RELATIONSHIP BETWEEN THE 30 TO 60 DAY OSCILLATION OF ATMOSPHERIC HEAT SOURCE AND THE DROUGHT AND FLOOD EVENTS IN JUNE IN THE SOUTH OF CHINA

Based on the NCEP/NCAR reanalysis data and the observed precipitation data in the south of China from 1958 to 2000,the impact of 30 to 60 day oscillation of atmospheric heat sources on the drought and flood events in June in the south of China is discussed.During the flood(drought) events,there exists an anomalous low-frequency anticyclone(cyclone) at the low level of the troposphere over the South China Sea and the northwestern Pacific,accompanied with anomalous low-frequency heat sinks(heat sources),while there exists an anomalous low-frequency cyclone(anticyclone) with anomalous heat sources(sinks) over the area from the south of China to the south of Japan.On average,the phase evolution of the low-frequency in drought events is 7 to 11 days ahead of that in flood events in May to June in the south of China.In flood events,low-frequency heat sources and cyclones are propagated northward from the southern South China Sea,northwestward from the warm pool of the western Pacific and westward from the northwestern Pacific around 140°E,which have very important impact on the abundant rainfall in June in the south of China.However,in drought events,the northward propagations of the low-frequency heat sources and cyclones from the South China Sea and its vicinity are rather late compared with those in flood events,and there is no obvious westward propagation of the heat sources from the northwestern Pacific.The timing of the low-frequency heat source propagation has remarkable impact on the June rainfall in the south of China.

Change of Flood Patterns in China under the Influences of Climate Change and Human Activities

Based on the flood affected area （FA） data of the provinces in China from 1950 to 2005, the article discusses the change of the flood patterns in China, and investigates its relationship with climate change and human activities. The flood center shifted from North China and the Yangtze-Huaihe basin in the 1950s towards the south, north and west of China, and located in the south of the Yangtze River and South China after the 1990s. The FA in the western provinces was continuously on the rise since the 1950s. There are two characteristics for the future flood pattem in China. The pattern of ＂flood in the south and drought in the north＂ depends on the north-south shift of the maximum rainfall region in eastern China. The flood intensification to the west of Hu Huanyong＇s line mainly results from the increase of rainfall, extreme precipitation and the melting of glaciers under the background of human activity magnification.

ANALYSIS ON EFFECT OF SOUTH ASIA HIGH ON MID-SUMMER EXTREME DROUGHT AND FLOOD IN SICHUAN-CHONGQING REGION

NCEP/NCAR data are utilized to analyze an extreme flood year(1998) and an extreme dry year(2006) in the Sichuan-Chongqing region(SCR) and the results are as follows. The positive divergence of South Asia High(SAH) is stronger in the flood year; the position of the ridge line of SAH is southward compared with the annual average; Western Pacific Subtropical High(WPSH) extends westward and its ridge line is southward. In the drought year, the positive divergence of SAH is weaker, its ridge line is northward, and the position of WPSH is also northward. As shown in the dynamics, in drought(flood) years, negative(positive) vorticity advection in the upper atmosphere can cause the atmosphere to ascend(descend), and anomalous circulation of SAH displays divergence(convergence), and anomalous circulation of the lower atmosphere shows convergence(divergence). Thermal structure of the atmosphere shows that there is warm(cold) temperature advection in the lower atmosphere, and the vertical distribution of diabetic heating causes SAH's local circulation to display convergence(divergence) and affects vertical motion of the lower atmosphere circulation eventually. To some extent, the two extreme years in the SCR is closely related to the vertical motion of atmosphere circulation and the variation of such vertical motion is caused by differences of interactions between SAH and lower atmosphere circulations.

NUMERICAL SIMULATION OF THE IMPACT OF LATENT HEAT FLUX ANOMALY IN THE TROPICAL WESTERN PACIFIC ON PRECIPITATION OVER SOUTH CHINA IN JUNES

Based on composite analysis and numerical simulations using a regional climate model(RegCM3), this paper analyzed the impact of the LHF anomaly in the tropical western Pacific on the precipitation over the south of China in June. The results are as follows.(1) Correlation analysis shows that the SC precipitation in June is negatively correlated with the LHF of the tropical western Pacific in May and June, especially in May. The SC precipitation in June appears to negatively correlate with low-level relative vorticity in the abnormal area of LHF in the tropical western Pacific.(2) The LHF anomaly in the tropical western Pacific is a vital factor affecting the flood and drought of SC in June. A conceptual model goes like this: When the LHF in the tropical western Pacific is abnormally increased(decreased), an anomalous cyclone(anticyclone) circulation is formed at the low-level troposphere to its northwest. As a result, an anomalous northeast(southwest) air flow affects the south of China, being disadvantageous(advantageous) to the transportation of water vapor to the region. Meanwhile, there is an anomalous anticyclone(cyclone) at the low-level troposphere and an anomalous cyclone(anticyclone) circulation at the high-level troposphere in the region, which is advantageous for downdraft(updraft) there. Therefore a virtual circulation forms updraft(downdraft) in the anomalous area of LHF and downdraft(updraft) in the south of China, which finally leads to the drought(flood) in the region.

Relation among Summer Rainfall in South Shandong and High Pressure in South Asia and Atmospheric Circulation

[Objective] The aim was to study the relation among summer rainfall in south Shandong and high pressure in South Asia and atmospheric circulation.[Method] Taking the precipitation in south Shandong along the Yellow River and Huaihe River,using the NCEP/NCAR data and summer rainfall data in south Shandong in summer from 1961 to 2005,the characteristics of high pressure in South Asia and atmospheric circulation in drought year and flood year in summer in south Shandong Province were expounded.The mechanism of 100 hPa pressure in South Asian influencing precipitation in south Shandong Province was discussed.The interaction of different equipment,different altitude and different system of atmosphere circulation in low and high layer was expounded.[Results] The first mode of EOF decomposition of precipitation in summer in south Shandong Province explained above 63% variances and reflected universal form of precipitation.The difference of central position of the central position of height field of high pressure in South Asia in drought and flood year was small.But the wind field center was inconsistent.As the area of SAH was smaller and its eastern ridge line stretched to the Western Pacific between the middle of south Shandong and Changjiang Estuary,flood summer occurred when there was an unusual cyclone lied in the east of 90° E and south of Lake Baikal.The area of SAH was larger and its eastern ridge line stretching to the Western Pacific over Changjiang Estuary,drought summer occurred,when there was an unusual anticyclone lied in the east of 90° E and south of Lake Baikal.SAH and summer rainfall also had close relationship with Tele-connection Patterns over the Eurasia continent and EAP Tele-connection.When the height anomaly was in '+-+' form in the north of the Caspian Sea,around Lake Baikal and Kamchatka,and when the height anomaly in East Asia-West Pacific area was in '-+' form from low altitude to high altitude,there was much precipitation in summer;and conversely,it was drought in summer in south Shandong.[Conclusion] It provided the oretical basis for summer rainfall in south Shandong.

DIAGNOSIS OF SUMMERTIME FLOODS/DROUGHTS AND THEIR ATMOSPHERIC CIRCULATION ANOMALIES OVER NORTH CHINA

Proposed are a set of new regional flood/drought indices and a scheme of grading their severity whereby 1951-2000 summer wet/dry events are investigated for North China (NC) in terms of 160 station monthly precipitation data from NCC (China National Center of Climate).Results suggest that 7 heavy droughts during 1951-2000 are 1965,1968,1972,1980,1983,1997 and 1999,while 6 heavy floods are 1954,1956,1959,1964,1973 and 1996. Based on 1951-2000 summer flood/drought severity graded by the new scheme,atmospheric circulation characteristics associated with the disasters over the NC are addressed in terms of monthly NCEP (National Centers for Environmental Prediction) reanalysis of geopotential heights,winds,surface temperature and PW (precipitable water).Evidences suggest that prominent anomalies benefiting to the heavy droughts occur over the Northern Hemisphere.The variations over middle-high latitudes especially the negative ones on Ural Mountain to western Siberia deepen the normal trough there and are indicative of stronger than normal cold air activity. At middle latitudes,remarkable positive anomalies present on the south to Baikal lead to the fact that the normal ridge shifts eastward over NC concomitant with anomaly sinking motion in the whole troposphere,which is helpful for the maintenance of the continent high.And the opposed ones over Korea and Japan force the trough moving eastward running against northwestward shifting of the western Pacific subtropical high.In addition,the anomaly west-east pressure gradient at middle latitudes profits northerly flow there.The southerly monsoon flow at low levels is weaker than normal with weak East Asian summer monsoon,and the related water vapor transportation is also weak with deficit PW over NC.Besides,sea surface temperature (SST) rises in the equatorial eastern and central Pacific and associated convective region moves to the east accordingly companied with weak Walker circulation in the droughts.And the opposed situations will occur during the floods.

Simulation and Analysis about the Effects of Geopotential Height Anomaly in Tropical and Subtropical Region on Droughts or Floods in the Yangtze River Valley and North China

Previous study comes to the conclusion： based on the anomalies of the South Asian high （SAH）, 100-hPa geopotential height, and 100-hPa circulation over tropical and subtropical regions, we can predict precipitation anomaly in the Yangtze River Valley and North China. To test its validity, a series of experiments have been designed and operated, which include controlled experiment, sensitivity experiment （which has added anomalies into 100-hPa geopotential height and wind field）, and four-composite experiments. Experiments based on the composed initial field such as EPR-CF, EPR-CD, EPR-HF, and EPR-HD, can reproduce the floods or droughts in the Yangtze River Valley and North China. It suggests that anomalies of the SAH, 100- hPa geopotential height, and circulation over tropical and subtropical regions may probably imply summer precipitation anomalies in the two regions. Sensitivity experiment results show that anomalies of the SAH, 100-hPa geopotential height, and southwest flow in the previous period is a signal of droughts or floods for the following summer in the Yangtze River Valley and North China. And it is also one of the factors that have impact on summer precipitation anomaly in the two regions. Positive anomaly of 100-hPa geopotential height and the anomalous intensifying of the SAH and southwest flow will induce floods in the Yangtze River Valley and droughts in North China; while negative anomaly of 100-hPa geopotential height and anomalous weakening of the SAH and southwest flow will induce droughts in the Yangtze River Valley and floods in North China.

Relationships Between Anomalies of Land-Sea Thermal Contrast in North Africa and Summer Flood and Drought Across the Jianghuai Region of China

The flood and drought across the Yangtze and Huaihe River（Jianghuai） areas are frequent in summer, especially in June and July.Therefore,predicting the summer flood and drought in the Jianghuai region is always one of the key topics concerned by meteorologists in China.Previous studies focused more on the skin temperature anomalies in a local area,and paid less attention to the connections between the anomalies of land-sea thermal contrast in remote continents and the summer flood and drought in Jianghuai areas of China.By using the US NCEP/NCAR monthly mean reanalysis data and the rainfall data at 743 stations in China,based on the interdecadal variation characteristics of the flood and drought index（FDI） during 51 yr （1954-2004） in the Jianghuai region of China in summer,the North African areas have been selected as the key regions for the correlation analysis.The results show that the surface temperature anomalies in the key regions have good continuity in winter,and the winter North Atlantic Oscillation（NAO） is perhaps one of the important factors that bring about the continuity of the anomalies.By a singular value decomposition（SVD） analysis between the skin temperature in the previous winter in North Africa and the summer rainfall in the Jianghuai region,it is found that when the North African continent is colder（warmer） and its northwestern sea is warmer（colder）,the rainfall increases（decreases） in the Jianghuai region in summer generally.Further analysis finds that the anomaly of the surface temperature contrast between sea and land in North Africa has a good indication for the summer flood and drought in the Jianghuai areas of China.Therefore,a sea and land thermal contrast index（SLTCI） is defined to reflect the intensity of the large-scale land-sea thermal contrast.A positive correlation between the SLTCI in North Africa and the summer FDI in Jianghuai areas is identified,and it can well indicate the extreme flood and drought situations in the Jianghuai region of China.

2-D NUMERICAL SIMULATION OF FLOODING EFFECTS CAUSED BY SOUTH-TO-NORTH WATER TRANSFER PROJECT

Since the General Channel designed for the South-to-North Water Transfer Project in China has to cross many rivers and streams flowing from west to east, there are potentially serious effects additional flooding on the western side of the project alignment. Therefore, a 2-D numerical model for forecasting basin flood disasters was established and verified using historical flood data. The model was applied to researching the interaction between the proposed Project and flooding events for 5 streams in the Anyang River reach as a representative case study. Simulated results indicate that the model could correctly forecast the flood, submerged area and depths, and water surface elevations along the left side of the channel. The discharge capacity and location of hydraulic structures in the transfer canal alignment were analyzed. Then adjustments to the dimensions and positioning of proposed hydraulic structures were recommended at intersections, especially the addition of a channel to transfer flood water from one stream to another, which can effectively limit the sluice and protect the Anyang City from flooding.

南水北调中线工程水源区和受水区的降水时空变化特征及丰枯遭遇

[目的]基于1950—2022年时间序列ERA5再分析降水数据,探究南水北调中线工程水源区和受水区降水量时空变化特征,旨在保障跨流域调水的可行性和持续性,为南水北调中线工程水资源调度和运行管理提供科学参考。[方法]采用Mann-Kendall检验、小波分析、云模型以及Copula函数等方法,分析水源区和受水区降水变化趋势以及丰枯遭遇特征。[结果]①1950—2022年南水北调工程水源区和受水区分别按照年降水量34.75和39.35mm/(10a)幅度减少,水源区年降水量存在43a的主周期,受水区年降水量存在55a的主周期。②云模型隶属云图夏季降水量分布不均且离散程度较高,而冬季分布均匀且较集中。③调水不利的组合汛期和非汛期遭遇频率分别为23.15%和25.92%。[结论]水源区和受水区年降水量均呈现明显减少趋势,水源区丰枯交替变化显著;南北丰枯异步概率大于丰枯同步,同枯频率不高,有利于调水工程的安全性和可靠性。研究结果可为制定南水北调中线工程相关政策和技术指导提供理论依据,有助于工程的规划和管理,提高水资源利用效率和可持续性。

南水北调中线工程交叉河流防洪“四预”应用

为满足南水北调中线工程典型交叉河流防洪管理需求,确保中线工程防洪安全,构建索河交叉河流数据底板,以及洪水预报、洪水演进、风险分析等模型,结合2022和2023年汛期典型降雨洪水对模型参数进行率定,基于2021年郑州“7·20”暴雨对现状索河交叉河流进行预演分析,结果表明索河渡槽槽身存在阻水风险,中线工程安全风险较大。通过对防洪“四预”应用深入研究,实现南水北调中线工程交叉河流降水—产流—汇流—演进的全过程模拟,研究成果可进一步提升南水北调中线工程防灾减灾能力,为沿线交叉河流防洪应用提供参考。

HEC-RAS模型在南水北调中线洪水推求中的运用

为提高南水北调中线工程防洪度汛科学性,提升防汛抢险决策水平,有效计算交叉河流过流情况,最大限度减少南水北调交叉河道洪水造成的人员伤亡和工程损失,保障工程正常运行,开展南水北调中线工程交叉河道水文计算工作非常有必要。以南水北调中线交叉河道净肠河倒虹吸河道计算为例,运用HEC-RAS水动力模型对净肠河倒虹吸河道水文复核,根据24小时洪水情况,对交叉河道断面部位洪水流量及水位进行计算,复核验算结论超标洪水问题,为南水北调工程防洪度汛提供信息支撑,以期对其他类似工程科学预防,避免水毁、减小损失能够有所帮助。

汉江流域骤旱演变规律及其对南水北调中线工程的响应分析

通过识别汉江流域骤旱事件,分析了汉江流域在南水北调中线工程实施前后其骤旱事件特征的差异,并且使用偏最小二乘回归(Partial Least Squares Regression, PLSR)、支持向量机(Support Vector Machine, SVM)和随机森林(Random Forest, RF)3种机器学习方法建立了骤旱历时回归模型,定量分析了南水北调中线工程对骤旱历时的影响。结果表明:(1)南水北调中线工程实施后,汉江流域骤旱频次从平均每年0.7次上升至0.9次,骤旱历时从平均每年24.6 d上升至34.5 d,但在空间上,上游骤旱历时显著延长,中游增加幅度减弱,下游骤旱历时却缩短;(2)骤旱爆发历时在空间分布上由5~6 d均匀分布变为上下游不均分布,上游爆发历时缩短至3~5 d,而中下游地区延长至7~9 d;(3)随机森林对骤旱历时模拟效果最好,气候条件对骤旱历时影响最大,相对重要性为0.34,其中降水为0.14,净辐射为0.12,相对湿度为0.08,南水北调中线工程延长了汉江流域中下游地区的骤旱历时,相对重要性为0.12。

南水北调东线工程江苏段沿线洪水风险区划

南水北调东线工程承担供水任务,兼具防洪除涝、调水、航运等多种功能,为提高南水北调东线工程江苏段沿线洪水风险管理水平,掌握沿线洪水风险分布情况,开展了洪水风险区划。以南水北调东线工程江苏段沿线区域为研究对象,分析南水北调东线工程江苏段沿线区域洪涝特性,开展区划单元划分、区划分析方案、区划分析模型构建及风险要素分析计算,采用综合风险度计算方法,将研究区域洪水风险等级划分为极高、高、中和低4个等级。研究结果表明,极高、高风险区面积共计3481 km2,占比21.20%,主要分布在洪泽湖周边、白马湖高宝湖区洼地区域及里下河区腹部湖泊湖荡周边等区域。成果反映了区域洪水风险总体状况,明确了区域内部洪水风险程度,支撑了洪水防灾减灾规划、防汛调度管理与预案制定。

华南汛期旱涝急转特征及其与海温异常的关系

【目的】探究华南汛期旱涝异常时空特征,旱涝急转前/后大气环流和水汽输送的变化,及其与赤道太平洋、南海海温异常的关系。【方法】基于华南地区129个站点的降水资料、NCEP/NCAR再分析资料和海温资料,采用合成分析、相关分析等统计学方法,拉格朗日后向气流轨迹模式(HYSPLIT_4.9),分析华南汛期旱涝急转特征及其成因。【结果与结论】涝转旱事件旱期相比于涝期,西太平洋副热带高压西伸加强,水汽辐散,来源于洋面的水汽贡献率减少,不利于降水的形成。旱转涝期涝期相较于旱期,副热带高压强度无太大变化,但副热带高压位置南落至华南地区南侧,水汽辐合,来自洋面的水汽通道向西偏移至印度洋,来自印度洋的水汽贡献增多,西南到偏南风向华南输送充足水汽,有利于降水产生。赤道中东太平洋海温正异常引起华南地区气旋式环流异常,有利于将暖湿气流输送至华南地区,使得华南地区降水异常偏多,而ENSO正位相减弱渐变为负位相的过程中,华南地区降水相应减少,会发生涝转旱事件。反之,赤道中东太平洋海温负异常易造成旱转涝事件的发生。

超标准暴雨洪水条件下南水北调中线工程沿线各单元风险评估

对南水北调中线工程沿线639个单元工程风险进行评估,并对灌渠河、贾河、金水河、石门河和沙沟5个典型单元进行重点分析。结果表明:中线工程沿线639个评估单元中,风险事件可能性指数为1的单元占比最大,风险等级以一般风险为主;在5个典型单元中,极端降雨移用(植)后在交叉断面处形成的洪水基本上呈现流域面积越小洪水量级越大的特点。此外,灌渠河排水倒虹吸发生渠顶漫溢的风险低;贾河梁式渡槽、沙沟排洪渡槽和石门河渠道倒虹吸均存在较高的工程风险;金水河渠道倒虹吸在超标准洪水下河道冲刷满足要求,工程风险较低。研究成果对提前部署南水北调中线工程防御超标准暴雨洪水具有指导意义。

南水北调东线二期半专道输水方案对淮河入江水道行洪能力影响研究

淮河入江水道是淮河下泄洪水的主要通道,承担来自淮河中上游约66%~79%的洪水,是淮河行洪的安全阀。为了研究南水北调东线二期半专道输水方案对淮海入江水道行洪的影响,建立北抵洪泽湖三河闸、南至长江三江营的一二维水动力数学模型,在设计洪水工况下分析工程方案对入江水道行洪的影响。结果表明:工程实施后,入江水道上段、下段行洪水位低于设计值;中段邵伯湖由于东侧修建隔堤侵占河道,行洪时部分区域水位有少量抬高,当对新民滩南部实施切滩补偿后,可有效消除其影响。

南水北调备用水源地上游水生态环境综合治理探讨

南召县地处河南省西南部,伏牛山南麓,南阳盆地北缘,南水北调备用水源地鸭河口水库上游,属于长江流域。由于鸭河口水库上游河道存在防洪标准低、水体污染、水环境差等问题,通过实施水系连通、河道防洪治理、河道生态修复、水土保持及水源涵养、污水处理及管网建设等综合治理措施,提高河道防洪能力及河湖生态修复能力,保障水质安全,创建宜居生态,实现了流域水生态环境可持续发展和南水北调后续高质量发展。

中国东部夏季主要降水型的年代际变化及趋势分析

针对中国东部夏季降水存在着20世纪50和60年代为“南旱北涝”、80和90年代为“南涝北旱”的相反形势,该文应用小波分析等方法对华北和长江中下游地区近54 a来的夏季降水进行分析,发现二者都具有不同周期的年代际变化。对于周期小于24 a的年代际变化,其morlet小波分析表明两地夏季降水的位相关系并不是固定的。对于更长的时间尺度,用正交小波分析了周期大于28 a的年代际变化,这种长周期的年代际变化能较好地体现出“南旱北涝”和“南涝北旱”两种形势,说明两地降水还存在着更长时间尺度的准周期变化。对于两地降水的这种长周期变化,分别选用不同位相的17 a为代表进行海温、环流形势的合成分析,对比发现两阶段所对应的海温、环流形势具有极明显的差异。最后,用近期的海温、环流形势与上述两种位相的海温、环流形势进行对比,讨论了未来降水型的可能演变趋势。

东亚副热带西风急流位置变化及其对中国东部夏季降水异常分布的影响

利用1951-2004年美国国家环境预报中心（NCAR/NCEP）再分析候平均资料和同期中国740站日降水资料,分析了东亚副热带西风急流轴位置的变化趋势及其对中国东部夏季降水分布的影响,结果表明：（1）夏季7-8月东亚副热带西风急流位置和形态在1975-1980年间出现转折,1980年后西风急流中心逐渐向西移动的同时伴随有西风急流向南偏移。（2）在1980年以后华北地区降水量减少和降水强度减弱,雨季开始时间推迟、雨季变短,而长江中下游地区入梅提前、梅雨期变长,降水量增加,从而形成南涝北旱的降水分布形势。（3）1957-1964年华北典型多雨时期,西风急流呈纬向分布,在华北地区有高低空急流耦合,强辐合上升区正好位于华北,并有充足的水汽条件供应,使得华北降水偏多。而1980-1987年和1997-2002年华北地区为典型少雨时期,1980-1987年西风急流中心位置偏南和1997-2002年急流位置显著偏西,在华北地区均无高低空急流耦合的环流形势,水汽辐合区位于长江流域,强辐合上升区位置在30°N以南区域,有利于江南地区降水增加而华北地区少雨,这表明西风急流位置变化导致环流调整对中国东部降水分布有显著影响。因而,在讨论东亚副热带西风急流位置与中国东部地区降水异常的关系时,不仅要考虑西风急流南北位置变化,还需要综合分析西风急流的东西位置和形态的变化。