Projection and possible causes of summer precipitation in eastern China using self-organizing map

Self-organizing map (SOM) is used to simulate summer daily precipitation over the Yangtze–Huaihe river basin in Eastern China, including future projections. SOM shows good behaviors in terms of probability distribution of daily rainfall and spatial distribution of rainfall indices, as well as consistency of multi-model simulations. Under RCP4.5 Scenario, daily rainfall at most sites (63%) is projected to shift towards larger values. For the early 21st century (2016–2035), precipitation in the central basin increases, yet decreases occur over the middle reaches of the Yangtze River as well as a part of its southeast area. For the late 21st century (2081–2100), the mean precipitation and extreme indices experience an overall increase except for a few southeast stations. The total precipitation in the lower reaches of the Yangtze River and in its south area is projected to increase from 7% at 1.5 °C global warming to 11% at 2 °C, while the intensity enhancement is more significant in southern and western sites of the domain. A clustering allows to regroup all SOM nodes into four distinct regimes. Such regional synoptic regimes show remarkable stability for future climate. The overall intensification of precipitation in future climate is linked to the occurrence-frequency rise of a wet regime which brings longitudinally closer the South Asia High (eastward extended) and the Western Pacific Subtropical High (westward extended), as well as the reduction of a dry pattern which makes the two atmospheric centers of action move away from each other.     

A possible abrupt change in summer precipitation over eastern China around 2009

Historical studies have shown that summer rainfall in eastern China undergoes decadal variations, with three apparent changes in the late 1970s, 1992, and the late 1990s. The present observational study indicates that summer precipitation over eastern China likely underwent a change in the late 2000s, during which the main spatial pattern changed from negative–positive–negative to positive–negative in the meridional direction. This change in summer precipitation over eastern China may have been associated with circulation anomalies in the middle/upper troposphere. A strong trough over Lake Baikal created a southward flow of cold air during 2009–15, compared with 1999–2008, while the westward recession of the western Pacific subtropical high strengthened the moisture transport to the north, creating conditions that were conducive for more rainfall in the north during this period. The phase shift of the Pacific Decadal Oscillation in the late 2000s led to the Pacific–Japan-type teleconnection wave train shifting from negative to positive phases, resulting in varied summer precipitation over eastern China.     

1960-2005年长江中下游极端降水指数变化特征分析

本文利用长江中下游流域内的81个气象站,对长江中下游极端降水指数的时空变化特征进行分析.结果发现,在时间尺度上,长江中下游地区近46 a来极端降水指数呈上升趋势,其中降水强度上升趋势最明显,各个极端降水指数在年代际尺度上具有相同的变化特征,均存在着12 a左右的周期振荡,在年际尺度上,各极端降水指数变化周期并不一致.大雨日数与其他指数相比突变时间比较早,发生在1979年,其他几个指数突变时间比较接近,出现在1990年前后.在空间变化上,除极端湿天降水量在全区均为上升趋势外,其他几种极端降水指数在江苏东部地区、湖北西北部都存在着极端降水指数的负变化趋势,高值区主要分布在江西大部、湖北东南部、湖南东北部地区.     

A relation between extreme daily precipitation and extreme short term precipitation

The Royal Netherlands Meteorological Institute (KNMI) has published the KNMI’06 climate scenarios in 2006. These scenarios give the possible states of the climate in The Netherlands for the next century. Projections of changes in precipitation were made for a time scale of 1 day. The urban drainage sector is, however, more interested in projections on shorter time scales. Specifically, time scales of 1 h or less. The aim of this research is to provide projections of precipitation at these shorter time scales based on the available daily scenarios. This involves an analysis of climate variables and their relations to precipitation at different time scales. On the basis of this analysis, one can determine a numeric factor to translate daily projections into shorter time scale projections.     

Decreasing precipitation occurs in daily extreme precipitation intervals across China in observations and model simulations

Extreme precipitation response to increasing temperature includes not only changes of frequency and intensity, but also changes of extreme precipitation interval (EPIV) and the precipitation during the neighboring daily extreme precipitations interval (EPIP). These changes have not been fully evaluated yet in observations or climate model simulations although they are very useful to understand variations of extreme precipitation. We used daily precipitation data from 669 meteorological stations during the past five decades across China and projections of 19 general circulation models from CMIP5 under the RCP4.5 and RCP8.5 scenarios to investigate variations of EPIV and EPIP. We found the national average annual EPIV increased across China during the last five decades, while annual EPIP significantly decreased. The decreases mainly occurred in southwest China, east China, and southeast China. At national and regional scales, the average annual EPIV and EPIP showed greater decreases under the RCP8.5 scenario than those under the RCP4.5 scenario from 2006 to 2100. Annual EPIP showed a stronger correlation with extreme precipitation intensity than EPIV. The national average annual EPIP had a significant positive correlation with the Western Pacific Subtropical High Area Index. The abnormal geopotential heights over western Mongolia and the western Pacific at 500 hpa as well as the abnormal SSTs in Japan Sea and the western of Pacific in rainy seasons would result in abnormal annual EPIVs and EPIPs in China. This study may provide references for flooding prediction, water resources management, and disaster prevention and mitigation.     

Cutoff low systems and their relevance to large-scale extreme precipitation in the European Alps

In this paper, we attempt to highlight the relevance of cutoff low systems (CoLs) to large-scale heavy precipitation events within the Alpine region which often lead to catastrophic flooding. The main results of this study are (1) a detailed climatology (1971–1999) of CoLs for the European region, (2) contribution of CoLs to extreme precipitation events in the European Alpine region, (3) identification of regions within the European Alps most affected by extreme precipitation caused by CoLs, and (4) identification of regions where presence of CoLs is related to extreme precipitation in the Alpine region. The findings of this paper suggest that CoLs have a significant correlation with extreme precipitation events and strongly influence the climate of the Alpine region. The total contribution of CoLs to large-scale heavy precipitation events ranges between 20 and 95 % and is most pronounced in the northern and eastern parts of the Alps. More than 80 % of the events occur in the summer season. The area around the Alps and West of Spain (over the Atlantic Ocean) is the most affected region. The location of the center of CoLs that affect the Alpine region most occur on the northern and southern sides of the Alpine ridge.     

华南后汛期极端降水特征及变化趋势

利用中国国家气象信息中心提供的华南89个代表站1969-2008年逐日降水资料,对华南后汛期（7-月）极端降水时空演变特征进行研究。结果表明：多年平均总降水量、强降水量、降水频率、强降水频率以及暴雨频率与强降水量阈值空间分布一致,广东和广西的南部及福建西南部为大值区;强降水量、降水频率及暴雨频率很大程度影响着华南后汛期总降水量的空间分布。强降水量、强降水频率、暴雨频率对后汛期总降水量的时间变化有很好的指示意义。华南后汛期极端降水在1992--1993年发生一次明显的年代际转折,1993年以来各指标的均值（除降水频率外）和方差均显著增加,华南发生极端旱涝的情况增多。另外,转折前后两个时段的总降水量、降水频率均呈减小趋势,但减小显著的区域有一定差异。     

Anthropogenic influence on the extreme drought in eastern China in 2022 and its future risk

2022年夏季中国东部地区遭遇了一次持续性极端高温干旱事件.本文利用CMIP6检测归因比较计划(DAMIP)数据,量化了人为强迫对类2022年极端干旱事件发生概率的影响,并基于未来不同增暖情景试验给出了此类极端干旱事件的未来变化预估.通过分析不同外强迫因子作用下此类极端干旱事件的发生概率变化,发现人为强迫使此类极端干旱事件的发生概率提高约56％,这主要与人为强迫下中国东部平均水汽减少和平均上升运动减弱有关.进一步通过分析此类极端干旱事件对不同温室气体排放情景(SSP1-2.6,SSP2-4.5,SSP5-8.5)的响应,发现在低排放情景下类2022年极端干旱事件的发生概率较当今气候显著下降,这主要与中国东部平均水汽的增加和平均环流的变化有关,而在高排放情景(SSP5-8.5)下,此类极端干旱事件的发生概率较当今气候增加约79％,这主要与高排放情景下平均下沉运动增强有关.该研究表明,人为强迫通过调制气候平均背景场从而引起极端事件发生频次的变化是人类活动影响极端气候事件的重要途径之一,极端干旱事件对温室气体排放量的响应可能是非线性的.     

中国区域性极端降水事件及人口经济暴露度研究

This paper tests the idea of substituting the atmospheric observations with atmospheric reanalysis when setting up a coupled data assimilation system.The paper focuses on the quantification of the effects on the oceanic analysis resulted from this substitution and designs four different assimilation schemes for such a substitution.A coupled Lorenz96 system is constructed and an ensemble Kalman filter is adopted.The atmospheric reanalysis and oceanic observations are assimilated into the system and the analysis quality is compared to a benchmark experiment where both atmospheric and oceanic observations are assimilated.Four schemes are designed for assimilating the reanalysis and they differ in the generation of the perturbed observation ensemble and the representation of the error covariance matrix.The results show that when the reanalysis is assimilated directly as independent observations,the root-mean-square error increase of oceanic analysis relative to the benchmark is less than 16%in the perfect model framework;in the biased model case,the increase is less than 22%.This result is robust with sufficient ensemble size and reasonable atmospheric observation quality(e.g.,frequency,noisiness,and density).If the observation is overly noisy,infrequent,sparse,or the ensemble size is insufficiently small,the analysis deterioration caused by the substitution is less severe since the analysis quality of the benchmark also deteriorates significantly due to worse observations and undersampling.The results from different assimilation schemes highlight the importance of two factors:accurate representation of the error covariance of the reanalysis and the temporal coherence along each ensemble member,which are crucial for the analysis quality of the substitution experiment.     

The increasing predominance of extreme precipitation in Southwest China since the late 1970s

Extreme precipitation events cause severe environmental and societal damage worldwide. Southwest China (SWC) is sensitive to such effects because of its overpopulation, underdevelopment, and fragile ecosystems. Using daily observations from 108 rain-gauge stations, the authors investigated the frequency of extreme precipitation events and their contribution to total precipitation in SWC since the late 1970s. Results indicate that total precipitation is decreasing insignificantly, but rainfall-events frequency is decreasing significantly, whereas the region is experiencing more frequent and intense extreme precipitation events. Note that although fewer stations are statistically significant, about 60% of the rain-gauge stations show an increasing trend in the frequency and intensity of extreme precipitation. Furthermore, there is an increasing trend in the contribution of total extreme precipitation to total precipitation, with extreme precipitation becoming dominant in the increasingly arid SWC region. The results carry important implications for policymakers, who should place greater emphasis on extreme precipitation and associated floods and landslides when drafting water-resource management policies.摘要本文分析了中国西南20世纪70年代末以来极端降水事件的频率, 强度及其对总降水的贡献. 结果表明, 该地区约60%的降水站点极端降水的频率和强度正在增加, 而大多数站点总降水频率明显减少. 同时极端降水总量对总降水量的贡献有显著增加的趋势, 极端降水在日益干旱的中国西南地区变得更具主导性. 研究结果提醒应更加重视极端降水及其可能引发的次生灾害, 如洪水, 山体滑坡等.     

Role of land-atmosphere coupling in persistent extreme climate events in eastern China in summer 2022

2022年暖季,中国东部地区遭受持续性高温,少雨和土壤干旱的复合极端事件.特征分析指出,在研究时段内,中国东部地区的气温,降水和土壤湿度呈现明显的季节内变化和南北差异.由1940-2022年的气候态可知,长江流域和东南地区的土壤含水充足,蒸散主要受限于陆面有效能量.然而,潜在机制研究指出,2022年土壤湿度对蒸散的限制作用在上述区域异常偏强.土壤湿度与气候要素之间的强反馈可能在2022年复合极端事件的演变和持续中发挥了关键作用.     

Future evolution of extreme precipitation in the Mediterranean

Mediterranean basins can be impacted by severe floods caused by extreme rainfall, and there is a growing awareness about the possible increase in these heavy rainfall events due to climate change. In this study, the climate change impacts on extreme daily precipitation in 102 catchments covering the whole Mediterranean basin are investigated using nonstationary extreme value model applied to annual maximum precipitation in an ensemble of high-resolution regional climate model (RCM) simulations from the Euro-CORDEX experiment. Results indicate contrasted trends, with significant increasing trends in Northern catchments and conversely decreasing trends in Southern catchments. For most cases, the time of signal emergence for these trends is before the year 2000. The same spatial pattern is obtained under the two climate scenarios considered (RCP4.5 and RCP8.5) and in most RCM simulations, suggesting a robust climate change signal. The strongest multi-model agreement concerns the positive trends, which can exceed +?20% by the end of the twenty-first century in some simulations, impacting South France, North Italy, and the Balkans. For these areas, society-relevant strong impacts of such Mediterranean extreme precipitation changes could be expected in particular concerning flood-related damages.     

Spatial autocorrelation analysis of extreme precipitation in Iran

Spatial variations in extreme precipitation events make hydrological, climatological, social, environmental and agricultural effects on a country. This study presents the spatiotemporal autocorrelation analysis of extreme precipitation events over Iran using gridded data on daily precipitation for the period 1961–2010. The 95th percentile is considered as extreme precipitation factor. The spatial autocorrelation of extreme precipitation is examined by three commonly used spatial autocorrelation statistics, the G _i statistic index, Moran’s I global index, and Local Moran’s I (LISA) index, at the 95 and 99% significant confidence level. The results showed a strong significant spatial autocorrelation for extreme precipitation events with the highest Moran’s I value in January. The positive significant autocorrelation of extreme precipitation is observed over the southern parts of the Caspian Sea and Zagros Mountains ranges, while the negative significant autocorrelation is observed over the central and eastern parts of country. In spring and summer the positive autocorrelation cores displace from the Zagros Mountains ranges to the northwestern and southeastern parts.     

Trends in extreme rainfall events of Bangladesh

Previous studies have shown that urbanization in the Pearl River Delta (PRD) region in southern China can have significant effect on the land-sea breeze circulation in the region. In this study, some characteristics of this urban heat island (UHI) effect are further investigated by some idealized numerical experiments. This study focuses on three aspects of the UHI effects. Firstly, the interaction of UHI effects associated with different city clusters in the PRD region. Secondly, the UHI effect and the strength of the background wind field. Thirdly, seasonal variation of the UHI effect. Experiment results suggest that the interaction of the UHI effects due to the closely distributed city clusters in the PRD region is not significant, while the UHI effect in the region may depend on the strength of the background wind. The seasonal-mean UHI effect in the region shows a significant variation associated with the change of seasons.     

未来气候情景下中国东部极端降水和气温的危险性特征

基于BCC-CSM11模式降尺度预估结果,通过构建极端天气气候事件的危险性指数,考察和分析了中国东部极端降水和气温未来气候情景下可能的变化趋势和危险性分布格局。结果表明: 1）在中等排放情景（RCP4.5）下,近期（2021—2050年）极端降水和极端高温危险性呈现增强趋势,危险性指数增幅分别约为2%和10%,而极端低温危险性则呈减弱趋势,危险性指数降幅约为4%。21世纪末期（2070—2099年）,极端降水和气温危险性均基本保持现有水平,未有明显趋势。在高等排放情景（RCP8.5）下,极端降水和极端高温危险性将持续增强,至21世纪末危险性指数增幅分别约为5%和60%;极端低温危险性持续减弱,危险性指数降幅约为5%。2）在未来气候情景下,中国东部极端高温的危险性以全域持续增强为主要特征,特别是西南地区、长江以南地区和东南沿海危险性增强最为显著。至21世纪末,在高排放情景下的危险性指数增幅为30%—60%。极端降水危险性在黄河上游、长江上游和下游以及东北地区中南部等地区呈增强趋势,危险性指数增幅为3%—5%。极端低温危险性全域呈减弱趋势,至21世纪末期高等排放情景下的危险性指数最高降幅为7%—9%。     

Spatial and temporal trends of extreme temperature and precipitation in the Daqing River Basin,North China

Theoretical and Applied Climatology - According to the daily maximum and minimum temperature and the precipitation at 40 meteorological stations in the Daqing River Basin of China during...