Enhanced rainfall in North China in July 2021:Role of the North Pacific SST gradient

2021年7月,华北地区的降水量突破1980年以来的极值,这与太平洋海温异常的经向梯度相吻合.研究表明,西北太平洋正海温异常与热带中太平洋负海温异常之间的梯度是2021年7月华北地区降水异常偏多的主要原因,该梯度使得西北太平洋反气旋及其南部的气旋北移,汇合的偏东风将大量水汽从海洋输送到华北地区,使得华北地区降水偏多.同时,太平洋海温异常的经向梯度可能在对流层低层触发异常的反向哈德利环流,伴随着南风异常,使得季风环流增强,雨带偏北.

A Possible Linkage in the Interdecadal Variability of Rainfall over North China and the Sahel

The instrumental records of precipitation, including some historical documentary evidence, show that the rainfall in North China during the rainy season (July and August) exhibits an interdecadal variability similar to the Sahelian rainfall. Both these areas exhibited a weak interdecadal rainfall variability prior to the 1950s, and experienced a long-lasting drought since the 1960s, with two rainfall decreasing transitions, one around the year 1965 and another in the late 1970s. NCEP/NCAR reanalysis data are used to analyze the associated changes in atmospheric circulation during the second decrease transition. The changes of local atmospheric circulation at the end of the 1970s, at both lower and upper levels, contribute to the less precipitation in North China and the Sahel.

Impacts of Two Types of Northward Jumps of the East Asian Upper-tropospheric Jet Stream in Midsummer on Rainfall in Eastern China

The East Asian upper-tropospheric jet stream （EAJS） typically jumps north of 45~N in midsummer. These annual northward jumps are mostly classified into two dominant types： the first type corresponds to the enhanced westerly to the north of the EAJS＇s axis （type A）, while the second type is related to the weakened westerly within the EAJS＇s axis （type B）. In this study, the impacts of these two types of northward jumps on rainfall in eastern China are investigated. Our results show that rainfall significantly increases in northern Northeast China and decreases in the Yellow River-Huaihe River valleys, as well as in North China, during the type A jump. As a result of the type B jump, rainfall is enhanced in North China and suppressed in the Yangtze River valley. The changes in rainfall in eastern China during these two types of northward jumps are mainly caused by the northward shifts of the ascending air flow that is directly related to the EAJS. Concurrent with the type A （B） jump, the EAJS-related ascending branch moves from the Yangtze-Huai River valley to northern Northeast （North） China when the EAJS＇s axis jumps from 40~N to 55~N （50~N）. Meanwhile, the type A jump also strengthens the Northeast Asian low in the lower troposphere, leading to more moisture transport to northern Northeast China. The type B jump, however, induces a northwestward extension of the lower-tropospheric western North Pacific subtropical high and more moisture transport to North China.

INSTABILITY OF THE TELECONNECTION OF SUMMER RAINFALLS BETWEEN NORTH CHINA AND INDIA

Summer rainfall variations in North China closely relate to that in India. It seems that an alternation of signs of“＋, -, ＋” exists in the geographical pattern of the correlation in summer rainfall from North China to India through the Tibetan Plateau. However, it appears that the teleconnection of summer rainfall variations between North China and India is unstable. Over 1945 - 1974, the correlation coefficient （hereafter as CC） is as large as 0.7. In contrast, the CC is about -0.3 over 1827-1856. Further studies, based on observations starting from 1813, showed that the correlation is strong when summer rainfalls in both North China and India are large, and vice versa. In order to find what induce the change of the teleconnection, variations of summer rainfall in both North China and India, mean sea surface temperature （SST） in the eastern equatorial Pacific and the frequency of ENSO events were examined in relation to the change of the teleconnection. The result showed that the teleconnection appears weak when the mean SST is high and the frequency ofLa Nifia events is low; the teleconnection is strong when the mean SST is low and the frequency ofLa Nina events is high. At last, it is notable that La Nifia happens in only 3 years during the recent 30 years from 1976 to 2005 and the teleconnection becomes weak too.

Slope runoff study in situ using rainfall simulator in mountainous area of North China

Simulated rainfall is a valid tool to examine the runoff generation on the slope.13 simulated rainfall experiments with different rainfall intensities and durations are completed in a 5 m ×10 m experimental plot in mountainous area of North China.Simultaneously,rainfall,surface runoff,soil-layer flow,mantel-layer flow and soil moisture are monitored respectively.From the results,it is found that the hydrographs in all layers have the characteristics of rapid rise and fall.The recessions of surface flow and soil-layer flow are much faster than that of mantel-layer flow.Surface flow,the main contributor,makes up more than 60% of the total runoff in the study area.It even exceeds 90% in the cases of high intensity rainfall events.Runoff coefficient(ratio of total runoff to rainfall amount) is mainly influenced by rainfall amount,rainfall intensity and antecedent soil moisture,and the relationship can be well expressed by a multiple linear regression function α = 0.002P + 0.182i + 4.88Wa-0.821.The relation between the rainfall intensity and the lag time of three flows(surface runoff,soil-layer flow and mantel-layer flow) is shown to be exponential.Then,the result also shows that the recession constant is 0.75 for surface runoff,is 0.94 for soil-layer and mantel-layer flow in this area.In this study area,the dominant infiltration excess runoff is simulated by Horton model.About 0.10 mm/min percolation is observed under the condition of different rainfall intensities,therefore the value is regarded as the steady infiltration rate of the study area.

The ENSO’s Effect on Eastern China Rainfall in the Following Early Summer

ENSO＇s effect on the rainfall in eastern China in the following early summer is investigated by using station precipitation data and the ERA-40 reanalysis data from 1958 to 2002. In June, after the E1 Nifio peak, the precipitation is significantly enhanced in the Yangtze River valley while suppressed in the Huaihe River-Yellow River valleys. This relationship between ENSO and the rainfall in eastern China is established possibly through two teleconnections： One is related to the western North Pacific （WNP） anticyclonic anomaly in the lower troposphere leading to enhanced precipitation in the Yangtze River valley, and the other is related to the southward displacement of the Asian jet stream （AJS） in the upper troposphere resulting in suppressed precipitation in the Huaihe River-Yellow River valleys. This southward displacement of the AJS is one part of ENSO＇s effect on the zonal flow in the whole Northern Hemisphere. After the E1 Nifio peak, the ENSO-related warming in the tropical troposphere persists into the following early summer, increasing the meridional temperature gradient and through the thermal wind balance, leads to the enhancement of westerly flow in the subtropics south of the westerly jet stream and results in a southward displacement of the westerly jet stream.

Revisiting the Second EOF Mode of Interannual Variation in Summer Rainfall over East China

The second EOF（EOF2） mode of interannual variation in summer rainfall over East China is characterized by inverse rainfall changes between South China（SC） and the Yellow River-Huaihe River valleys（YH）.However,understanding of the EOF2 mode is still limited.In this study,the authors identify that the EOF2 mode physically depicts the latitudinal variation of the climatological summer-mean rainy belt along the Yangtze River valley（YRRB）,based on a 160-station rainfall dataset in China for the period 1951-2011.The latitudinal variation of the YRRB is mostly attributed to two different rainfall patterns：one reflects the seesaw（SS） rainfall changes between the YH and SC（SS pattern）,and the other features rainfall anomalies concentrated in SC only（SC pattern）.Corresponding to a southward shift of the YRRB,the SS pattern,with above-normal rainfall in SC and below-normal rainfall in the YH,is related to a cyclonic anomaly centered over the SC-East China Sea region,with a northerly anomaly blowing from the YH to SC;while the SC pattern,with above-normal rainfall in SC,is related to an anticyclonic anomaly over the western North Pacific（WNP）,corresponding to an enhanced southwest monsoon over SC.The cyclonic anomaly,related to the SS pattern,is induced by a near-barotropic eastward propagating wave train along the Asian upper-tropospheric westerly jet,originating from the mid-high latitudes of the North Atlantic.The anticyclonic anomaly,for the SC pattern,is related to suppressed rainfall in the WNP.

What Caused the Differences between the July 2023 and August 1996 Extreme Rainfall Events in North China under Similar Synoptic Background?

This study examined the rainfall characteristics and related synoptic processes of two extreme rainfall events that affected North China during 29 July–1 August 2023(“23·7”rainstorm)and 3–5 August 1996(“96·8”rainstorm),respectively.A stable dual-typhoon circulation pattern was observed in both rainstorm events.The surviving vortex of a landed typhoon,slowly approaching the rainstorm region,was blocked by a high-pressure system as it moved northwestward.Meanwhile,the second typhoon over the western Pacific Ocean facilitated remote northward transport of moisture.The low-level jet between the surviving vortex and the western Pacific subtropical high relayed moist warm air from the area of the South China Sea and western Pacific into the rainstorm region.Although the circulation patterns are similar,the stratification conditions,driving factors,and moisture budget of the two rainstorms differed during the main period of rainfall.The“23·7”rainstorm was categorized as warm-sector rainfall,as a result of the lifting of warm moist air over the eastern foothills of Taihang Mountains.In comparison with the situation of the“96·8”rainstorm,the surviving vortex of the“23·7”rainstorm traveled further northeastward and directly impacted the occurrence and progression of the rainfall,leading to relative northward displacement of the rainfall center,while the stronger net inward moisture flux caused greater regional average rainfall.The“96·8”rainstorm was broadly analogous to precipitation of a cold front,and the rainfall center was observed in the convergence area of warm and cold air masses before the mountains;the surviving vortex did not exert direct impact on the rainfall;and the more unstable stratification led to stronger hourly rainfall.The results derived through comparison of the two rainstorms could serve as valuable scientific reference for operational forecasting of heavy rainfall under similar environmental conditions over North China.

“23.7”华北特大暴雨过程小时强降水时空分布特征

由于西北太平洋副热带高压、2023年第5号台风“杜苏芮”、第6号台风“卡努”等共同作用,2023年7月29日08时—8月2日08时(北京时)华北地区发生极端特大暴雨,造成了重大社会影响。利用中国气象局国家气象信息中心提供的国家级气象站逐小时降水资料,分析了此次特大暴雨过程中的小时强降水(Hourly Heavy Rainfall,HHR)时空分布特征,并对不同历时类型强降水事件(Heavy Rainfall Event,HRE)的统计特征进行了对比。结果表明:1)此次特大暴雨HHR强度高、局地性明显,其对总降水量的贡献超过20%,北京西部、河北中部和西南部等太行山东麓为HHR降水量大值区和降水频次活跃区,双台风将水汽源源不断地输送到华北平原,受到太行山等山脉阻挡抬升,利于HHR增幅和持续。2)HHR降水量、降水频次经历6次峰值后减弱,其中第3次峰值时段的HHR降水量最大、降水频次最多、持续时间最长;而在“杜苏芮”残涡螺旋影响、暖式切变线和偏东风影响以及偏南或西南急流影响的3个主要降水阶段中,“杜苏芮”残涡螺旋影响阶段HHR最为活跃,共发生257次HHR,HHR最大降水量达73.5 mm。3)在3种类型HRE中,长历时(>12 h)最多,占比54.5%,短历时(1~6 h)次之,占比32.9%,中历时(7~12 h)最少,占比12.6%;长历时HRE降水量多>180 mm,短历时、中历时HRE的降水量多为[20,60)mm;HRE的最大降水量表现为短历时<中历时<长历时,而最大降水强度表现正相反,3种历时HRE的最大小时降水量多为[20,30)mm。4)不同类型HRE降水量、频次和平均降水强度的空间分布显示,长历时HRE因频次高于短历时和中历时,加之历时长,其降水量也高于后两种类型,北京、河北等地是长历时HRE降水量和频次大值区。

华北平原及沿山和沿海五个城市夏季短历时降水过程的日变化分析

利用华北平原294个国家级气象站1980—2022年夏季(6—8月)逐时降水资料,分析了华北平原夏季短历时降水过程的日变化时、空分布特征,结合地面气温和风的日变化,综合分析了4个沿山城市(北京、石家庄、济南、郑州)和1个沿海城市(天津)在城市发展前期(1980—1989年)和后期(2013—2022年)短历时降水过程日变化特征及差异。结果表明:华北平原短历时降水过程整体呈现傍晚单峰的日变化特征;平原站点的达峰时间自东南向西北推后。5个主要城市在城市发展后期,短历时降水过程的日振幅都出现不同程度的减小;沿山城市短历时降水过程日峰值出现时间提前了1—2 h,而沿海城市天津无明显峰值时间,且高值时段提前;沿山城市50 mm以上短历时降水过程概率均有所升高,其中北京和石家庄40 mm以上的降水过程概率分别升高了2倍和5倍,而沿海城市天津40 mm以上的降水过程概率下降了43%。华北平原短历时降水过程的日变化与局地地形的热、动力作用和地面盛行风有关。城市局地增温效应导致沿山、沿海城市与周围地形热力差异更加明显,增强了沿山城市的山风和沿海城市的海风;同时,城市增温导致城市午后地面风辐合增强。近地面层上述热力差异变化导致的风场变化可能是沿山城市短历时降水过程日峰值提前、沿海城市短历时降水过程集中时段变为白天的重要原因。

CMIP6模式对中国北方季风区盛夏降水的模拟能力评估

基于站点降水观测资料和多种模式评估方法,评估了参加国际耦合模式比较计划第六阶段(CMIP6)的30个全球模式对1961~2014年我国北方季风区盛夏(7~8月)降水气候态和主模态的模拟能力,结果表明:(1)大多数模式均能再现北方季风区盛夏气候态降水呈东南向西北递减的空间分布特征,对空间分布模拟能力最好的4个模式为MPI-ESM-1-2-HAM、MPI-ESM1-2-HR、MPI-ESM1-2-LR和E3SM-1-0。(2)综合衡量对气候态、第一模态及其年际变率模拟能力最好的4个模式为NESM3、E3SM-1-0、GFDL-ESM4和MPI-ESM1-2-LR。(3)综合衡量对气候态、第二模态及其年际变率模拟能力最好的5个模式为EC-Earth3-Veg、EC-Earth3、E3SM-1-0、NESM3和MPI-ESM1-2-HR。(4)多模式集合平均的模拟能力好于大部分模式。

“23·7”华北极端暴雨精细特征和天气学成因分析

2023年7月29日至8月1日,华北京津冀地区遭受极端特大暴雨袭击,导致区域性洪涝灾害,造成重大经济损失和人员伤亡。为探明此次“23·7”华北极端降水过程的基本特征和形成原因,基于地面自动气象站分钟级降水资料、风廓线雷达和雨滴谱仪观测资料及ERA5再分析数据,揭示了“23·7”华北极端暴雨的精细特征和天气学成因。结果表明:(1)此次降水过程持续时间长、累计雨量极大,特大暴雨落区、突破降水量历史极值的国家级观测站站点密集分布在京津冀太行山近山地区,降水呈现显著极端性、区域性差异和阶段性变化。河北和北京太行山东侧迎风坡地区出现持续性、强度相对稳定的强降水过程,雨滴谱分布偏向于高雨滴数密度、小雨滴直径的海洋型对流降水;京津冀平原地区过程雨量明显小于山区,但中尺度对流雨带活跃,降水阵性、对流性特征明显。(2)在“杜苏芮”台风残涡、西太平洋副热带高压、大陆高压等天气系统协同作用下,中层位势高度场出现罕见的“北高南低”+“西低东高”的稳定天气形势。北上台风“杜苏芮”残涡在高压坝以及太行山、燕山地形影响下移动缓慢,残涡中心北侧的地形障碍流与低空东南风之间形成稳定倒槽,京津冀地区出现持续性低层辐合,同时配合来自南海季风和东部洋面台风“卡努”的双路水汽输送,导致京津冀地区出现长历时、区域性极端暴雨过程。(3)在阶段性降水发展过程中,与台风残涡相关的倒槽、暖切变线、低空急流等精细结构调控了中尺度对流系统的组织和发展特征,7月31日早晨至上午京津冀地区出现超过22 m/s的超低空东南风急流,配合对流和地形抬升等因素影响,降水强度明显增强,北京西部山前出现超100 mm/h的极端雨强。此次极端降水事件中的地形影响、中小尺度特征以及极端暴雨可预报性等问题尚待更深入研究。

1632年华北地区暴雨洪涝事件研究

以历史文献为主要代用资料,重建了1632年华北地区暴雨洪涝事件的雨情、水情、灾情等,从而识别出1632年8月河南地区暴雨事件的时空范围,并与有器测记录的2021年河南极端降水事件进行对比。1632年华北地区大规模降水始于7月17日,从6月18日—11月11日,整个华北不同区域皆存有长时间降雨记录,且有多次强降水记录,造成黄淮地区雨涝,并使得黄淮交溃,对社会经济造成较大影响。梳理降水和洪涝的过程,区分了本地降水与客水的分布。将1632年8月河南暴雨洪涝与2021年7月河南极端降水事件对比发现,强降水位置相似,降水前后都有两个台风输送水汽,且1632和2021年华北地区夏季降水皆受到拉尼娜的影响,整体偏涝。

Winter Asia Jetstream and Seasonal Precipitation in East China

The monthly mean geostrophic wind fields for January during 1951 - 1990 period are calculated by using data of500 hpa monthly mean height. The relation between Asia jetstream in winter and the important seasonal precipitationin East China is analysed. The analysis shows that the south branch of jetstream is stronger (weaker) in winter, therainfall will be more (less) than normal in the subsequent spring in South China, and summer rainfall in North Chinawill be more (less). too; these important rainy seasons are related to each other; the indian summer monsoon is notonly related to the summer rainfall in North China, but also related to the spring rainfall in South China and thesouth branch of jetstream in winter.

Impact of El Nio on atmospheric circulations over East Asia and rainfall in China： Role of the anomalous western North Pacific anticyclone

This paper presents a review on the impact of El Nio on the interannual variability of atmospheric circulations over East Asia and rainfall in China through the anomalous anticyclone over western North Pacific(WNPAC). It explains the formation mechanisms of the WNPAC and physical processes by which the WNPAC affects the rainfall in China. During the mature phase of El Nio, the convective cooling anomalies over western tropical Pacific caused by the weakened convections trigger up an atmospheric Rossby wave response, resulting in the generation of the WNPAC. The WNPAC can persist from the winter when the El Nio is in its peak to subsequent summer, which is maintained by multiple factors including the sustained presence of convective cooling anomalies and the local air-sea interaction over western tropical Pacific, and the persistence of sea surface temperature anomalies(SSTA) in tropical Indian and tropical North Atlantic. The WNPAC can influence the atmospheric circulations over East Asia and rainfall in China not only simultaneously, but also in the subsequent summer after an El Nio year, leading to more rainfall over southern China. The current paper also points out that significant anomalies of atmospheric circulations over East Asia and rainfall over southern China occur in El Nio winter but not in La Nio winter, suggesting that El Nio and La Nio have an asymmetric effect. Other issues, including the impact of El Nio diversity and its impact as well as the relations of the factors affecting the persistence of the WNPAC with summer rainfall anomalies in China, are also discussed. At the end of this paper some issues calling for further investigation are discussed.

Linear relationship between the interdecadal and interannual variabilities of North China rainfall in rainy season

Interdecadal and interannual timescales are dominant in the North China rainfall in rainy season (July and August). On the interdecadal timescale, the North China rainfall exhibited an abrupt decrease at the end of 1970s. In this study, we examined the effect of this abrupt rainfall decrease on the association between rainfall and circulation on the interannual timescale, and found that the interdecadal variation does not change the physical mechanism responsible for the interannual variation of North China rainfall. There is a linear relationship between the interdecadal and interannual variabilities of North China rainfall in rainy season.

Interannual variation of North China rainfall in rainy season and SSTs in the equatorial eastern Pacific

The rainfall in North China during rainy sea-son (July and August (JA)) exhibits a strong interannual variability. In this study, the atmospheric circulation and SST anomalies associated with the interannual variation of JA North China rainfall are examined. It is found that on the interannual timescale, the JA North China rainfall is associ-ated with significant SST anomalies in the equatorial eastern Pacific, and the North China rainfall and SST anomaly in the equatorial eastern Pacific correspond to the similar variation of the upper-level westerly jet stream over East Asia. A pos-sible mechanism is proposed for the influence of the SST anomalies in the equatorial eastern Pacific on the North China rainfall.

“23·7”华北特大暴雨过程的基本特征与成因初探

2023年7月29日08时—8月2日08时(北京时),华北地区出现历史罕见的特大暴雨,造成严重灾害。利用常规高空和地面、FY-4A卫星红外云顶亮温(TBB)等观测资料以及ERA5再分析资料对此次天气过程的降水实况特征和成因进行了分析。结果表明:此次过程的累计降水量、降水持续时间等在华北地区均具有显著极端性;北京西部、河北中部和西南部等多地出现特大暴雨,累计降水量超过400 mm,局地达1000 mm以上,降水极值多分布于太行山东麓;根据FY-4A卫星TBB、逐小时降水量和主要影响系统的演变特征,强降水可分为三个阶段,第一阶段降水范围和累计降水量最大,第二阶段对流性最强,多站出现100 mm·h^(-1)以上的极端强降水。此次特大暴雨过程发生在台风残涡北上停滞的稳定环流背景下,台风“杜苏芮”和“卡努”提供了持续而充沛的水汽和能量,高空辐散与低层强辐合提供了稳定而强大的动力条件,偏东气流在太行山东麓强迫抬升,使降水增幅并持续。

太行山地形在“7·19”华北持续性低涡暴雨中的作用

本文对2016年“7·19”华北特大暴雨进行观测分析和数值模拟,并设置了改变地形高度的敏感性试验,以探究该过程降水系统的发生发展机制以及太行山地形的作用。结果表明:(1)本次强降水过程发生在“东高西低”的有利环流形势下,受太行山地形和平原环流系统影响,低层东风急流造成强的对流性降水和低涡作用的叠置造成“7·19”华北地区持续性暴雨的维持和加强;(2)第一阶段为对流性降水,太行山东麓大气对流不稳定能量释放,大气逐渐转为稳定层结;第二阶段为低涡降水,涡度收支分析表明水平散度项和扭转项对低涡维持和发展起到了主要的正贡献,同时伴随有较强的上升运动和垂直风切变,垂直风切变的增强促使水平涡度向垂直涡度转变;(3)太行山地形在持续性暴雨中对两阶段降水、低涡和水汽的作用存在差异。地形高度敏感性试验中,地形高度增高对低层气流的阻挡和强迫抬升作用增强,使得地形降水增强,低涡路径东移,且强度增大。水平散度项使得对流层低层辐合上升运动增强,造成涡度的垂直输送,这是低涡发展和维持的重要原因之一。太行山地形阻挡截留东部平原水汽,且水汽回流加强,有利于太行山东麓水汽的输送与辐合。

东亚副热带季风季节循环年际变化对华北夏季降水的影响

由于受到东亚副热带夏季风复杂变化的影响,华北夏季降水的年际变化预测一直是我国季节气候预测的难点。本文采用1979~2020年中国站点日降水数据和CRA-40大气再分析资料,通过谐波分解、MV-EOF分析等统计方法,研究了东亚副热带季风季节循环年际变化对华北夏季降水的影响。结果表明:在气候态上,华北降水相关的季节循环在850 hPa风场上主要表现为东南风和西南风主导两种模态,其中,西南风主导的降水在7月初达到峰值,而东南风主导的降水在7月底达到峰值。在年际尺度上,季节循环前两个主模态中的华北降水虽然表现为区域一致性变化特点,但在夏季先后受到西南风和东南风的影响。通过对季节循环华北夏季风雨季的开始时间(P1)、峰值时间(P2)、结束时间(P3)、持续时间(Dur)和振幅(Amp)与夏季(6~8月)降水年际变化分析,我们发现东南风主导的季节循环的P1、P3和Amp与华北夏季降水异常呈显著正相关,而P2和Dur与华北夏季降水异常呈负相关。西南风主导的季节循环的P2、P3和Amp与华北夏季降水异常均呈显著正相关。其中,东南风季节循环的位相(P1、P2、P3)变化与夏季西南风的强度有关,而其幅度(Amp)变化主要取决于东南风的强度。由于东南风主导的夏季降水开始于每年的4~5月份,其建立的早晚为6~8月夏季降水的季节预测提供了一个新的参考指标。