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

This paper constructs annual homogenised precipitation series for 13 locations in Scotland (1861–1991) from non-standard sources. Whilst the British Rainfall series represents the backbone of most UK rainfall records before the middle of the twentieth century, this paper returns to the original records to examine spatial and temporal variations in Scottish precipitation. The records were subjected to trend analysis and principal components analysis (PCA), and then related to atmospheric circulation parameters to understand the synoptic- scale mechanisms involved. Upward (downward) trends in precipitation were noted in western (eastern) Scotland over the period 1870–1991. Precipitation variability in western (eastern) Scotland is best explained by fluctuations in the westerly circulation (pressure), with the west- east precipitation gradient closing in those years with below average pressure, and a greater frequency of northerly and/or easterly winds. Precipitation trends cannot be explained by changes in atmospheric circulation patterns, suggesting that other factors must be involved. [ABSTRACT FROM AUTHOR]

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

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

There is an urgent need to understand and predict how extreme precipitation events (EPEs) will change at high latitudes, both for local climate change adaptation plans and risk mitigation and as a potential proxy "anticipating" the impact of climate change elsewhere in the world. This paper illustrates that a combination of non‐stationary modelling approaches can be adopted to evaluate trends in EPEs under uncertainty. A large database of daily rainfall events from 281 sparsely distributed weather stations in Finland between 1961 and 2016 was analysed. Among the tested methods, Poisson distributions provided a powerful method to evaluate the impacts of multiple physical covariates, including temperature and atmospheric circulation patterns (ACPs), on the resulting trends. The analysis demonstrates that non‐stationarity is statistically valid for the majority of observations, independently of their location in the country and the season of the year. However, subsampling can severely hinder the statistical validity of the trends, which can be easily confused with random noise and therefore complicate the decision‐making processes regarding long‐term planning. Scaling effects have a strong impact on the estimates of non‐stationary parameters, as homogenizing the data in space and time reduces the statistical validity of the trends. Trends in EPE statistics (mean, 90 and 99% percentiles) and best‐fitted Generalized Pareto parameters in the tails of the distributions appear to be stronger when approaching the Polar region (Lapland) than away from it, consistent with the Arctic amplification of climate change. ACPs are key covariates in physically explaining these trends. In particular, the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) can explain statistically significant increases in extreme precipitation in Lapland, Bothnian and South regions of Finland, particularly during summer and fall seasons. Non‐stationarity is statistically valid in the majority of observations but masked by subsampling and spatio‐temporal heterogeneity. Trends on extremes and non‐stationary generalized Pareto parameters stronger when approaching the Polar region (Lapland). Artic Oscillation (AO) and North Atlantic Oscillation (NAO) can explain statistically significant trends. [ABSTRACT FROM AUTHOR]

Precipitation changes in a warming climate have been examined with a focus on either mean precipitation or precipitation extremes, but changes in the full probability distribution of precipitation have not been well studied. This paper develops a methodology for the quantile-conditional column moisture budget of the atmosphere for the full probability distribution of precipitation. Analysis is performed on idealized aquaplanet model simulations under 3-K uniform SST warming across different horizontal resolutions. Because the covariance of specific humidity and horizontal mass convergence is much reduced when conditioned onto a given precipitation percentile range, their conditional averages yield a clear separation between the moisture (thermodynamic) and circulation (dynamic) effects of vertical moisture transport on precipitation. The thermodynamic response to idealized climate warming can be understood as a generalized "wet get wetter" mechanism, in which the heaviest precipitation of the probability distribution is enhanced most from increased gross moisture stratification, at a rate controlled by the change in lower-tropospheric moisture rather than column moisture. The dynamic effect, in contrast, can be interpreted by shifts in large-scale atmospheric circulations such as the Hadley cell circulation or midlatitude storm tracks. Furthermore, horizontal moisture advection, albeit of secondary role, is important for regional precipitation change. Although similar mechanisms are at play for changes in both mean precipitation and precipitation extremes, the thermodynamic contributions of moisture transport to increases in high percentiles of precipitation tend to be more widespread across a wide range of latitudes than increases in the mean, especially in the subtropics. [ABSTRACT FROM AUTHOR]

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Summer precipitation anomalies over eastern China are characterized spatially by meridionally banded structures fluctuating on interannual and interdecadal timescales, leading to regional droughts and floods. In addition to long-term trends, how these patterns may change under global warming has important implications for agricultural planning and water resources over this densely populated area. Using the latest Hadley Centre climate model, HadGEM3-GC2, this paper investigates the potential response of summer precipitation patterns over this region, by comparing the leading modes between a 4×CO2 simulation and the model’s pre-industrial control simulation. Empirical Orthogonal Function (EOF) analyses show that the first two leading modes account for about 20% of summer rainfall variability. EOF1 is a monopole mode associated with the developing phase of ENSO events and EOF2 is a dipole mode associated with the decaying phase of ENSO. Under 4×CO2 forcing, the dipole mode with a south-north orientation becomes dominant because of a strengthened influence from excessive warming of the Indian Ocean. On interdecadal time scales, the first EOF looks very different from the control simulation, showing a dipole mode of east-west contrast with enhanced influence from high latitudes. [ABSTRACT FROM AUTHOR]

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

This paper analyses the main geographic patterns of rainfall variability over Central and Southern Scotland at the daily timescale during the autumn and extended winter (SONDJFM) period. Principal component analysis (PCA) is used to identify the most important rainfall patterns, with the first eight principal components (PC) accounting for 85.2% of the variance. The most important pattern (PC 1) is associated with a cyclonic W/SW flow and produces the wettest conditions in Renfrewshire and Dumbartonshire (PC 1); rainfall under the PC 1 pattern is widespread, with even the drier sites in eastern Scotland recording comparatively high totals. PC 2 produces wetter than average conditions in eastern Scotland (especially the Scottish Borders region) and so reduces the average rainfall gradient between the west (wetter) and the east (drier). All eight significant PCs are explained in terms of the atmospheric circulation and the geography of Scotland. Cluster analysis is then used to identify coherent rainfall regions. The regions on easterly days show the most difference from the other directions and the regions derived across all days. [ABSTRACT FROM AUTHOR]

A new approach to forecast the middle-lower reaches of the Yangtze River Valley summer rainfall in June-August (JJA) is proposed in this paper. The year-to-year increment of the middle-lower reaches of the Yangtze River Valley is forecasted and hence the summer precipitation could be predicted. In this paper, DY is defined as the difference of a variable between the current year and the preceding year (year-to-year increment). YR denotes the seasonal mean precipitation rate of the middle-lower reaches of the Yangtze River Valley summer rainfall. After analyzing the atmospheric circulation anomalies in winter and spring that were associated with the DY of YR, six key predictors for the DY of YR have been identified. Then the forecast model for the DY of YR is established by using the multi-linear regression method. The predictors for the DY of YR are Antarctic Oscillation, the meridional wind shear between 850hPa and 200hPa over the Indo-Australian region, and so on. The prediction model shows a high skill for the hindcast during 1997-2006, with the average relative root mean square error is at 18%. The model can even reproduce the upward and downward trends of YR during 1984-1998 and 1998-2006. Considering that the current operational forecast models of the summer precipitation over the China region have the average forecast scores at 60%-70% and that the prediction skill for the middle-lower reaches of Yangtze River Valley summer precipitation remains quite limited up to now, thus this new approach to predict the year-to-year increment of the summer precipitation over the Yangtze River Valley (and hence the summer precipitation itself) has the potential to significantly increase the operational forecast skill of the summer precipitation. [ABSTRACT FROM AUTHOR]

A 117 year series of hourly precipitation data from Kraków, South Poland and the Niedźwiedź classification of synoptic weather types were the basis of a detailed study of diurnal precipitation variation and atmospheric circulation types. The analysis of the relationship between precipitation and circulation types was restricted to the warm half of the year (May–October). During this period precipitation ispredominantly liquid. This paper discusses the role of theCarpathian orographic barrier in the development of the 24 hour precipitation pattern. The diurnal cycle of precipitation shows a stronger relationship with the direction of air mass advection than with pressure system type. Circulation types linked with cold and moist air mass advection cause a more uniform precipitation variation. Conversely, diurnal precipitation patterns in circulation types bringing continental Polar air are affected to a greater extent by thermal than dynamic factors. This influence is manifest by a greater amplitude of the diurnal cycle of precipitation frequency. [ABSTRACT FROM AUTHOR]

Global warming substantially intensifies hydrologic cycles, causing increasing frequency and magnitude of catastrophic floods and droughts. Understanding the patterns and mechanisms of precipitation in historical periods is pivotal for regional disaster prevention and mitigation. Here, we analyzed the daily precipitation of six stations at the Shimantan Reservoir from 1952 to 2013 to examine precipitation characteristics at different time scales. The Mann–Kendall test, moving t-test, and Innovative Polygon Trend Analysis (IPTA) were employed to detect the trends and change points in total precipitation amount, frequency, and duration. Influences of atmospheric circulations on precipitation were then explored via cross-wavelet analysis. Our results showed increased average precipitation and decreased precipitation days annually at the Shimantan Reservoir in the past decades. Specifically, increased seasonal precipitation was only detected in summer, while precipitation days were mainly reduced in winter. There was a noticeable increasing to decreasing transition trend in precipitation from July to August, and a transition from decreasing to increasing from June to July in precipitation days. Summer rainfall was predominantly moderate and light, accompanied by shortening and highly fluctuating rainstorm durations. July exhibited the highest precipitation frequency and always experienced rainstorms. The Arctic Oscillation and East Asian summer monsoon showed positive and negative correlations, respectively, with the changes in precipitation at the Shimantan Reservoir. Our analyses provide a fine-scale portrait of precipitation patterns and mechanisms under a changing climate and benefit regional flood control and sustainable development. [ABSTRACT FROM AUTHOR]

The physical mechanisms and predictability associated with extreme daily rainfall in southeastern South America (SESA) are investigated for the December-February season in a two-part study. Through a k-mean analysis, this first paper identifies a robust set of daily circulation regimes that are used to link the frequency of rainfall extreme events with large-scale potential predictors at subseasonal-to-seasonal scales. This represents a basic set of daily circulation regimes related to the continental and oceanic phases of the South Atlantic convergence zone (SACZ) and wave train patterns superimposed on the Southern Hemisphere polar jet. Some of these recurrent synoptic circulation types are conducive to extreme rainfall events in the region through synoptic control of different mesoscale physical features and, at the same time, are influenced by climate phenomena that could be used as sources of potential predictability. Extremely high rainfall (as measured by the 95th and 99th percentiles) is associated with two of these weather types (WTs), which are characterized by moisture advection intrusions from lower latitudes and the Pacific Ocean; another three WTs, characterized by above-normal moisture advection toward lower latitudes or the Andes, are associated with dry days (days with no rain). The analysis permits the identification of several subseasonal-to-seasonal scale potential predictors that modulate the occurrence of circulation regimes conducive to extreme rainfall events in SESA. It is conjectured that a cross-time scale interaction between the different climate drivers improves the predictive skill of extreme precipitation in the region. [ABSTRACT FROM AUTHOR]

This contribution provides the basics of the climatology of the Polish Tatra Mountains in a nutshell, with particular reference to intense precipitation and its relation to atmospheric circulation. Variability of various precipitation characteristics, including selected indices of intense precipitation in Zakopane and at Kasprowy Wierch, is illustrated in this paper. None of the trends in these characteristics and indices calculated for the entire time interval exhibit a statistical significance, but short-time fluctuations are evident. The occurrence of intense precipitation in the Tatra Mountains is strongly related to three circulation types. These situations (Nc, NEc, Bc) are associated with cyclones following track Vb after van Bebber. In addition to changing frequencies of circulation, this study also reveals an increase in the frequency of the circulation types associated with extreme precipitation. [ABSTRACT FROM AUTHOR]

This study examines spiral rainbands in a numerical simulation of Hurricane Bill (2009). This paper, the first part of the study, evaluates the structures of spiral rainbands and compares them to previous observations. Four types of spiral rainbands are identified: principal, secondary, distant, and inner rainbands. Principal rainbands tend to be stationary relative to the storm center, while secondary rainbands are more transient and move around the storm center. Both principal and secondary rainbands are tilted radially outward with height and have many of the commonly observed kinematic features, such as overturning secondary circulation and enhanced tangential velocity on their radially outward sides. Principal rainbands are bounded by very dry air on their radially outward sides. Distant rainbands are radially inward-tilting convective features that have dense cold pools near the surface. Inner rainbands are made of shallow convection that appears to have originated from near the eyewall. Differences in the structures of spiral rainbands between observations and the Hurricane Bill simulation are noted. The second part of the study investigates how inner rainbands propagate and makes comparison with previously proposed hypotheses such as vortex Rossby waves. [ABSTRACT FROM AUTHOR]

During the recent decades, summer precipitation has increased over the western and northeastern Tibetan Plateau (TP) and decreased over the southeastern TP. This study investigates the effects of anthropogenic forcing and internal variability on summer rainfall changes over the TP during 1979–2013. It is shown that the variations in atmospheric circulation and precipitation over the TP are largely contributed by the anthropogenic forcing and oceanic boundary conditions and that the Interdecadal Pacific Oscillation (IPO) plays an important role in TP summer rainfall changes in recent decades. The anthropogenic forcing contributes to the drying trends over the southeastern TP and the wetting trends over the western and northeastern TP. The negative sea surface temperature (SST) anomalies in the tropical central-eastern Pacific induce a wave train consisting of anomalous cyclones and anticyclones over Eurasia. The anomalous anticyclone to the northeast TP weakens westerlies, facilitating the convergence of water vapor over the TP. Further analysis indicates that the IPO-related SST anomalies contribute partly to the uncertainty in simulations of summer rainfall changes over the TP in recent decades using a 40-member ensemble of the Community Earth System Model. Consequently, the IPO can be used as one of the predictive factors of the near-term TP summer rainfall projection. [ABSTRACT FROM AUTHOR]

The response over West Africa to uniform warming of the Atlantic Ocean is analyzed using idealized simulations with a regional climate model. With warming of 1 and 1.5 K, rainfall rates increase by 30%-50% over most of West Africa. With Atlantic warming of 2 K and higher, coastal precipitation increases but Sahel rainfall decreases substantially. This nonlinear response in Sahel rainfall is the focus of this analysis. Atlantic warming is accompanied by decreases in low-level geopotential heights in the Gulf of Guinea and in the large-scale meridional geopotential height gradient. This leads to easterly wind anomalies in the central Sahel. With Atlantic warming below 2 K, these easterly anomalies support moisture transport from the Gulf of Guinea and precipitation increases. With Atlantic warming over 2 K, the easterly anomalies reverse the westerly flow over the Sahel. The resulting dry air advection into the Sahel reduces precipitation. Increased low-level moisture provides moist static energy to initiate convection with Atlantic warming at 1.5 K and below, while decreased moisture and stable thermal profiles suppress convection with additional warming. In all simulations, the southerly monsoon flow onto the Guinean coast is maintained and precipitation in that region increases. The relevance of these results to the global warming problem is limited by the focus on Atlantic warming alone. However, confident prediction of climate change requires an understanding of the physical processes of change, and this paper contributes to that goal. [ABSTRACT FROM AUTHOR]

This paper constitutes a step towards the understanding of some characteristics associated with high rainfall amounts and flooding on Madeira Island. The high precipitation events that occurred during the winter of 2009/2010 have been considered with three main goals: to analyze the main atmospheric characteristics associated with the events; to expand the understanding of the interaction between the island and the atmospheric circulations, mainly the effects of the island on the generation or intensification of orographic precipitation; and to evaluate the performance of high resolution numerical modeling in simulating and forecasting heavy precipitation events over the island. The MESO-NH model with a horizontal resolution of 1 km is used, as well as rain gauge data, synoptic charts and measurements of precipitable water obtained from the Atmospheric InfraRed Sounder (AIRS). The results confirm the influence of the orographic effects on precipitation over Madeira as well as the tropical-extratropical interaction, since atmospheric rivers were detected in six out of the seven cases analyzed, acting as a low level moisture supplier, which together with the orographic lifting induced the high rainfall amounts. Only in one of the cases the presence of a low pressure system was identified over the archipelago. [ABSTRACT FROM AUTHOR]

Postglacial expansion of deciduous oak woodlands of the Zagros-Anti-Taurus Mountains, a major biome of the Near East, was delayed until the middle Holocene at ~6300 cal. yr BP. The current hypotheses explain this delay as a consequence of a regional aridity during the early Holocene, slow migration rates of forest trees, and/or a long history of land use and agro-pastoralism in this region. In the present paper, support is given to a hypothesis that suggests different precipitation seasonalities during the early Holocene compared with the late Holocene. The oak species of the Zagros-Anti-Taurus Mts, particularly Quercus brantii Lindl., are strongly dependent on spring precipitation for regeneration and are sensitive to a long dry season. Detailed analysis of modern atmospheric circulation patterns in SW Asia during the late spring suggests that the Indian Summer Monsoon (ISM) intensification can modify the amount of late spring and/or early summer rainfall in western/northwestern Iran and eastern Anatolia, which could in turn have controlled the development of the Zagros-Anti-Taurus deciduous oak woodlands. During the early Holocene, the northwestward shift of the Inter-Tropical Convergence Zone (ITCZ) could have displaced the subtropical anticyclonic belt or associated high pressure ridges to the northwest. The latter could, in turn, have prevented the southeastward penetration of low pressure systems originating from the North Atlantic and Black Sea regions. Such atmospheric configuration could have reduced or eliminated the spring precipitation creating a typical Mediterranean continental climate characterized by winter-dominated precipitation. This scenario highlights the complexity of biome response to climate system interactions in transitional climatic and biogeographical regions. [ABSTRACT FROM AUTHOR]

This paper investigates the relationship between maximum precipitation in Krakow and types of atmospheric circulation in southern Poland, as classified by Niedźwiedź. Three characteristics were used to define this relationship: maximum precipitation (Pmax), its duration (d) and probability of exceedance (p). The input data came from Kraków's uniquely long and homogenous pluviographic record spanning the period 1886-2007. Hourly precipitation values for the maximum precipitation events were identified and arranged in 1-24 hour intervals. They were then processed using the generalized extreme value (GEV) distribution to produce quantiles of maximum precipitation totals in each of the intervals and broken down by the corresponding circulation type. Differences between the development mechanisms are manifested in relationships between precipitation characteristics and their measure of randomness, i.e. exceedance probability. This paper demonstrates that maximum precipitation events depend on their duration d and atmospheric circulation. The maximum short-duration (one-hour) events occur primarily in either of two circulation types: (i) cyclonic with advection from the east and from the southeast or (ii) low-pressure centre and cyclonic trough. Maximum long-duration precipitation events (24 hour), on the other hand, occur in the cyclonic type of circulation with advection from the north and from the northeast. [ABSTRACT FROM AUTHOR]

There is a widely held view that the Pampa region (PR) dry and wet periods are predominantly a consecuence of the El Niño-Southern oscillation (ENSO) phenomenom. The current paper focuses on non-ENSO rainfall anomalies for the period 1948–2000, the more recent of which have had catastrophic consequences throughout the region. We analyze horizontal water vapor transport, pressure and circulation anomalies occurring in Southern South America (SSA) during this type of event. Positive and negative (wet and dry) extreme events during the rainy and dry seasons in the region were registered. Based on NCEP reanalysis data it was established that under rainfall deficit, anomalies of similar intensity occurred simultaneously in the PR and in central Chile, whereas under excess rainfall the anomalies were mostly confined to the PR. The existence of a cyclone-anticyclone pair in the anomalous circulation pattern over mid latitudes of the Atlantic and Pacific oceans and straddling the southern portion of the continent maintains an intense and extense meridional circulation over the continental plains, which leads to the abnormal values in moisture transport and rainfall rate. The atmospheric water balance equation calculated for the PR indicates that anomalous water vapor is carried in from the continental equatorial region and from the subtropical Atlantic, its magnitude varying in accordance with the season and the sign of the anomaly. Furthermore, evidence of the important role of transient terms corroborates their contribution to the anomalous total moisture flux divergence under rainfall deficit during the dry season. The mean sea-level pressure anomaly fields of the extreme cases were further examined by principal component analysis to discern those circulation features directly linked to rainfall deviations. [ABSTRACT FROM AUTHOR]

Storm separation is a key step when carrying out storm transposition analysis for Probable Maximum Precipitation (PMP) estimation in mountainous areas. The World Meteorological Organization (WMO) has recommended the step-duration-orographic-intensification-factor (SDOIF) method since 2009 as an effective storm separation technique to identify the amounts of precipitation caused by topography from those caused by atmospheric dynamics. The orographic intensification factors (OIFs) are usually developed based on annual maximum rainfall series under such assumption that the mechanism of annual maximum rainfalls is close to that of the PMP-level rainfall. In this paper, an alternative storm separation technique using rainfall quantiles, instead of annual maximum rainfalls, with rare return periods estimated via Regional L-moments Analysis (RLMA) to calculate the OIFs is proposed. Based on Taiwan's historical 4- and 24-h precipitation data, comparisons of the OIFs obtained from annual maximum rainfalls with that from extreme rainfall quantiles at different return periods, as well as the PMP estimates of Hong Kong from transposing the different corresponding separated nonorographic rainfalls, were conducted. The results show that the OIFs obtained from rainfall quantiles with certain rare probabilities are more stable and reasonable in terms of stability and spatial distribution pattern. [ABSTRACT FROM AUTHOR]

A regional climate model with 90-km horizontal resolution on a large domain is used to predict and analyze precipitation changes over East Africa caused by greenhouse gas increases. A pair of six-member ensembles is used: one representing the late twentieth century and another the mid-twenty-first century under a midline emissions scenario. The twentieth-century simulation uses boundary conditions from reanalysis climatology, and these are modified for the mid-twenty-first-century simulation using output from coupled GCMs. The twentieth-century simulation reproduces the observed climate well. In eastern Ethiopia and Somalia, the boreal spring rains that begin in May are cut short in the mid-twenty-first-century simulation. The cause is an anomalous dry, anticyclonic flow that develops over the Arabian Peninsula and the northern Arabian Sea as mass shifts eastward near 20°N in response to strong warming over the Sahara. In Tanzania and southern Kenya, the boreal spring's long rains are reduced throughout the season in the future simulation. This is a secondary response to precipitation enhancement in the Congo basin. The boreal fall 'short rains' season is lengthened in the twenty-first-century simulation in the southern Kenya and Tanzania region in association with a northeastward shift of the South Indian convergence zone. [ABSTRACT FROM AUTHOR]

Large-scale cloud features referred to as cloudbands are known to be related to widespread and heavy rain via the transport of tropical heat and moisture to higher latitudes. The Australian northwest cloudband is such a feature that has been identified in simple searches of satellite imagery but with limited investigation of its atmospheric dynamical support. An accurate, long-term climatology of northwest cloudbands is key to robustly assessing these events. A dynamically based search algorithm has been developed that is guided by the presence and orientation of the subtropical jet stream. This jet stream is the large-scale atmospheric feature that determines the development and alignment of a cloudband. Using a new 40-yr dataset of cloudband events compiled by this search algorithm, composite atmospheric and ocean surface conditions over the period 1979–2018 have been assessed. Composite cloudband upper-level flow revealed a tilted low pressure trough embedded in a Rossby wave train. Composites of vertically integrated water vapor transport centered around the jet maximum during northwest cloudband events reveal a distinct atmospheric river supplying tropical moisture for cloudband rainfall. Parcel backtracking indicated multiple regions of moisture support for cloudbands. A thermal wind anomaly orientated with respect to an enhanced sea surface temperature gradient over the Indian Ocean was also a key composite cloudband feature. A total of 300 years of a freely coupled control simulation of the ACCESS-D system was assessed for its ability to simulate northwest cloudbands. Composite analysis of model cloudbands compared reasonably well to reanalysis despite some differences in seasonality and frequency of occurrence. [ABSTRACT FROM AUTHOR]

The unique precipitation patterns over the Serengeti National Park in East Africa form the foundation of an internationally important ecosystem. Quantifying these precipitation patterns, and identifying the causal atmospheric processes, can improve understanding of past and future changes to regional rainfall. Precipitation and reanalysis datasets (CHIRPS v2.0, TRMM 3B42, ERA5) were used to quantify the regional climatic conditions on annual, monthly and hourly timescales. Hierarchical cluster analysis identified regions with distinct annual cycles of precipitation. Annual and monthly precipitation over the wider Serengeti domain (1°–4°S, 33°–37°E) was spatially heterogeneous. Cluster analysis identified five sub‐regions with distinct annual cycles, with differing rainfall totals during January–February and June–September, wet season peak rainfall months, and rainfall peak symmetry. Seasonality was broadly controlled by the biannual passage of the tropical rainfall belt. Low‐level wind, humidity and convergence patterns were impacted by the topography and Lake Victoria. An afternoon convergence zone between tropical easterlies and lake breeze winds always ran through the park and was associated with ascending motion and convection. The spatial progression of diurnal rainfall over the Serengeti followed the direction of 750 hPa tropical easterlies. The majority of the park received a late‐afternoon rainfall peak, but from October to March an early afternoon peak was present between the wind convergence line over the central Serengeti and the rift topography. We propose that interactions between tropical easterlies, lake breeze westerlies and the topography control the spatial distribution of Serengeti precipitation, and that the seasonally changing rainfall gradient over the Serengeti may be generated by storms forming at the lake front and propagating in the direction of tropospheric easterlies. We suggest the early precipitation peak in the eastern Serengeti may be due to variability in the position of the lake front, or small storms generated by localized solar heating. [ABSTRACT FROM AUTHOR]

This study investigates the applicability of five cumulus parameterization schemes (CPSs) for precipitation simulation in East Asia with the atmospheric model ECHAM6.3. Besides the built-in Tiedtke–Nordeng–Tiedtke (TNT) scheme, four updated schemes are implemented in ECHAM6.3: two modified Tiedtke schemes, the simplified Arakawa–Schubert scheme (SAS), and the Betts–Miller–Janjić scheme (BMJ). An overall assessment of seasonal climatology shows that the SAS and the two modified Tiedtke schemes have clear advantages over the default TNT scheme. The spatial pattern of precipitation climatology and seasonal variations are successfully reproduced by all CPSs. Biases of rainfall distribution are strongly modulated by the simulation of atmospheric circulation systems over East Asia. The main characteristics of summer monsoonal precipitation, including the increase in rainfall and rain belt migration under the influence of the East Asian summer monsoon (EASM), are successfully captured. However, there are large model diversities in the location and intensity of rain centers around Southeast China and the Korean Peninsula. In addition, all simulations of Meiyu rain band migration are farther north than observed. The SAS scheme is the best of the five CPSs in simulating the timing and intensity of seasonal rainfall variations. The results also suggest that the environmental feedback of a CPS is a key factor that determines the applicability of the CPS in a particular climate model. [ABSTRACT FROM AUTHOR]

Enhanced summer insolation during the early and mid-Holocene drove increased precipitation and widespread expansion of vegetation across the Sahara during the African humid period (AHP). While changes in atmospheric dynamics during this time have been a major focus of palaeoclimate modelling efforts, the transient nature of the shift back to the modern desert state at the end of this period is less well understood. Reconstructions reveal a spatially and temporally complex end of the AHP, with an earlier end in the north than in the south and in the east than in the west. Some records suggest a rather abrupt end, whereas others indicate a gradual decline in moisture availability. Here we investigate the end of the AHP based on a transient simulation of the last 7850 years with the comprehensive Earth system model MPI-ESM1.2. The model largely reproduces the time-transgressive end of the AHP evident in proxy data, and it indicates that it is due to the regionally varying dynamical controls on precipitation. The impact of the main rain-bringing systems, i.e. the summer monsoon and extratropical troughs, varies spatially, leading to heterogeneous seasonal rainfall cycles that impose regionally different responses to the Holocene insolation decrease. An increase in extratropical troughs that interact with the tropical mean flow and transport moisture to the western Sahara during the mid-Holocene delays the end of the AHP in that region. Along the coast, this interaction maintains humid conditions for a longer time than further inland. Drying in this area occurs when this interaction becomes too weak to sustain precipitation. In the lower latitudes of west Africa, where the rainfall is only influenced by the summer monsoon dynamics, the end of the AHP coincides with the retreat of the monsoonal rain belt. The model results clearly demonstrate that non-monsoonal dynamics can also play an important role in forming the precipitation signal and should therefore not be neglected in analyses of north African rainfall trends. [ABSTRACT FROM AUTHOR]

Extreme events are widely studied across the world because of their major implications for many aspects of society and especially floods. These events are generally studied in terms of precipitation or temperature extreme indices that are often not adapted for regions affected by floods caused by snowmelt. The rain on snow index has been widely used, but it neglects rain-only events which are expected to be more frequent in the future. In this study, we identified a new winter compound index and assessed how large-scale atmospheric circulation controls the past and future evolution of these events in the Great Lakes region. The future evolution of this index was projected using temperature and precipitation from the Canadian Regional Climate Model large ensemble (CRCM5-LE). These climate data were used as input in Precipitation Runoff Modelling System (PRMS) hydrological model to simulate the future evolution of high flows in three watersheds in southern Ontario. We also used five recurrent large-scale atmospheric circulation patterns in north-eastern North America and identified how they control the past and future variability of the newly created index and high flows. The results show that daily precipitation higher than 10 mm and temperature higher than 5 ∘ C were necessary historical conditions to produce high flows in these three watersheds. In the historical period, the occurrences of these heavy rain and warm events as well as high flows were associated with two main patterns characterized by high Z500 anomalies centred on eastern Great Lakes (HP regime) and the Atlantic Ocean (South regime). These hydrometeorological extreme events will still be associated with the same atmospheric patterns in the near future. The future evolution of the index will be modulated by the internal variability of the climate system, as higher Z500 on the east coast will amplify the increase in the number of events, especially the warm events. The relationship between the extreme weather index and high flows will be modified in the future as the snowpack reduces and rain becomes the main component of high-flow generation. This study shows the value of the CRCM5-LE dataset in simulating hydrometeorological extreme events in eastern Canada and better understanding the uncertainties associated with internal variability of climate. [ABSTRACT FROM AUTHOR]

This study uses an atmospheric general circulation model to examine the relative effects of Maritime Continent (MC) orography, surface roughness, and land–sea contrast on the three cross-equatorial flows (CEF) north of Australia, including the South China Sea (SCS), Celebes-Moluccas (CM), and New Guinea (NG) CEFs, and Asian monsoon precipitation during boreal summer. Four experiments are conducted: with islands, with islands without orography, with islands with ocean roughness and no orography, and with ocean only in the MC region. At the approximately 1° horizontal resolution of these sensitivity experiments, results indicate that the land–sea contrast and orography in the MC have complicated impacts on the CEFs. The land–sea contrast creates the three CEFs. The orography is dominant in deepening, concentrating, and strengthening the CM CEF and modulating the longitudinal location of the NG CEF. For the intensity and depth of the SCS and NG CEFs, the surface roughness over the flat MC and orography are both important. In addition, the MC modulates the monsoon rainfall in tropical Asia. The decreased rainfall (by roughly 57% and 21.4% over South Asia and the SCS, respectively) is dominated by the reduced moisture availability resulting from the presence of the land–sea contrast, thereby intercepting the westward propagating quasi-biweekly convection. The surface roughness over the MC is key in reducing precipitation through reducing moisture convergence over Sumatra, Borneo, and northeastern New Guinea. However, the orography controls the intense precipitation over southwestern New Guinea and the adjacent seas through enhancing the moisture transport carried by the CM and NG CEFs. [ABSTRACT FROM AUTHOR]

During the period from June 30th to July 6th, 2016, a heavy rainfall event affected the middle and lower reaches of the Yangtze River valley in eastern China. The event was characterized by high‐intensity, long‐duration (lasted more than 6 days) precipitation and huge amounts (over 600.0 mm) of rainfall. The rainfall moved eastward from the Sichuan basin to the middle Yangtze River valley during the first 2 days, then Mei‐yu front formed and circulations became more "quasi‐stationary". During the second‐phase, successive heavy rainfall systems occurred repeatedly over the same areas along the front, leading to widespread and catastrophic flooding. In this study, limited‐area convection‐permitting models (CPMs) covering all of eastern China, and global‐model simulations from the Met Office Unified Model are compared to investigate the added values of CPMs on the veracity of short‐range predictions of the heavy rainfall event. The results show that all the models can successfully simulate the accumulated amount and the evolution of this heavy rainfall event. However, the global model produces too much light rainfall (10.0 mm/day), fails to simulate the small‐scale features of both atmospheric circulations and precipitation, and tends to generate steady heavy rainfall over mountainous region. Afternoon precipitation is also excessively suppressed in global model. By comparison, the CPMs add some value in reproducing the spatial distribution of precipitation, the smaller‐scale disturbances within the rain‐bands, the diurnal cycle of precipitation and also reduce the spurious topographical rainfall, although there is a tendency for heavy rainfall to be too intense in CPMs. [ABSTRACT FROM AUTHOR]

According to Iran geographical location and the general circulation of the atmosphere (or atmospheric circulation), rainfall starts in many parts early in autumn and ends in mid or late spring based on the latitude. Due to the dominating synoptic conditions during some years, rainfall starts earlier than the normal conditions, and during some years, rainfall starts later than the normal conditions. Such synoptic conditions showed that the start of rainfall, especially in the south and southwest of Iran, is closely related to the location and displacement of Arabian subtropical high pressure in autumn. A study conducted on rainfall beginning day of 30 stations in a statistical period in 36 regions in south and southwest of Iran showed that during 12 years of rainfall began later than normal conditions. Arabian subtropical high pressure was extracted 1000, 850, and 700 hPa at four sea levels since September 15 to the beginning day of rainfall. This study showed that during those years when rainfall began later than normal, there were three general states. During some years, the high pressure system has been out of the southwest of Iran later than normal conditions. In a number of years, the high pressure system has left the area, but has moved westward, or the southern border of the Persian Gulf. The governing synoptic conditions of these three patterns have made it impossible for any rainfall system entering the region. In these patterns, Sudan low pressure system has been blocked as the most effective rainfall system in the southern latitude on Sudan and Ethiopia, and it is not possible to expand on the region or move to another route. In the three patterns, Arabian subtropical high pressure generally has an orbital expansion, and in this way, Mediterranean trough has no potential to expand southward. As a result, instability cannot be transferred to the region. [ABSTRACT FROM AUTHOR]

The present study analyzes spatial patterns of precipitation Concentration Index (CI) in Puerto Rico considering the daily precipitation data of 20 precipitation-gauging stations during 1971–2010. The South and East interior parts of Puerto Rico are characterized by higher CI and the West and North-West parts show lower CI. The annual CI and the rainy season CI show a gradient from South-East to North-West and the dry season CI shows a gradient from South to North. Another difference between the rainy season CI and dry season CI is that the former shows the lowest values of CI while the latter shows the highest values of CI. The different types of seasonal precipitation seem to play a major role on the spatial CI distribution. However, the local relief plays a major role in the spatial patterns due to the effect of the air circulation by the mountains. These findings can contribute to basin-scale water resource management (flooding, soil erosion, etc.) and conservation of the ecological environment. [ABSTRACT FROM AUTHOR]

The precipitation and diabatic heating resulting from moist convection make it a key component of the atmospheric water budget in the tropics. With convective parameterization being a known source of uncertainty in global models, convection-permitting (CP) models are increasingly being used to improve understanding of regional climate. Here, a new 10-yr CP simulation is used to study the characteristics of rainfall and atmospheric water budget for East Africa and the Lake Victoria basin. The explicit representation of convection leads to a widespread improvement in the intensities and diurnal cycle of rainfall when compared with a parameterized simulation. Differences in large-scale moisture fluxes lead to a shift in the mean rainfall pattern from the Congo to Lake Victoria basin in the CP simulation—highlighting the important connection between local changes in the representation of convection and larger-scale dynamics and rainfall. Stronger lake–land contrasts in buoyancy in the CP model lead to a stronger nocturnal land breeze over Lake Victoria, increasing evaporation and moisture flux convergence (MFC), and likely unrealistically high rainfall. However, for the mountains east of the lake, the CP model produces a diurnal rainfall cycle much more similar to satellite estimates, which is related to differences in the timing of MFC. Results here demonstrate that, while care is needed regarding lake forcings, a CP approach offers a more realistic representation of several rainfall characteristics through a more physically based realization of the atmospheric dynamics around the complex topography of East Africa. [ABSTRACT FROM AUTHOR]

This study investigates the decadal variations in summer precipitation over eastern China (SPEC) and its response to the decadal change of sensible heat (SH) flux over the Tibetan Plateau (TP). The results show that SPEC experiences a significant decadal change in approximate 2003, demonstrating positive rainfall anomalies over central China and negative anomalies over northeast, north, and south China. Meanwhile, a noticeable increase in SH over the TP occurs in the early 2000s at the decadal time scale. Further analysis reveals that when spring SH over the TP is abnormally strong, there will be a stronger atmospheric heat source and development of deep convection over the TP in the forthcoming summer. The positive thermal forcing induces an abnormal anticyclonic circulation and airflow divergence in the upper layer and cyclonic circulation and airflow convergence in the mid‐layer over the TP. And the negative vorticity and positive geopotential height anomalies in the upper layer over the TP and south China leads to the strengthening of the South Asian High (SAH) and the northwestern Pacific subtropical high (NWPSH). As a result, cold and dry air from higher latitudes and warm and humid air from the tropics converge over central China and diverge over north and south China. In addition, there exist a strong ascending motion over central China and an anomalous descending motion over south China due to the positive diabatic heating and horizontal temperature advection anomalies over central China and negative anomalies over south China. On the other hand, the circulation conditions strengthen the East Asian summer monsoon and weaken the South China Sea summer monsoon. Ultimately, more moisture is transported into central China, but less into south China, which give rise to positive and negative rainfall amounts in the corresponding areas, respectively. (a) Time series of averaged summer rainfall (unit: mm/day) over five regions in eastern China. (b) Time series of regional averaged sensible heat (SH) over the Tibetan Plateau (TPSH) in spring (unit: W/m2) calculated by the China Meteorological Administration (CMA) data set, and the reanalysis data sets from 1982 to 2015 (series for the JRA55 data set is from 1982 to 2012). (c) The 11‐year moving correlations between spring TPSH and summer rainfall over five regions in eastern China from 1982 to 2015. The value corresponding to 1987 is the correlation coefficient during the period 1982–1992. The curves represent the correlation results, and the horizontal dashed lines indicate the 95% confidence levels. Summer precipitation over eastern China (SPEC) experienced significant decadal changes in approximately 2003, demonstrating positive rainfall anomalies over central China and negative rainfall anomalies over north and south China. Meanwhile, a noticeable increase in SH over the TP occurs in the early 2000s at the decadal time scale. When spring TPSH increases, precipitation increases over central China and decreases over south China according to the corresponding correlations. [ABSTRACT FROM AUTHOR]

Climate model faces considerable difficulties in simulating the rainfall characteristics of southwest summer monsoon. In this study, the dynamical downscaling of European Centre for Medium-Range Weather Forecast's (ECMWF's) ERA-Interim (EIN15) has been utilized for the simulation of Indian summer monsoon (ISM) through the Regional Climate Model version 4.3 (RegCM-4.3) over the South Asia Co-Ordinated Regional Climate Downscaling EXperiment (CORDEX) domain. The complexities of model simulation over a particular terrain are generally influenced by factors such as complex topography, coastal boundary, and lack of unbiased initial and lateral boundary conditions. In order to overcome some of these limitations, the RegCM-4.3 is employed for simulating the rainfall characteristics over the complex topographical conditions. For reliable rainfall simulation, implementations of numerous lower boundary conditions are forced in the RegCM-4.3 with specific horizontal grid resolution of 50 km over South Asia CORDEX domain. The analysis is considered for 30 years of climatological simulation of rainfall, outgoing longwave radiation (OLR), mean sea level pressure (MSLP), and wind with different vertical levels over the specified region. The dependency of model simulation with the forcing of EIN15 initial and lateral boundary conditions is used to understand the impact of simulated rainfall characteristics during different phases of summer monsoon. The results obtained from this study are used to evaluate the activity of initial conditions of zonal wind circulation speed, which causes an increase in the uncertainty of regional model output over the region under investigation. Further, the results showed that the EIN15 zonal wind circulation lacks sufficient speed over the specified region in a particular time, which was carried forward by the RegCM output and leads to a disrupted regional simulation in the climate model. [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) summer monsoon onset has been regarded as the beginning of the East Asian summer monsoon. In this study, we investigated the impacts of the SCS monsoon onset on the transition from the spring persistent rainfall to the summer Meiyu in the middle and lower reaches of the Yangtze River basin (MLYZB). It is found that rainfall in the MLYZB reduces after the SCS monsoon onset. This reduction in rainfall persists until the onset of the Meiyu and is accompanied by a weakening of southwesterlies to the south of the MLYZB. These features exist in both climatology and interannual variability. Rainfall increases significantly over the SCS and the subtropical western North Pacific after the SCS monsoon onset. The latent heating of the increased rainfall can excite an anomalous cyclone over the western North Pacific, which weakens the mean southwesterlies to the south of the MLYZB and decreases water vapor entering the MLYZB. It also generates descending motion over southeastern China. Thus, the SCS monsoon onset could suppress rainfall over the MLYZB by the latent heating induced changes in circulation. Compared to increased rainfall over the SCS, the latent heating of increase rainfall over the subtropical western North Pacific plays a more important role in the reduction of rainfall over the MLYZB. As the SCS monsoon onset affects the timing of the reduction of rainfall in the MLYZB, an early SCS monsoon onset is accompanied by below-normal May rainfall in the MLYZB, while a late SCS monsoon onset is accompanied by above-normal May rainfall. [ABSTRACT FROM AUTHOR]

Previous research has suggested that the anomalous western North Pacific anticyclone (WNPAC) can generally persist from an El Niño mature winter to the subsequent summer, influencing southern China precipitation significantly, where southern China includes the Yangtze River valley and South China. Since the late 1970s, three extreme El Niño events have been recorded: 1982/83, 1997/98, and 2015/16. There was a sharp contrast in the change in southern China rainfall and corresponding atmospheric circulations in the decaying August between the 2015/16 extreme El Niño event and the earlier two extreme El Niño events. Enhanced rainfall in the middle and upper reaches of the Yangtze River and suppressed rainfall over South China resulted from basinwide warming in the tropical Indian Ocean induced by the extreme El Niño in August 1983 and 1998, which was consistent with previous studies. However, an anomalous western North Pacific cyclone emerged in August 2016 and then caused positive rainfall anomalies over South China and negative rainfall anomalies from the Yangtze River to the middle and lower reaches of the Yellow River. Without considering the effect of the long-term global warming trend, in August 2016 the negative SST anomalies over the western Indian Ocean and cooling in the north tropical Atlantic contributed to the anomalous western North Pacific cyclone and a rainfall anomaly pattern with opposite anomalies in South China and the Yangtze River region. Numerical experiments with the CAM5 model are conducted to confirm that cooler SST in the western Indian Ocean contributed more than cooler SST in the north tropical Atlantic to the anomalous western North Pacific cyclone and anomalous South China rainfall. [ABSTRACT FROM AUTHOR]

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

The sensitivity of the representation of the global monsoon annual cycle to horizontal resolution is compared in three AGCMs: the Met Office Unified Model-Global Atmosphere 3.0; the Meteorological Research Institute AGCM3; and the Global High Resolution AGCM from the Geophysical Fluid Dynamics Laboratory. For each model, we use two horizontal resolution configurations for the period 1998-2008. Increasing resolution consistently improves simulated precipitation and low-level circulation of the annual mean and the first two annual cycle modes, as measured by the pattern correlation coefficient and equitable threat score. Improvements in simulating the summer monsoon onset and withdrawal are region-dependent. No consistent response to resolution is found in simulating summer monsoon retreat. Regionally, increased resolution reduces the positive bias in simulated annual mean precipitation, the two annual-cycle modes over the West African monsoon and Northwestern Pacific monsoon. An overestimation of the solstitial mode and an underestimation of the equinoctial asymmetric mode of the East Asian monsoon are reduced in all high-resolution configurations. Systematic errors exist in lower-resolution models for simulating the onset and withdrawal of the summer monsoon. Higher resolution models consistently improve the early summer monsoon onset over East Asia and West Africa, but substantial differences exist in the responses over the Indian monsoon region, where biases differ across the three low-resolution AGCMs. This study demonstrates the importance of a multi-model comparison when examining the added value of resolution and the importance of model physical parameterizations for simulation of the Indian monsoon. [ABSTRACT FROM AUTHOR]

During extended winter (November-April), 43% of the intraseasonal rainfall variability in China is explained by three spatial patterns of temporally coherent rainfall. These patterns were identified with empirical orthogonal teleconnection (EOT) analysis of observed 1982-2007 pentad rainfall anomalies and connected to midlatitude disturbances. However, examination of individual strong EOT events shows that there is substantial inter-event variability in their dynamical evolution, which implies that precursor patterns found in regressions cannot serve as useful predictors. To understand the physical nature and origins of the extratropical precursors, the EOT technique is applied to six simulations of the Met Office Unified Model at horizontal resolutions of 200-40 km, with and without air-sea coupling. All simulations reproduce the observed precursor patterns in regressions, indicating robust underlying dynamical processes. Further investigation into the dynamics associated with observed patterns shows that Rossby wave dynamics can explain the large inter-event variability. The results suggest that the apparently slowly evolving or quasi-stationary waves in regression analysis are a statistical amalgamation of more rapidly propagating waves with a variety of origins and properties. [ABSTRACT FROM AUTHOR]

Recent work has shown the dominance of the Himalaya in supporting the Indian summer monsoon (ISM), perhaps by surface sensible heating along its southern slope and by mechanical blocking acting to separate moist tropical flow from drier midlatitude air. Previous studies have also shown that Indian summer rainfall is largely unaffected in sensitivity experiments that remove only the Tibetan Plateau. However, given the large biases in simulating the monsoon in CMIP5 models, such results may be model dependent. This study investigates the impact of orographic forcing from the Tibetan Plateau, Himalaya and Iranian Plateau on the ISM and East Asian summer monsoon (EASM) in the UK Met Office’s HadGEM3-GA6 and China’s Institute of Atmospheric Physics FGOALS-FAMIL global climate models. The models chosen feature oppositesigned biases in their simulation of the ISM rainfall and circulation climatology.The changes to ISM and EASM circulation across the sensitivity experiments are similar in both models and consistent with previous studies. However, considerable differences exist in the rainfall responses over India and China, and in the detailed aspects such as onset and retreat dates. In particular, the models show opposing changes in Indian monsoon rainfall when the Himalaya and Tibetan Plateau orography are removed. Our results show that a multi-model approach, as suggested in the forthcoming Global Monsoon Model Intercomparison Project (GMMIP) associated with CMIP6, is needed to clarify the impact of orographic forcing on the Asian monsoon and to fully understand the implications of model systematic error. [ABSTRACT FROM AUTHOR]

Rainfall in the Levant drylands is scarce but can potentially generate high-magnitude flash floods. Rainstorms are caused by distinct synoptic-scale circulation patterns: Mediterranean cyclone (MC), active Red Sea trough (ARST), and subtropical jet stream (STJ) disturbances, also termed tropical plumes (TPs). The unique spatiotemporal characteristics of rainstorms and floods for each circulation pattern were identified. Meteorological reanalyses, quantitative precipitation estimates from weather radars, hydrological data, and indicators of geomorphic changes from remote sensing imagery were used to characterize the chain of hydrometeorological processes leading to distinct flood patterns in the region. Significant differences in the hydrometeorology of these three flood-producing synoptic systems were identified: MC storms draw moisture from the Mediterranean and generate moderate rainfall in the northern part of the region. ARST and TP storms transfer large amounts of moisture from the south, which is converted to rainfall in the hyperarid southernmost parts of the Levant. ARST rainfall is local and intense, whereas TP rainfall is widespread and prolonged due to high precipitation efficiency and large-scale forcing. Thus, TP rainfall generates high-magnitude floods in the largest catchments; integration of numerous basins leads to sediment feeding from the south into the Dead Sea, exhibited in large sediment plumes. Anecdotal observations of the channel with the largest catchment in the region (Nahal HaArava) indicate that TP floods account for noticeable geomorphic changes in the channel. It provides insights into past intervals of increased flash flood frequency characterized by episodes of marked hydrogeomorphic work; such an increase is especially expected during intervals of southerly situated and southwesterly oriented STJs. [ABSTRACT FROM AUTHOR]

In the east Pacific (EP) intertropical convergence zone (ITCZ), Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) latent heating retrievals suggest a top-heavy structure; however, light precipitation and its associated low-level heating are underestimated by the PR. This study uses stratiform and deep convective precipitation from the TRMM PR and shallow precipitation from the more sensitive CloudSat radar to assess the seasonal latent heating structure in the EP ITCZ (130°–90°W) for 1998–2014. This study also uses reanalyses (MERRA-2, ERA-Interim, and NCEP–NCAR) to analyze the meridional circulation linked to variations in ITCZ heating. The TRMM/CloudSat heating profiles suggest a distinct seasonality. During DJF, latent heating peaks at 800 hPa because of the predominance of shallow convection and rises to 700 hPa during MAM as the contribution from deep convective rain increases. During JJA and SON, stratiform precipitation increases and the latent heating has a double peak at 700 and 400 hPa. Additionally, the EP ITCZ heating has a meridional slope throughout most of the year as a result of the prevalence of shallow (deep) convection in the southern (northern) part of the ITCZ. While the reanalyses agree that the most bottom-heavy heating occurs in DJF and the most top-heavy heating occurs in JJA, they underestimate heating aloft compared to the satellite retrievals throughout the year and show varying ability in representing the shallow meridional circulation and deeper Hadley cell overturning. [ABSTRACT FROM AUTHOR]

East African hydroclimate exhibits considerable variability across a range of timescales, with implications for its population that depends on the region’s two rainy seasons. Recent work demonstrated that current state-of-the-art climate models consistently underestimate the long rains in boreal spring over the Horn of Africa while overestimating the short rains in autumn. This inability to represent the seasonal cycle makes it problematic for climate models to project changes in East African precipitation. Here we consider whether this bias also has implications for understanding interannual and decadal variability in the East African long and short rains. Using a consistent framework with an unforced multi-century global coupled climate model simulation, the role of Indo-Pacific variability for East African rainfall is compared across timescales and related to observations. The dominant driver of East African rainfall anomalies critically depends on the timescale under consideration: Interannual variations in East African hydroclimate coincide with significant sea surface temperature (SST) anomalies across the Indo-Pacific, including those associated with the El Niño-Southern Oscillation (ENSO) in the eastern Pacific, and are linked to changes in the Walker circulation, regional winds and vertical velocities over East Africa. Prolonged drought/pluvial periods in contrast exhibit anomalous SST predominantly in the Indian Ocean and Indo-Pacific warm pool (IPWP) region, while eastern Pacific anomalies are insignificant. We assessed dominant frequencies in Indo-Pacific SST and found the eastern equatorial Pacific dominated by higher-frequency variability in the ENSO band, while the tropical Indian Ocean and IPWP exhibit lower-frequency variability beyond 10 years. This is consistent with the different contribution to regional precipitation anomalies for the eastern Pacific versus Indian Ocean and IPWP on interannual and decadal timescales, respectively. In the model, the dominant low-frequency signal seen in the observations in the Indo-Pacific is not well-represented as it instead exhibits overly strong variability on subdecadal timescales. The overly strong ENSO-teleconnection likely contributes to the overestimated role of the short rains in the seasonal cycle in the model compared to observations. [ABSTRACT FROM AUTHOR]

Heavy rainfalls and consequent floods are the most important extreme events in Central Europe in terms of frequency and economic losses. The purpose of the study is to evaluate regional extreme precipitation events in the Polish Carpathian Mountains as well as to determine the circulation basis for these events. Extreme precipitation values were identified based on daily precipitation totals measured at almost 70 weather stations in the Polish Carpathians between 1951 and 2015. The analysis concentrates on daily mean areal precipitation totals in excess of 100 mm per day as well as 50 and 30 mm, which causes flooding in the Carpathians. All extreme precipitation data were analyzed from a synoptic perspective, with special attention paid to pressure patterns over Central Europe. The analysis confirms that the heaviest rainfalls are affected by the situations with the NNE airflow. However, analyses with the application of the upper tropospheric data brought some new findings on the precipitation extreme topic. It turned out that most cases with significant rains occurred when the mid-tropospheric advection was from the southern sector (during the typical N and NE advections in the lower part). [ABSTRACT FROM AUTHOR]

Continuous 305-year (1711-2016) monthly rainfall series (IoI_1711) is created for the Island of Ireland. The post 1850 series draws on an existing quality assured rainfall network for Ireland, while pre-1850 values come from instrumental and documentary series compiled, but not published by the UK Met Office. The series is evaluated by comparison with independent long-term observations and reconstructions of precipitation, temperature and circulation indices from across the British-Irish Isles. Strong decadal consistency of IoI_1711 with other long-term observations is evident throughout the annual, boreal spring and autumn series. Annually, the most recent decade (2006-2015) is found to be the wettest in over 300 years. The winter series is probably too dry between the 1740s and 1780s, but strong consistency with other long-term observations strengthens confidence from 1790 onwards. The IoI_1711 series has remarkably wet winters during the 1730s, concurrent with a period of strong westerly airflow, glacial advance throughout Scandinavia and near unprecedented warmth in the Central England Temperature record - all consistent with a strongly positive phase of the North Atlantic Oscillation. Unusually wet summers occurred in the 1750s, consistent with proxy (treering) reconstructions of summer precipitation in the region. Our analysis shows that inter-decadal variability of precipitation is much larger than previously thought, while relationships with key modes of climate variability are time-variant. The IoI_1711 series reveals statistically significant multicentennial trends in winter (increasing) and summer (decreasing) seasonal precipitation. However, given uncertainties in the early winter record, the former finding should be regarded as tentative. The derived record, one of the longest continuous series in Europe, offers valuable insights for understanding multi-decadal and centennial rainfall variability in Ireland, and provides a firm basis for benchmarking other long-term records and reconstructions of past climate. Correlation of Irish rainfall with other parts of Europe increases the utility of the series for understanding historical climate in further regions. [ABSTRACT FROM AUTHOR]

Two extreme, high-impact events of heavy rainfall and severe floods in West African urban areas (Ouagadougou on 1 September 2009 and Dakar on 26 August 2012) are investigated with respect to their atmospheric causes and statistical return periods. In terms of the synoptic–convective dynamics, the Ouagadougou case is truly extraordinary. A succession of two slow-moving African easterly waves (AEWs) caused record-breaking values of tropospheric moisture. The second AEW, one of the strongest in recent decades, provided the synoptic forcing for the nighttime genesis of mesoscale convective systems (MCSs). Ouagadougou was hit by two MCSs within 6 h, as the strong convergence and rotation in the AEW-related vortex allowed a swift moisture refueling. An AEW was also instrumental in the overnight development of MCSs in the Dakar case, but neither the AEW vortex nor the tropospheric moisture content was as exceptional as in the Ouagadougou case. Tropical Rainfall Measuring Mission (TRMM) 3B42 precipitation data show some promise in estimating centennial return values (RVs) using the "peak over threshold" approach with a generalized Pareto distribution fit, although indications for errors in estimating extreme rainfall over the arid Sahel are found. In contrast, the Precipitation Estimation from Remotely Sensed Information Using Artificial Neural Networks–Climate Data Record (PERSIANN-CDR) dataset seems less suitable for this purpose despite the longer record. Notably, the Ouagadougou event demonstrates that highly unusual dynamical developments can create extremes well outside of RV estimates from century-long rainfall observations. Future research will investigate whether such developments may become more frequent in a warmer climate. [ABSTRACT FROM AUTHOR]

The purpose of this study is to use the PERSIANN-Climate Data Record (PERSIANN-CDR) dataset to evaluate the ability of 32 CMIP5 models in capturing the behavior of daily extreme precipitation estimates globally. The daily long-term historical global PERSIANN-CDR allows for a global investigation of eight precipitation indices that is unattainable with other datasets. Quantitative comparisons against CPC daily gauge; GPCP One-Degree Daily (GPCP1DD); and TRMM 3B42, version 7 (3B42V7), datasets show the credibility of PERSIANN-CDR to be used as the reference data for global evaluation of CMIP5 models. This work uniquely defines different study regions by partitioning global land areas into 25 groups based on continent and climate zone type. Results show that model performance in warm temperate and equatorial regions in capturing daily extreme precipitation behavior is largely mixed in terms of index RMSE and correlation, suggesting that these regions may benefit from weighted model averaging schemes or model selection as opposed to simple model averaging. The three driest climate regions (snow, polar, and arid) exhibit high correlations and low RMSE values when compared against PERSIANN-CDR estimates, with the exceptions of the cold regions showing an inability to capture the 95th and 99th percentile annual total precipitation characteristics. A comprehensive assessment of each model's performance in each continent-climate zone defined group is provided as a guide for both model developers to target regions and processes that are not yet fully captured in certain climate types, and for climate model output users to be able to select the models and/or the study areas that may best fit their applications of interest. [ABSTRACT FROM AUTHOR]