear side of the subtropical jet, where the gradient of the zonal wind is found to be high. RWB is responsible for the southward movement of high potential vorticity air from the westerly jet, leading to the formation of a blocking high over the Arabian region. In turn, this blocking high advects and causes the descent of upper tropospheric cold and dry air towards Central India. Such an air mass with low moist static energy inhibits deep monsoonal convection and thereby leads to a dry spell. In fact, we find that RWB induced blocking over West Asia to be one of the major causes of dry spell/break episodes in ISM. Additionally, the presence of cold air over Central India reduces the north-south thermal contrast over the monsoon region thereby modifying the local Hadley circulation over the region. [ABSTRACT FROM AUTHOR]

An accurate seasonal prediction of Indian summer monsoon rainfall (ISMR) is intriguing as well as the most challenging job for monsoon meteorologists. As there is a cause and effect relationship between clouds and precipitation, the modulation of cloud formation in a dynamical model affects profoundly on ISMR. It has already been established that the critical relative humidity (CRH) plays a crucial role on the realistic cloud formation in a general circulation model. Hence, it may be hypothesized that the proper choice of CRH can be instrumental in driving the large scale Indian monsoon by modulating the cloud formation in a global climate model. An endeavor has been made for the first time to test the above hypothesis on the NCEP-CFSv2 model in the perspective of seasonal prediction of ISMR by modifying the CRH profile. The model sensitivity experiments have been carried out for two different CRH profiles along with the existing profile during the normal (2003) and deficient (2009) monsoon years. First profile is the constant CRH following the existing one but with increased magnitude and the second one is the variable CRH at different cloud levels based on the observations and MERRA reanalysis. The ensemble mean of model runs for four initial conditions of each year has revealed that the variable CRH profile in CFSv2 represents seasonal ISMR and its variability best among the three CRH experiments linking with the thermodynamical and dynamical parameters like precipitable water, tropospheric temperature and its gradient, cloud structure and radiation, water vapour flux, systematic error energy with its nonlinear error growth and the length of the rainy seasons during the contrasting years. It has also been shown that the improved depiction of seasonal ISMR has been achieved without disturbing much the forecast biases at other global tropical regions. The indigenous part of this paper is that the CRH modification can play a seminal role in modulating the large scale system like Indian monsoon by representing the realistic variability of cloud formation in CFSv2 and that proves the hypothesis. This work creates an avenue for further development of CFSv2 approaching towards an accurate seasonal forecast of ISMR. [ABSTRACT FROM AUTHOR]

The eastern United States is vulnerable to flooding from tropical cyclone rainfall. Understanding how both the frequency and intensity of this rainfall will change in the future climate is a major challenge. One promising approach is the dynamical downscaling of relatively coarse general circulation model results using higher-resolution regional climate models (RCMs). In this paper, we examine the frequency of landfalling tropical cyclones and associated rainfall properties over the eastern United States using Zetac, an 18-km resolution RCM designed for modeling Atlantic tropical cyclone activity. Simulations of 1980-2006 tropical cyclone frequency and rainfall intensity for the months of August-October are compared against results from previous studies and observation-based datasets. The 1980-2006 control simulations are then compared against results from three future climate scenarios: CMIP3/A1B (late twenty-first century) and CMIP5/RCP4.5 (early and late twenty-first century). In CMIP5 early and late twenty-first century projections, the frequency of occurrence of post-landfall tropical cyclones shows little net change over much of the eastern U.S. despite a decrease in frequency over the ocean. This reflects a greater landfalling fraction in CMIP5 projections, which is not seen in CMIP3-based projections. Average tropical cyclone rain rates over land within 500 km of the storm center increase by 8-17 % in the future climate projections relative to control. This is at least as much as expected from the Clausius-Clapeyron relation, which links a warmer atmosphere to greater atmospheric water vapor content. Over land, the percent enhancement of area-averaged rain rates from a given tropical cyclone in the warmer climate is greater for larger averaging radius (300-500 km) than near the storm, particularly for the CMIP3 projections. Although this study does not focus on attribution, the findings are broadly consistent with historical tropical cyclone rainfall changes documented in a recent observational study. The results may have important implications for future flood risks from tropical cyclones. [ABSTRACT FROM AUTHOR]

In this paper, the east-west tropical 'Walker circulation' and its linear association with sea surface temperature (SST) of the Nino 3 region and Indian summer monsoon rainfall (ISMR) have been investigated. 'Walker circulation'/'reverse Walker circulation' is primarily forced by SSTs of the equatorial Pacific Ocean. In this study, velocity potential field of 0.21 sigma level over the tropics was considered as proxy of the zonal tropical circulation ('Walker circulation'/'reverse Walker circulation'). Principal component analysis of the monsoon season tropical velocity potential data of 0.21 sigma level for the two periods 1951-1980 and 1981-2010, was done separately. We find that earlier, two different patterns of the velocity potential field, forced by probably two distinct modes of El Nino episodes, were associated with the ISMR. These two El Nino episodes, respectively, correspond to the strong El Nino events, where in warming was extended up to the date line (primarily zonal) and the moderate El Nino events in which, warming having north south extension, was limited to the eastern Pacific Ocean only. However, in recent years, only the first pattern of the velocity potential field, induced by the strong El Nino events (warming extending up to the date line), was correlated with the ISMR. Further, in the later period (1981-2010), velocity potential field at 0.21 sigma level over the tropical Pacific and Indian Oceans, which appeared to be primarily driven by SST anomalies of the equatorial Pacific Ocean in the first period, was found to be significantly correlated with the extra tropical circulation anomalies also. Therefore, modulation of the ISMR through velocity potential field over the tropical Indian and Pacific Oceans, in the later period, may have additional significant impact of the extra-tropical circulation anomalies. This might have led weak correlation between the ISMR and SSTs of the Nino 3 region, which is actually being observed in recent years. [ABSTRACT FROM AUTHOR]

This paper investigates the reversibility of CO induced climate change and in particular the potential impacts of different rates of CO reduction using a coupled climate model. Atmospheric CO concentration is ramped up by 0.5 %/year from the preindustrial value to 4×CO and then ramped down from 2×CO to 4×CO with different rates. How the response of the climate system is affected by the peak atmospheric CO concentration and the rate of long term decline is vital information for those considering hypothetical geoengineering options to remove CO. Major components of the climate system including global mean surface air temperature and precipitation, contribution of thermal expansion to global sea level rise, loss of the Arctic sea ice, weakening of the Atlantic meridional overturning circulation (AMOC) and the South Asia monsoon are analyzed. We have found no 'tipping points' or thresholds beyond which CO induced climate change in these components become irreversible within this model under the specific scenarios. However, there are strong inertias and path-dependent hysteresis in the climate system linked through oceanic memory. Initially the strengthened global hydrological cycle accelerates further in response to a CO ramp-down before weakening. Thermal expansion of the oceans continues for many decades after CO concentration starts to decrease. A 0.5 %/year reduction from 4×CO could see a further 25 % sea level rise. The weakening of the AMOC is reversible, but the build-up of highly saline subtropical waters during global warming drives an overshoot of the AMOC after the CO ramp-down and extends the warming of the northern high latitudes by many decades. The South Asia monsoon strengthens in response to a CO ramp-up marked by an increase in summer monsoon rainfall. This increase reverses rapidly following a CO ramp-down, displaying an undershoot in monsoon rainfall for rapid CO reductions. [ABSTRACT FROM AUTHOR]

In this paper we analyze the multi-week prediction bias and skill from the National Centers for Environment Prediction (NCEP) Climate Forecast System version 2 (CFSv2) based on its hindcasts for 1999-2012. The analyses focus on the prediction of the rainfall variability over South-East Asia during boreal warm seasons and the dependence of the prediction on the activity of intrasesaonal leading modes. It is shown that the prediction skill measured by anomaly correlation is comparable between the total anomalies and intraseasonal anomalies during the first 2 weeks. After week 2, the prediction skill drops substantially and the skill for total anomalies is largely from the prediction for the interannual variability. Moreover, the forecast skill tends to be higher when the amplitude of the Madden-Julian Oscillation and the Boreal Summer Intraseasonal Oscillation (BSISO) is larger, especially for the BSISO. It is noted that the prediction skill over South-East Asia depends on the phase of the BSISO. One deficiency in the CFSv2 is that the northward propagation of the forecast BSISO is generally slower than the observed. [ABSTRACT FROM AUTHOR]

A systematic analysis of the main weather types influencing southern Australian rainfall is presented for the period 1979–2015. This incorporates two multi-method datasets of cold fronts and low pressure systems, which indicate the more robust fronts and lows as distinguished from the weaker and less impactful events that are often indicated only by a single method. The front and low pressure system datasets are then combined with a dataset of environmental conditions associated with thunderstorms, as well as datasets of warm fronts and high pressure systems. The results demonstrate that these weather types collectively account for about 86% of days and more than 98% of rainfall in Australia south of 25° S. We also show how the key rain-bearing weather systems vary throughout the year and for different regions, with the co-occurrence of simultaneous lows, fronts and thunderstorm conditions particularly important during the spring and summer months in southeast Australia. [ABSTRACT FROM AUTHOR]

A hidden Markov model (HMM) with four hidden states is used to describe the daily precipitation occurrence and wet day intensities of a gridded precipitation dataset over the Northeast Brazil (NEB) for 35 austral summer seasons (1980/81–2014/15). Results show that States 1 and 3 correspond to the wet versus dry conditions associated with the interplay between the Intertropical Convergence Zone (ITCZ) and South American Monsoon System (SAMS). States 2 and 4 feature a southwest-northwest seesaw of precipitation occurrence, associated with antisymmetric conditions between the ITCZ and South Atlantic Convergence Zone (SACZ). The Madden Julian Oscillation (MJO) seems to affect the development of the SACZ in States 3 and 4. In State 4, the MJO influenced the convection in the northwest NEB during phases 8-1-2. During easterly Quasi-Biennial Oscillation (EQBO), the convective activity in the SACZ regions is intensified in States 3 and 4 during MJO phase 1, and weakened in State 4 during phases 1, 3, and 4. In the interannual timescales, States 1 and 3 are preconditioned by a positive and a negative South Atlantic Ocean Dipole (SAOD), respectively. State 2 is modulated by a long and positive El Niño-Southern Oscillation (ENSO) that triggered extratropical teleconnection patterns in both hemispheres. State 4 is preconditioned by a negative ENSO that produces changes in the Walker and Hadley circulations. These findings enhance the understanding of the dynamic processes of summer rainfall over the NEB and their association with potentially predictable intraseasonal and interannual teleconnections on seasonal timescales. [ABSTRACT FROM AUTHOR]

This study focuses on regional extreme precipitation (REP) in North China. We found a trend turning in summer (July to August) REP frequencies and intensities from a decrease trend in 1961–2002 to an increase trend in 2003–2020, accompanied by a southward shift of the extreme rain belt, and an enhanced connection with the Ural blocking (UB) and the Western Pacific Subtropical High Pressure (WPSH) in 2003–2020. Rains in North China are accompanied by a west–east low–high dipole at upper troposphere. During the REP, the high of the low–high dipole at Northeast China (the NEH) is strongly amplified from a pre-existing stationary ridge over northeast Asia under the influence of eastward propagating Rossby wave energies along the subpolar/subtropical wave guide over Eurasia. For the REP years, an enhanced stationary ridge over the Ural Mountains in the period 2003–2020 replaces the stationary Ural trough in the period 1961–2002, favouring the development of the UB and leading to a change of the Rossby wave propagation path from along the subtropical waveguide in 1961–2002 to along the polar wave guide in 2003–2020. Therefore, a connection between the NEH and the UB forms, which may lead to a higher probability of extreme precipitation in North China since blocking is a major source of strong circulation anomalies. The mean summer WPSH expands more westward-northward in 2003–2020 than in 1961–2002, which provides a background conditions for a further westward-northward expanding of the daily WPSH under the influence of the NEH leading to a strong moisture transport from north Pacific. As a result, the intensity and probability of extreme precipitation over southern North China increase. [ABSTRACT FROM AUTHOR]

Moisture flux and precipitable water content (PWC) analysis are carried out over Himalayan state of Uttarakhand, India during the Indian summer monsoon season for the period 1959–2021 (1959–2001: P1; 2002–2021: P2) to understand their distribution pattern. The intense moisture convergence is seen over the hilly region in the recent past (P2) through the rotational and divergent components of moisture flux analysis. The analysis indicates an increasing trend of PWC with values 1.2, 1.9, 2.0 and 2.5 mm/decade during June to September months respectively. The spatial distribution of rainfall shows a stronger magnitude in the eastern part and relatively weaker magnitude in the western part, during June to August since 2002. Convective available potential energy (CAPE) is increased in the low altitude region in the summer months except July and shows maximum during September (~ 100 J/kg) since 2002. However, the study of vertically integrated moisture flux divergence (VIMFD) shows stronger (weaker) convergence in the windward side (leeward side) in Uttarakhand. These climatological distributions of PWC, CAPE, rainfall, and VIMFD analysis implicate a signature of cloud burst/heavy precipitation region in the recent past in Uttarakhand. [ABSTRACT FROM AUTHOR]

The first rainy season (FRS) in South China from late-spring to early-summer represents the beginning of the summer monsoon rainy season over eastern China. This study investigates the interannual relationship between the South Pacific meridional sea surface temperature (SST) dipole (SPMD) and the concurrent FRS precipitation anomalies over South China during April–June (AMJ) without El Niño/Southern Oscillation (ENSO) impact. During AMJ, after removing the concurrent ENSO signal through the linear regression, the first empirical orthogonal function (EOF) mode of SST anomalies in the South Pacific exhibits a meridional dipole with warming in the tropical southeastern Pacific and cooling in mid-latitudes of the South Pacific. The first EOF mode is referred to as the SPMD pattern. The off-equatorial enhanced diabatic heating associated with the northeastern pole of the SPMD triggers low-level cross-equatorial northerlies in the tropical north central Pacific (TNCP) through the Matsuno–Gill type response. The anomalous northerly and its associated TNCP SST cooling lead to a Northwest Pacific anomalous anticyclone (NWP-AAC) via the wind-evaporation-SST feedback. The atmospheric general circulation model sensitivity experiments further suggest that the SPMD can induce the NWP-AAC directly or collaboratively with the TNCP SST cooling. Additionally, anomalous moisture can be transported from the tropical western Pacific into South China, giving rise to enhancement of the FRS precipitation. The Liang–Kleeman information flow for causality analysis and ensemble mean of 30-member atmospheric model simulation results further suggest a robust physical pathway that the SPMD is related to the FRS precipitation over South China when ENSO absents. Our results may deepen the understanding of interannual rainfall variability in South China in non-ENSO years. [ABSTRACT FROM AUTHOR]

Interannual variability of the Indian summer monsoon rainfall (ISMR) is explained through the total column water vapor in the atmosphere, primarily controlled by the incoming zonal moisture flux over the Arabian Sea ( F W ) and outgoing flux over the Bay of Bengal ( F E ). In this study, we discern the underlying mechanisms driving F W and F E leading to ISMR droughts in the observations and seasonal hindcasts by the Climate Forecast System version 2 (CFSv2) model. In observations, a reduction in F W is essential for droughts to occur. In addition, an increase in F E results in a severe drought. On the contrary, droughts in CFSv2 primarily occur due to an enhancement in F E , seldom accompanied by a decrease in F W . This hypersensitivity of the CFSv2 ISMR to F E is further explained using Matsuno–Gill response to moist convection. During El Nino droughts, precipitation decreases over the equatorial western Pacific and eastern Indian Oceans. The resulting anomalous diabatic cooling increases local surface pressure ( SP ), intensifying meridional SP gradient, and thus, F E . The reduction in F W , however, is associated with a cooling of the central north Pacific Ocean in tandem with El Nino. During non-El Nino droughts, frequent occurrences of cold sea surface temperature over the western north Pacific Ocean are noticed. This cooling decreases SP over east Asia, resulting in an increase in F E . To summarize, droughts in CFSv2 are controlled by the pan-Pacific climate, significantly increasing F E but weakly decreasing F W . But in observations, a strong decrease in F W and a moderate increase in F E together lead to droughts. [ABSTRACT FROM AUTHOR]

Phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6) both grossly underestimate the magnitude of low-frequency Sahel rainfall variability; but unlike CMIP5, CMIP6 mean historical precipitation does not even correlate with observed multi-decadal variability. We demarcate realms of simulated physical processes that may induce differences between these ensembles and prevent both from explaining observations. We partition all influences on simulated Sahelian precipitation variability into (1) teleconnections from sea surface temperature (SST); (2) atmospheric and (3) oceanic variability internal to the climate system; (4) the SST response to external radiative forcing; and (5) the "fast" (not mediated by SST) precipitation response to radiative forcing. In a vast improvement from previous ensembles, the mean spectral power of Sahel rainfall in CMIP6 atmosphere-only simulations is consistent with observed low-frequency variance. Low-frequency variability is dominated by teleconnections from observed global SST, and the fast response only hurts the performance of simulated precipitation. We estimate that the strength of simulated teleconnections is consistent with observations using the previously-established North Atlantic Relative Index (NARI) to approximate the role of global SST, and apply this relationship to the coupled ensembles to infer that both fail to explain low-frequency historical Sahel rainfall variability mostly because they cannot explain the observed combination of forced and internal variability in North Atlantic SST. Yet differences between CMIP5 and CMIP6 in mean Sahel precipitation and its correlation with observations do not derive from differences in NARI, but from the fast response or the role of other SST patterns. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau vortex (TPV) is a key system triggering rainfall over the Tibetan Plateau (TP) during the boreal summer. The TPV genesis mechanisms are complicated and its classification is a great challenge. This study attempts to elucidate these aspects. By introducing the standardized index of 24-h increment of equivalent potential temperature Δ 24 h θ e at 500 hPa, all the TPV cases generated in June between 1980 and 2016 are classified as either positive or negative. Composite analysis is subsequently applied to the extremes of these two types, i.e., the first and last fifth-percentile cases with extremely negative and positive standardized Δ 24 h θ e , respectively. Results indicate that 70% of them occur in relatively warmer and wetter environments, with diabatic heating dominating the positive type and the dynamic effect of large-scale circulation dominating the negative type. For the extremely positive cluster, the geopotential-height increment over the TP exhibits a negative/west–positive/east dipole, which enhances the southerly flow over the western TP, while forming surface water–vapor convergence. Consequently, strong condensation heating occurs near the sub-cloud level, resulting in the development of potential vorticity below and eventually TPV genesis. For the negative cluster, local shear lines at 500 hPa and upstream troughs at 250 hPa occur at the TPV genesis location. In conjunction with anomalous westerlies, positive potential vorticity is generated in situ due to zonal advection. The retardation caused by the Kunlun Mountains on the impinging westerly flow associated with side-boundary friction also contributes to TPV genesis southeast of the mountains. [ABSTRACT FROM AUTHOR]

In this study, based on the ECMWF 20-year (1997 ~ 2016) hindcasts, the subseasonal prediction of the weekly summer rainfall anomaly over the middle and lower reaches of the Yangtze River (YR) were studied. The skill at 2-week lead time exhibits prominent interannual variation, which is significantly correlated with the preceding winter ENSO. That is, rainfall anomaly can be better predicted in El Niño decaying summer than in La Niña decaying summer. Observation analyses show that in El Niño decaying summer the intraseasonal variation of YR summer rainfall is featured by strong low-frequency (> 30 days) intraseasonal oscillation (ISO) partly associated with the first boreal summer ISO mode (BSISO1) activity, in comparison with La Niña decaying summer. This is possibly because El Niño-induced mean state western North Pacific (WNP) anti-cyclone blocks the northward propagation of convection over the WNP, resulting in BSISO1 stagnation in phases 3–4. The phase stagnation could force stable atmospheric teleconnection, which is favorable to sufficient moisture transportation to the YR and persistent rainfall formation. Finally, prediction verification showed that more accurate prediction for the middle-low-level circulation contribute to the better prediction of rainfall anomaly in El Niño decaying summer than in La Niña decaying summer. [ABSTRACT FROM AUTHOR]

Most state-of-the-art general circulation models cannot well simulate the diurnal cycle of precipitation, especially the nocturnal rainfall peak over land. This study assesses the diurnal cycle of precipitation simulated using the Global-to-Regional Integrated forecast SysTem (GRIST) in its numerical weather prediction (NWP) configuration at resolutions typical of current global climate models. In the refinement region, the variable-resolution model well distinguishes the distinct features of diurnal cycle. No apparent artificial features are observed in the transition zone of the variable-resolution mesh. The model also exhibits a similar diurnal cycle pattern to the observation in the coarse-resolution region. We further investigate the model behaviors of dynamics–physics interaction by analyzing hourly dynamical and thermodynamical diagnostics. Composite analysis based on rainfall peak time is applied to examine the model capability in distinguishing different precipitation processes of daytime and nighttime peaks. Over East Asia, the model well reproduces both the nocturnal-to-early-morning and the afternoon rainfall peaks. The model simulates the dominant contribution of large-scale upward moisture advection to the formation of stratiform-like rainfall peaks, and produces daytime surface-heating induced rainfall. Refinement of the resolution substantially increases the composited nighttime precipitation intensity but has little impact on the composite percentage. The model captures the realistic dynamical and thermodynamical conditions for the occurrence of nocturnal rainfall. These results demonstrate that the variable-resolution model is able to reproduce the diurnal cycle of climatological summer rainfall through realistic precipitation processes. [ABSTRACT FROM AUTHOR]