华南前汛期的锋面降水和夏季风降水Ⅰ. 划分日期的确定

前汛期暴雨常常引发华南地区的洪涝, 但是前汛期降水的预报能力却相当低.降水的预报在很大程度上依赖于对降水性质的理解, 而华南前汛期降水通常被认为只是锋面性质的降水.事实上, 南海夏季风在6月(甚至5月)就可以影响到华南地区并产生季风对流降水.因此, 华南前汛期包含了两种不同性质的降水, 即锋面降水和夏季风降水, 如何区分它们是非常重要的.为了区分它们, 利用NCEP/NCAR再分析资料、 CMAP资料和中国730站降水资料, 分析气候平均(1971～2000年)状态下锋面降水和季风降水期间大气性质和特征的差异, 得到华南前汛期夏季风降水开始的基本判据: 100 hPa纬向风由西风转为东风并维持5天以上.利用该判据得出气候平均条件下的华南夏季风降水开始于5月24日, 并得到1951～2004年逐年华南前汛期锋面降水和季风降水的划分日期.合成分析的结果表明, 得到的划分日期是基本合理的, 因为它将锋面降水和季风降水期间大气特点的显著差别区分开来.     

华南前汛期夏季风降水开始日期的确定

华南前汛期降水包含锋面降水和夏季风降水,提高对这2种不同性质降水的认识及如何区分是非常重要的。本文利用NCEP/NCAR再分析资料,分析了华南不同性质降水期间大气特性的差异,并采用集合经验模态分解(EEMD)方法,自适应地提取假相当位温θse的调制年循环变量(MAC),得到华南前汛期夏季风降水开始日期的划分标准。结果表明,当某年θse的MACδ标准差时,南海夏季风推进至华南地区,夏季风降水开始。利用该标准划分的华南前汛期夏季风降水开始日期平均为5月第6候,且具有2~3 a、13~15 a的年际、年代际变化周期。进一步对比分析表明,此标准划分的结果基本合理。     

华南前汛期的锋面降水和夏季风降水 II. 空间分布特征

利用中国730站降水资料和第I部分(郑彬等,2006)得到的华南前汛期锋面降水和季风降水的划分日期,计算出1958-2000年华南前汛期锋面降水量(强度)和季风降水量(强度)的序列,采用EOF和扩展EOF分析方法,得到华南前汛期降水的几个主要分布型,并探讨锋面降水与季风降水的可能联系。分析结果表明:华南前汛期的锋面降水和季风降水分布主要有三种类型——全区旱涝型、西南涝(旱)东北旱(涝)型、东南涝(旱)西北旱(涝)型。各分布型的时间系数与850 hPa风场的相关结果表明不同的分布对应着不同的低层环流形势。统计结果显示华南前汛期锋面降水的分布形式与季风降水的分布形式有一定的对应关系。     

华南前汛期的锋面降水和夏季风降水Ⅱ.空间分布特征

利用中国730站降水资料和第I部分 (郑彬等, 2006) 得到的华南前汛期锋面降水和季风降水的划分日期, 计算出1958～2000年华南前汛期锋面降水量 (强度) 和季风降水量 (强度) 的序列, 采用EOF和扩展EOF分析方法, 得到华南前汛期降水的几个主要分布型, 并探讨锋面降水与季风降水的可能联系.分析结果表明: 华南前汛期的锋面降水和季风降水分布主要有三种类型--全区旱涝型、西南涝 (旱) 东北旱 (涝) 型、东南涝 (旱) 西北旱 (涝) 型.各分布型的时间系数与850 hPa风场的相关结果表明不同的分布对应着不同的低层环流形势.统计结果显示华南前汛期锋面降水的分布形式与季风降水的分布形式有一定的对应关系.     

2012年华南前汛期降水特征及环流异常分析

2012年华南前汛期于4月第2候开始,6月第5候结束。前汛期降水经历了三个不同的阶段：第一阶段是4月第2候至5月第3候的降水集中期（锋面降水）,江南大部和华南大部降水偏多25％以上,第二阶段是5月第4候至6月第2候的少雨期,华南中部和东部降水偏少50％以上,第三阶段是6月第3—5候的第二个降水集中期（季风降水）,江南东南部至华南中西部降水偏多50％以上。对各阶段大气环流距平场的分析结果表明：华南前汛期开始后,偏强的乌拉尔山高压脊导致南下的冷空气偏强,偏强的低层副热带高压使得我国南方为整层水汽输送的异常辐合区,两者共同导致华南前汛期第一阶段的锋面降水较常年同期偏多;南海夏季风在爆发后偏弱和西北太平洋副热带高压（以下简称副高）持续3候异常偏北是导致第二阶段前汛期降水明显偏少的主要原因;第三阶段,南海夏季风异常偏强,副高南落并增强,以及孟加拉湾季风槽的偏强使得华南前汛期此阶段的季风降水偏多。     

华南旱、涝年前汛期水汽输送特征的对比分析

利用卫星遥感反演的降水资料及中国740个测站逐日降水资料,根据定义的旱涝指数,划分了1957—2002年期间的华南旱涝年份。利用NCEP/NCAR逐日再分析资料,分别讨论华南前汛期4~6月、4月及6月的水汽输送的气候特征。在此基础上,进一步研究了4~6月、4月及6月水汽输送及其源地在华南前汛期涝年和旱年的不同特征。结果表明:影响华南的水汽输送环流在南海夏季风建立前后具有明显不同的气候特征,华南前汛期(4~6月)降水应分为南海夏季风爆发前4月份至夏季风爆发与南海夏季风爆发至6月两个时段。来源于西太平洋的水汽输送变化和来自中国北方的水汽输送变化对华南降水异常有重要作用,而阿拉伯海—孟加拉湾地区的水汽输送变化对华南的降水异常影响不大。     

华南前汛期不同降水时段的特征分析

利用1957-2001年华南地区74个测站逐日降水资料和同期NCEP／NCAR逐日再分析格点资料，对华南前汛期(4-6月)不同降水时段的特征进行了比较。分析发现，华南前汛期降水由锋面降水和夏季风降水两个时段组成。锋面降水时段主要集中在4月，为典型的由冬到夏过渡的环流形势，华南地区高空为平直的副热带西风急流，大气层结稳定，水汽来源主要是阿拉伯海的西风输送和西太平洋副高南侧东风的转向输送；南海夏季风爆发前，副高仍控制南海地区，华南地区水汽输送主要来源于阿拉伯海的西风输送和西太平洋副高南侧东风的转向输送及孟加拉湾的西南输送；南海夏季风爆发后，副高东撤退出南海地区，南半球越赤道水汽输送加强并与孟加拉湾水汽输送连通，华南区域内对流发展；夏季风降水时段盛期主要集中在6月，此时南亚高压跃上高原，华南地区处于南亚高压东部，对流发展极其旺盛，强大的南半球越赤道水汽输送越过孟加拉湾和南海地区向华南地区输送。     

不同PDO背景下华南前汛期锋面和季风降水的水汽输送差异

基于华南地区60个站点的逐日降水资料及NCEP再分析资料,采用拉格朗日后向气流轨迹模式(HYSPLIT_4.9),分析了1960—2012年PDO (Pacific Decadal Oscillation,太平洋年代际振荡)不同相位期间,华南前汛期锋面和季风降水的水汽输送轨迹、主要源地及不同源地水汽的降水贡献率的差异。结果表明:1)锋面降水阶段,在PDO正相位期间,西太平洋-中国南海-孟加拉湾水汽较多;夏季风降水阶段,在PDO正位相期间,北印度洋-孟加拉湾-中国南海的整层水汽含量较多。2)锋面降水阶段,水汽主要来自西北太平洋与中国南海,在PDO正相位期间,副热带高压位置偏北,使得西太平洋水汽输送路径偏北,有更多水汽向华南输送,有利于华南季风降水形成,降水与PDO呈显著的正相关关系。3)季风降水阶段,在PDO正位相期间,尽管北印度洋-孟加拉湾-中国南海的整层水汽含量大,但并未都输送至华南,故形成的有效季风降水偏少,降水与PDO呈显著的负相关关系。     

华南前汛期降水与我国近海海温的SVD分析

利用国家气象局整编的中国160站月降水资料和英国气象局的全球逐月海温格点资料,分析了华南前汛期降水的异常特征,并用SVD方法分析华南前汛期降水与近海海温的关系。结果表明：近50年华南南部前汛期降水的总的趋势变化不明显。华南南部前汛期降水异常存在3 a、5 a、7 a和14 a的周期;用SVD方法分析发现：存在一个关键区（130~158 ?E,5~18 ?N）,华南前汛期降水异常关键影响时段是前一年5—8月,华南前汛期降水与该关键区海温有较好的负相关关系,即前一年5~8月关键区海温异常偏高（低）,次年华南前汛期降水偏少（多）。     

华南前汛期降水开始和结束日期确定方法综述

对目前有关前汛期开始和结束日期的研究工作进行了总结.提出应参照梅雨期和季风降水等有关日期划分问题的方法,使用比月平均资料更高精度的逐日(候)资料,选择能够表征华南前汛期降水特征的适宜区域和代表站点,综合考虑降水、环流及其它物理要素在前汛期开始和结束前、后的演变特征,从而客观、准确地划定各年前汛期的起始和结束日期,以用于华南前汛期问题的研究.     

江门前汛期不同降水时段特征

通过对江门地区1971～2007年3—6月候雨量、日雨量,2003～2007年南海和华南地区低层（850hPa）风场、向外长波辐射（OLR）场和水汽场在南海夏季风爆发前后差异的比较分析,发现：江门前汛期降水由锋面降水和夏季风降水2个时段组成,降水集中期分别为5月第2候和6月第2候。南海夏季风爆发后,江门第1次出现的降水可看作是夏季风降水的开始,南海夏季风的不同爆发类型对江门夏季风降水的开始时间有不同影响。江门前汛期的锋面降水为大尺度抬升凝结降水,而具有热带性质的夏季风降水为对流性降水;由于降水性质的不同,导致两者在降水持续时间、降水形式等方面表现出差异。     

ONSET AND RETREAT DATES OF THE SOUTH CHINA SEA SUMMER MONSOON AND THEIR RELATIONSHIPS WITH THE MONSOON INTENSITY IN THE CONTEXT OF CLIMATE WARMING

Global gridded daily mean data from the NCEP/NCAR Reanalysis(1948-2012) are used to obtain the onset date,retreat date and duration time series of the South China Sea summer monsoon(SCSSM) for the past 65 years.The summer monsoon onset(retreat) date is defined as the time when the mean zonal wind at 850 hPa shifts steadily from easterly(westerly) to westerly(easterly) and the pseudo-equivalent potential temperature at the same level remains steady at greater than 335 K(less than 335 K) in the South China Sea area[110-120°E(10-20°N)].The clockwise vortex of the equatorial Indian Ocean region,together with the cross-equatorial flow and the subtropical high,plays a decisive role in the burst of the SCSSM.The onset date of the SCSSM is closely related to its intensity.With late(early) onset of the summer monsoon,its intensity is relatively strong(weak),and the zonal wind undergoes an early(late) abrupt change in the upper troposphere.Climate warming significantly affects the onset and retreat dates of the SCSSM and its intensity.With climate warming,the number of early-onset(-retreat) years of the SCSSM is clearly greater(less),and the SCSSM is clearly weakened.     

The Potential Predictability of the South China Sea Summer Monsoon in a Dynamical Seasonal Prediction System

The potential predictability of climatological mean circulation and the interannual variation of the South China Sea summer monsoon (SCSSM) were investigated using hindcast results from the Institute of Atmospheric Physics Dynamical Seasonal Prediction System (IAP DCP),along with the National Centers for Environmental Prediction (NCEP) reanalysis data from the period of 1980-2000.The large-scale characteristics of the SCSSM monthly and seasonal mean low-level circulation have been well reproduced by IAP DCP,especially for the zonal wind at 850 hPa;furthermore,the hindcast variability also agrees quite well with observations.By introducing the South China Sea summer monsoon index,the potential predictability of IAP DCP for the intensity of the SCSSM has been evaluated.IAP DCP showed skill in predicting the interannual variation of SCSSM intensity.The result is highly encouraging;the correlation between the hindcasted and observed SCSSM Index was 0.58,which passes the 95% significance test.The result for the seasonal mean June-July-August SCSSM Index was better than that for the monthly mean,suggesting that seasonal forecasts are more reliable than monthly forecasts.     

华南前汛期持续暴雨环流分型初步研究

采用1961—2010年NCEP/NCAR逐日再分析资料和台站观测降水量资料,按一定标准选取了华南前汛期24个持续暴雨过程;并且按基本判据确定逐年华南夏季风降水开始日期。然后依据南亚高压环流型和相对于该年夏季风降水开始的早晚,将这些暴雨过程划分为夏季风降水前、后南亚高压东部型,夏季风降水后南亚高压带状、西部型共4个类型;其中,夏季风后南亚高压西部型次数最多、平均持续时间最长。所有类型持续暴雨的相同点是:广东东北部附近均为暴雨频率和雨量高值区;暴雨期间华南150 h Pa位势高度增加、500 h Pa位势高度减少;华南处在150 h Pa偏西风急流南侧辐散区中;850 h Pa华南沿海有明显的西南气流,低层辐合在华南东北部最明显;两广沿海为可降水量大值区;华南的整层水汽输送主要呈现西南向。不同点是:夏季风后南亚高压西部型平均雨量较小,夏季风后南亚高压带状型与西部型在印度洋上存在明显的偏东风高空急流;夏季风后南亚高压类型在两广沿海的可降水量数值较大。     

Seasonal Transition of Summer Rainy Season over Indochina and Adjacent Monsoon Region

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Seasonal transition of summer rainy season over indochina and adjacent monsoon region

The mean onset and withdrawal of summer rainy season over the Indochina Peninsula were investigated using 5-day averaged rainfall data (1975-87). The mean seasonal transition process during onset and retreat phases in Indochina, India and the South China Sea is also examined using 5-day mean OLR (1975-87) and 850 hPa wind (1980-88) date. It was found that the onset of summer rainy season begins earlier in the inland region of Indochina (Thailand) in late April to early May than in the coastal region along the Bay of Bengal. This early onset of rainy sea-son is due to pre-monsoon rain under the mid-latitude westerly wind regime. The full summer monsoon circulation begins to establish in mid-May, causing active convective activity both over the west coast of Indochina and the cen-tral South China Sea. In case of withdrawal, the earliest retreat of summer rainy season is found in the central northern part of Indochina in late September. The wind field, on the other hand, already changes to easterlies in the northern South China Sea in early September. This easterly wind system covers the eastern part of Indochina where post-monsoon rain is still active. In late October, the wind field turns to winter time situation, but post monsoon rain still continues in the southern part of the Indochina Peninsula until late November     

Reappraisal of Asian Summer Monsoon Indices and the Long-Term Variation of Monsoon

The Webster and Yang monsoon index (WYI)-the zonal wind shear between 850 and 200 hPa was calculated and modified on the basis of NCEP/NCAR reanalysis data. After analyzing the circulation and divergence fields of 150-100 and 200 hPa, however, we found that the 200-hPa level could not reflect the real change of the upper-tropospheric circulation of Asian summer monsoon, especially the characteristics and variation of the tropical easterly jet which is the most important feature of the upper-tropospheric circulation. The zonal wind shear U850-U(150 100) is much larger than U850-U200, and thus it can reflect the strength of monsoon more appropriately. In addition, divergence is the largest at 150 hPa rather than 200 hPa, so 150 hPa in the upper-troposphere can reflect the coupling of the monsoon system. Therefore, WYI is redefined as DHI, i.e., IDH=U850* - U(150 100)*, which is able to characterize the variability of not only the intensity of the center of zonal wind shear in Asia, but also the monsoon system in the upper and lower troposphere. DHI is superior to WYI in featuring the long-term variation of Asian summer monsoon as it indicates there is obvious interdecadal variation in the Asian summer monsoon and the climate abrupt change occurred in 1980. The Asian summer monsoon was stronger before 1980 and it weakened after then due to the weakening of the easterly in the layer of 150-100 hPa, while easterly at 200 hPa did not weaken significantly. After the climate jump year in general, easterly in the upper troposphere weakened in Asia, indicating the weakening of summer monsoon; the land-sea pressure difference and thermal difference reduced, resulting in the weakening of monsoon; the corresponding upper divergence as well as the water vapor transport decreased in Indian Peninsula, central Indo-China Peninsula, North China, and Northeast China, indicating the weakening of summer monsoon as well. The difference between NCEP/NCAR and ERA-40 reanalysis data in studying the intensity and long-term variation of Asian summer monsoon is also compared in the end for reference.     

Intraseasonal Oscillation of the South China Sea Summer Monsoon and Its Influence on Regionally Persistent Heavy Rain over Southern China

The intraseasonal oscillation（ISO） in the South China Sea summer monsoon（SCSSM） and its influence on regionally persistent heavy rain（RPHR） over southern China are examined by using satelhte outgoing long wave radiation,NCEP/NCAR reanalysis,and gridded rainfall station data in China from 1981 to 2010.The most important feature of the ISO in SCSSM,contributing to the modulation of RPHR,is found to be the fluctuation in the western Pacific subtropical high（WPSH）,along with a close link to the Madden-Julian oscillation（MJO）.Southern China is divided into three regions by using rotated empirical orthogonal functions（REOFs）for intraseasonal rainfall,where the incidence rate of RPHR is closely linked to the intraseasonal variation in rainfall.It is found that SCSSM ISOs are the key systems controlling the intraseasonal variability in rainfall and can be described by the leading pair of empirical orthogonal functions（EOFs） for the 850-hPa zonal wind over the SCS and southern China.Composite analyses based on the principal components（PCs） of the EOFs indicate that the ISO process in SCSSM exhibits as the east-west oscillation of the WPSH,which is coupled with the northward-propagating MJO,creating alternating dry and wet phases over southern China with a period of 40 days.The wet phases provide stable and lasting circulation conditions that promote RPHR.However,differences in the ISO structures can be found when RPHR occurs in regions where the WPSH assumes different meridional positions.Further examination of the meridional-phase structure suggests an important role of northward-propagating ISO and regional air-sea interaction in the ISO process in SCSSM.     

华南夏季风降水开始日的异常与前冬大气环流和海温的关系

利用1951～2004年NCEP/NCAR再分析资料及ERSST海温资料,研究了华南夏季风降水开始日期的变化特征及其与前期冬季大气环流和海温的关系。小波分析表明,夏季风降水开始日期具有明显的年际和年代际变化,年际变化以准2～3年变化为主,年代际变化周期约16年。华南夏季风降水开始偏早年在大气环流上的前兆信号表现为前期冬季乌拉尔山阻塞高压偏强、东亚大槽较深,阿留申低压偏强,冷空气活动偏强。冬季,鄂霍次克海附近的海温异常为华南夏季风降水开始迟早有物理意义的、稳定正相关前兆信号。合成分析表明,冬季鄂霍次克海附近SST正异常时,5月100 hPa青藏高压偏东偏北偏弱,异常偏西风控制华南;850 hPa环流在华南表现为东北风,华南受冷空气影响为主,华南夏季风降水开始日期偏晚。相反时,若冬季鄂霍次克海附近SST负异常,5月100 hPa青藏高压偏西偏南偏强,异常偏东风控制华南;850 hPa环流在华南表现为偏南风,华南受热带系统影响为主,华南夏季风降水开始日期偏早。并提出冬季中高纬度地区冷空气活动影响华南夏季风降水开始日异常的物理机制。