梅雨锋暴雨中尺度对流系统触发和组织化的观测分析

利用观测和NCEP再分析资料,对2015年6月26-28日江淮流域梅雨锋暴雨天气对流的触发和中尺度对流系统（MCS）的组织方式进行了分析。结果表明:梅雨锋附近发展的2个线状中尺度对流系统是暴雨的直接制造者。MCS2的发展有2种组织方式,26日夜间到27日凌晨,东西向雨带的不断后部建立和随后对流单体的列车效应是其发展的主要方式。27日凌晨到白天,初期新单体不断在线状MCS2的南缘触发,形成多个近乎平行的东北-西南向短雨带,后期梅雨锋锋面雨带从西部不断东移,经过强降水区;对流元有2种尺度的组织方式:新生对流单体沿着单个雨带向东北方向的列车效应以及东北-西南向雨带沿线状中尺度对流系统向东平移的"列车带"效应;持续的后部建立型和沿着同一路径不断的"列车带"效应使MCS2发展和维持。梅雨锋前不稳定空气的地形抬升和边界层辐合上升是初始对流的主要触发机制;26日夜间对流产生的冷池对对流的触发和MCS2的组织化及维持起重要作用,中尺度对流系统的组织特征和发生、发展受近地面环境场制约。      

Interdecadal variation of April mean rainfall in Korea

The statistical change-point analysis demonstrates that there is a climate regime shift in the April mean precipitation in Korea in 1981. The April mean precipitation in the years post-1981 showed a distinct decrease compared to the years pre-1981. This phenomenon was also noticed in China and Japan, excluding south China. One of the major causes for this decrease in April mean precipitation was the increased snow depth in the mid-latitude regions of continental East Asia. This resulted in a strengthened cold and dry anticyclone anomaly over continental East Asia and a relatively weakened subtropical anticyclone anomaly over the western North Pacific, thus forcing a continuation of the typical winter pressure pattern of “high-West and low-East” in East Asia in April. The strengthened northerly anomaly from this zonal pressure pattern anomaly played a significant role in restricting the northern movement of the subtropical anticyclone and preventing the inflow of warm and humid air into Korea.

     

黄淮地区一次冷锋暴雨天气过程的预报分析

利用常规天气图、地面加密资料、数值预报产品、卫星云图和多普勒天气雷达资料等,对2007年7月18-19日出现在黄淮地区的暴雨天气过程进行了综合分析.结果表明:暴雨是高空西风槽、冷锋和副热带高压边缘强盛的西南暖湿急流共同作用产生的;强降水产生在地面中尺度辐合线附近.在高层干冷、低层暖湿的大气不稳定条件下,地面冷空气的侵入触发了不稳定能量的释放,同时低层辐合、高空辐散的高低空配置,引发了强降水的产生.卫星云图和多普勒天气雷达资料在暴雨临近预报中起着重要作用.     

Changma onset definition in Korea using the available water resources index and its relation to the Antarctic oscillation

This study defines the Changma onset using the available water resources index (AWRI) for 25?years (1985–2009) and verifies the validity of this definition. The three conditions for defining the Changma onset are established as follows: (i) The first day exceeding the June AWRI (threshold) averaged over the 25-year period. (ii) The continuation of the value over the threshold for at least 1?week after the onset. (iii) After the continuation of more than 1?week, the non-continuation of the value under the threshold for at least 1?week. The 25-year average Changma onset date is 24 June with a standard deviation of 9?days. The defined Changma onset is verified through the analysis on the relationship with the Antarctic oscillation (AAO). AAO in June shows a high correlation with not only the Changma onset but also the June precipitation (AWRI) in Korea. These three variables are influenced by Mascarene and Australian (positive AAO pattern) highs from in the preceding March. When these two pressure systems develop, the cold cross-equatorial flow in the direction from the region around Australia to the equator is intensified, which in turn, forces a western North Pacific high (WNPH) to develop northward; this eventually drives the rain belt north. As a result, the Changma begins early in the positive AAO phase, and the June precipitation increases in Korea. In addition, a WNPH that develops more northward increases the landfalling frequency of tropical cyclones in Korea, which plays an important role in increasing the June precipitation.     

The significant increase of summer rainfall occurring in Korea from 1998

Using the techniques of empirical orthogonal function analysis and the change-point analysis to total summer rainfall from 60 weather observation stations, it was found that total summer (from June to September) rainfall in Korea has increased greatly since 1998. The increase level was higher in the season between Changma and late summer rainy season (from the end of July to early August) and in the season after late summer rainy season (after the early September). Among the reasons for increase of summer rainfall in Korea since 1998, the north-high and south-low pressure pattern formed around Korea drew attention. As northeasterlies and southeasterlies derived from these two pressure systems converged in Korea, rainfall and moisture convergence increased most in Korea of the East Asia regions (0–60°, 100–180° E). In addition, the atmosphere above Korea revealed that there were strong ascents from the ground to 200-hPa level with the warm air to 500-hPa level.     

北京降雨过程分型特征及短历时降雨重现期研究

利用北京观象台1961—2004年44 a逐分钟自记降雨资料,采用模糊识别法和统计分析法对北京市降雨过程进行雨型分型,并对5、10、15、20、30、45、60、90、120、180 min共10种不同短历时年最大降雨极值的概率分布利用指数分布、耿贝尔分布和皮尔逊Ⅲ型概率分布函数进行拟合。在此基础上,推算各历时降雨重现期极值。结果表明,暴雨天数年代际差异较大,暴雨量主要集中在50~100 mm范围内;不同月份雨型分布不一致,6月多双峰型,7—9月多单峰型雨型;总体上,北京降雨过程多为单峰型,占80%以上。最大降雨量主要出现在午后至傍晚和凌晨;北京短历时降雨极值的概率分布多数不服从指数分布而服从皮尔逊Ⅲ型分布;重现期降雨极值随历时增加而增大,2 a至100 a重现期内10 min降雨极值取值为10.4~22.6 mm,60 min降雨极值取值为22.3~59.2 mm,180 min取值为53.0~89.8 mm。     

梅雨锋的典型结构、多样性和多尺度特征

利用自动站、FY-2E卫星、多普勒雷达及NCEP再分析资料等, 对2015年7月23日皖江地区一次最强梅雨锋大暴雨过程成因进行分析, 重点分析此次过程中短时强降水中尺度特征。结果表明:(1)此次过程是一次典型的梅雨锋对流性暴雨, 贝加尔湖宽广的高空槽及副高和大陆高压的稳定维持是引发大暴雨过程的大尺度背景条件。高空槽、西南涡、地面β中尺度气旋的维持和加强为大暴雨提供了强劲的动力和水汽条件。(2)此次暴雨过程低层辐合明显、高层辐散显著、中层上升运动强盛, 非常有利于低层中尺度涡旋的发生发展。(3)短时强降水发生于边界层辐合由弱变强且趋于最强的时段内, 超低空风速的显著增大对暴雨有着重要作用, 边界层的垂直风切变和700 hPa的干侵入增强了层结的不稳定性。强降水发生时低层锋生明显加强, 锋生函数最强中心的位置与大暴雨区基本一致。(4)短时强降水发生于α尺度对流云团团状中心和皖江西部特殊喇叭口地形处, 且位于地面β中尺度气旋右侧南风气流中。(5)强度弱、底部低、平均切变值小的中γ尺度气旋使低质心、移动缓慢的对流风暴更具组织性, 持续时间明显增加, 更易产生短时强降水。

     

伴随韩国雨季开始和结束早晚的关联场分析

分析了与韩国雨季开始和结束的年际变化相关联的一些要素场发生的变化。分别对雨季开始和结束早晚的个例进行了合成分析。６月月平均资料用来分析与雨季开始早晚相关联的要素场变化，而在分析与雨季结束早晚相关联的要素场变化时则利用７月月平均资料。结果表明，对应雨季开始（或结束）早晚，大气环流和表面温度等要素不仅在东亚地区、而且在远离东亚的地方具有显著的差异。在东亚地区的显著差异主要是高空急流和西太平佯副热带高压。远离东亚的显著差异主要是印度季风和ＥＮＳＯ现象。印度季风与韩国雨季开始和结束均有关联，但ＥＮＳＯ现象只与雨季开始显著相关、而与结束并没有显著的关联。     

Assessment of the WRF model in reproducing a flash-flood heavy rainfall event over Korea

This study examines the ability of the cloud-resolving weather research and forecasting (WRF) model to reproduce the convective cells associated with the flash-flooding heavy rainfall near Seoul, South Korea, on 12 July 2006. A triply nested WRF model with the highest resolution of 3-km horizontal grid spacing was integrated with conventional analysis data. The WRF model simulated the initiation of isolated thunderstorms, and the formation of a convective band, cloud cluster, and squall line at nearly the right time. The corresponding precipitation simulation was also reasonably reproduced in its distribution, although the amount was underestimated. A sensitivity experiment that excludes the orography over the peninsula revealed that orographic forcing over the peninsula is responsible for about 20% increase in precipitation over the heavy rainfall region. It was identified that in addition to the up-lifting local orographic forcing to the west of the mountain range in South Korea, anticyclonic circulation due to the presence of the Gaema Heights in North Korea contribute to the confinement of convective activities in the heavy rainfall region.     

Investigation of aerosol indirect effects on simulated flash-flood heavy rainfall over Korea

This study investigates aerosol indirect effects on the development of heavy rainfall near Seoul, South Korea, on 12 July 2006, focusing on precipitation amount. The impact of the aerosol concentration on simulated precipitation is evaluated by varying the initial cloud condensation nuclei (CCN) number concentration in the Weather Research and Forecasting (WRF) Double-Moment 6-class (WDM6) microphysics scheme. The simulations are performed under clean, semi-polluted, and polluted conditions. Detailed analysis of the physical processes that are responsible for surface precipitation, including moisture and cloud microphysical budgets shows enhanced ice-phase processes to be the primary driver of increased surface precipitation under the semi-polluted condition. Under the polluted condition, suppressed auto-conversion and the enhanced evaporation of rain cause surface precipitation to decrease. To investigate the role of environmental conditions on precipitation response under different aerosol number concentrations, a set of sensitivity experiments are conducted with a 5?% decrease in relative humidity at the initial time, relative to the base simulations. Results show ice-phase processes having small sensitivity to CCN number concentration, compared with the base simulations. Surface precipitation responds differently to CCN number concentration under the lower humidity initial condition, being greatest under the clean condition, followed by the semi-polluted and polluted conditions.     

形成2015年浙江省梅汛期暴雨的控制环流及梅雨锋结构

本文利用NCEP/NCAR全球再分析逐日资料、地面观测资料和自动站降水资料,在分析了2015年浙江省梅汛期强降水特征、水汽输送和局地环流的基础上,从西南季风进退、副热带高压、南亚高压及西风带波动等方面对2015年形成梅汛期暴雨的控制环流进行了分析。结果表明：2015年整个浙江省梅汛期降水量较常年显著偏多,浙江中部地区降水量比历史同期偏多接近一倍。丰沛的水汽从孟加拉湾经中南半岛向东输送,与西太平洋副热带高压西侧的西南气流相合并,在梅雨锋南侧形成异常辐合,为强降水提供了水汽条件。这次持续强降水由三次强降水过程构成并由西风辐合型锋生引起。第二次强降水过程中大气强对流性不稳定利于梅雨锋上中尺度对流系统发展,导致强降水呈现明显的局地性。而第一次和第三次过程中梅雨带附近大气基本处于对流稳定或中性,以斜压性降水为主。在对流层低层,副高较常年偏东偏南,其西北侧西南暖湿气流与北侧冷空气交汇于浙江省,利于梅汛期强降水集中期的出现。在对流层上层的南亚高压较常年位置偏东,其北侧的西风急流强度偏强,东亚急流核入口区右侧的强辐散利于造成强烈的上升运动。在对流层中层,贝加尔湖阻高的东侧有明显的波动能量向东向南传播并在长江中下游积聚,利于浙江地区扰动的维持,形成持续稳定的梅雨锋和中低空切变线,造成梅雨强降水过程的持续。2015年春夏季热带中东太平洋海温正异常分布有利于梅汛期降水偏多的异常环流的形成。     

Characteristics of Eurasian snow depth with respect to Indian summer monsoon rainfall

Earlier studies show a strong negative relationship between Eurasian snow cover/depth and Indian summer monsoon rainfall (ISMR). In such studies, both the parameters snow and rainfall are seasonally averaged over large areas. Indian summer monsoon has its own characteristics of evolution such as onset, active, break and withdrawal phases which have been studied extensively. However, the evolution of Eurasian snow is yet to be examined. Further, it is interesting to explore the characteristics of evolution of snow over the different regions of Eurasia and their relationship with the evolution characteristics of summer monsoon. In this paper, a detailed examination has been done on the starting and the ending dates of snowfall over different regions of Eurasia and attempts have been made to explore any relationship with onset of ISMR. It is observed that the regions where snowfall started early, it also ended late. Further, in those regions maximum snow depth also occurred late. In some years, more snowfall in East Eurasia is followed by less snowfall in West Eurasia. Also snow depths particularly in the northernmost and southwest regions of East Eurasia are opposite in phase. The results of this study indicate a weak relationship between snow starting dates in Eurasia and summer monsoon onset dates in the Kerala coast. However, the relationship between the northernmost Eurasian snow depth and the summer monsoon precipitation in the Peninsular India is significant.     

Mixing at the front of gravity currents

Observations made with the help of a movable-bed tank designed and operated to freeze the motion of gravity current fronts indicate that both the scale of the apparatus and the magnitude of the current Reynolds number have a strong influence on mixing rates. Low mixing rates are associated with fronts in relatively shallow fresh water depths and low Reynolds numbers. Mixing rates increase more rapidly as the ratio of current thickness to fresh water depth increases. For the deepest fresh water flow conditions, when scale effects can be expected to be negligible, the dimensionless mixing rate is approximately equal to three times the relative current thickness and has an upper limit of about 0.3. Lower mixing rates are observed as the current Reynolds number decreases, suggesting that viscous effects can still exist even for deep fresh water conditions. Application of the experimental results to estimate some parameters of a hypothetical gravity current on the continental shelf yield reasonable values. However, it is clear that, owing to the scale and Reynolds number effects that might be present in laboratory experiments, particular care should be exercised when trying to extrapolate results to gravity currents in nature.     

不同微物理方案对一次梅雨锋暴雨过程模拟的影响

中尺度模式中描述湿物理过程的方案主要有对流参数化方案和云微物理方案,当网格距达到可以分辨积云对流尺度时,云微物理方案对描述云和降水物理过程的作用将变得更为重要.利用GRAPES高分辨率中尺度数值模式对2007年7月7-9日中国梅雨锋暴雨过程进行了数值模拟,从降水量、雷达回波、水成物分布方面结合观测资料,分析了NCFP简...     

江淮流域梅雨锋暴雨南北雨带干冷空气侵入作用的对比研究

对2007—2011年梅雨期江淮流域降水进行统计分析,确认此时段降水频次与降水量空间分布在淮河与长江中下游,形成两个分离的多降雨区。长江与淮河两分离雨带在盛行中尺度系统、空间结构、梅雨锋位置、水汽输送通道、暴雨落区、干冷气团入侵以及干冷动量南移等方面,均有各自特征。在主导高空环流下,更多低涡暴雨(77%)位于长江流域,更多辐合线暴雨(86%)发生在淮河流域,这些盛行的浅薄降水系统,造成梅雨暴雨雨强大、风速小的特征。长江流域低涡环流引导水汽输送来自南海与东海两地,因而暴雨范围更大,雨带更宽,而淮河流域辐合线主要水汽来源仅为南海。假相当位温表现的梅雨锋作为干冷空气的前缘,在淮河流域因辐合线结构垂直剖面陡峭,在长江流域因低涡环流,形成干冷和暖湿气团的层次叠加混合。在梅雨锋北侧,干冷空气从对流层中高层向下侵入,在淮河流域位置较长江流域偏高且狭窄。湿位涡垂直分布显示,暴雨区与湿位涡斜压项大于0的正涡度发展区对应。干冷动量中心的下传与湿位涡斜压项小于0对应,下传时向南移,其前缘对暴雨雨带位置有指示性;其南下速度,因纬度与密度差异,在淮河流域移速小于长江流域。     

Mesoscale dynamics and its application in torrential rainfall systems in China

Progress over the past decade in understanding moisture-driven dynamics and torrential rain storms in China is reviewed in this paper. First, advances in incorporating moisture effects more realistically into theory are described, including the development of a new parameter, generalized moist potential vorticity(GMPV) and an improved moist ageostrophic Q vector(Qum). Advances in vorticity dynamics are also described, including the adoption of a "parcel dynamic" approach to investigate the development of the vertical vorticity of an air parcel; a novel theory of slantwise vorticity development, proposed because vorticity develops easily near steep isentropic surfaces; and the development of the convective vorticity vector(CVV)as an effective new tool. The significant progress in both frontal dynamics and wave dynamics is also summarized, including the geostrophic adjustment of initial unbalanced flow and the dual role of boundary layer friction in frontogenesis, as well as the interaction between topography and fronts, which indicate that topographic perturbations alter both frontogenesis and frontal structure. For atmospheric vortices, mixed wave/vortex dynamics has been extended to explain the propagation of spiral rainbands and the development of dynamical instability in tropical cyclones. Finally, we review wave and basic flow interaction in torrential rainfall, for which it was necessary to extend existing theory from large-scale flows to mesoscale fields, enriching our knowledge of mesoscale atmospheric dynamics.     

The role of synoptic systems in the interannual variability of Sahel rainfall

Summary Components of the June-September climate over the Sahel are investigated in simulations with the GCM of the NASA/Goddard Institute for Space Studies, forced by SST observed during 1987 and 1988. The study analyzes the role of the synoptic patterns in determining precipitation differences between the two seasons, with special attention given to African wave disturbances (AWD). Emphasis is placed on deducing the characteristics of individual systems which may be missed by spectral and/or composite analyses. Results are derived from time-longitude cross-sections and spatial distributions of daily and weekly averages of key climatological variables. Despite the overall rainier season, rainless AWD are more prevalent in the simulations corresponding to June–September 1988 forcing than for 1987. Daily precipitation is shown to be highly correlated with mid-tropospheric vorticity, near surface convergence and 200 mb divergence. August daily rainfall was some-what better correlated with implied large scale vertical motion for 1988 forcing, emphasizing the dominance of broad circulation influences during that summer. While significant rainfall variability is attributed to AWD, quasistationary mechanisms cannot be ignored. In these simulations, upper tropospheric divergence modulated by changes in the Tropical Easterly Jet serves to both intensify the rainfall triggered by AWD and to sustain broader rainfall patterns during events of massive uplift.With 16 Figures     

Characteristics of mass concentrations depending on synoptic features during airborne dustfall episodes observed at Cheongwon in Korea in 2005

Sandstorms in the desert and loess regions of north China and Mongolia, as well as the associated dustfall episodes on the Korean Peninsula, were monitored in 2005. The ground mass concentrations of PM10 and PM2.5 were analyzed during dustfall episodes at Cheongwon, in central south Korea, based on synoptic features at surface, 850 hPa and 500 hPa levels. A total of seven dustfall episodes lasting eleven days were observed and the mass concentration ratios of PM2.5 and PM10 during dustfall episodes were classified into a severe dustfall episode (SDE) and a moderate dustfall episode (MDE) depending upon two synoptic features. The main synoptic feature was for SDEs, which occurred frequently under a surface anticyclone and cyclone located in the west and east of the Korean Peninsula with large amplitude trough at 500 hPa over the northern Korean Peninsula. The sandstorms at the source headed directly to Korea via a strong N-NW wind without passing through any large cities or industrial areas of east China. The PM10 mass concentration sharply increased during the SDEs; however, the fine aerosol fraction of PM2.5 levels was relatively low with 13.6% of the mass concentration. In a synoptic feature for MDEs, a slow moving cyclone headed to Korea via the industrial areas of northeastern China under a small amplitude trough at a 500 hPa level. A weak anticyclone was also located over China. MDEs showed low mass concentrations of coarse PM10 particles and large fraction of fine PM2.5 particles at 46.3%.     

江苏一次持续性梅雨锋暴雨过程诊断与分析

利用NCEP再分析资料,FY2E卫星的TBB资料,常规和加密气象站资料,对2012年7月2—4日,江苏省一次持续性梅雨锋暴雨过程进行了诊断和中尺度特征分析。结果表明:此次过程是东北冷涡槽东移与副热带高压西北侧暖湿气流交汇形成的。暴雨落区在低空西南急流的左侧和中高空急流的一、三象限,低层干线触发了不稳定能量的释放。经分析有7个中尺度云团造成了本次持续性暴雨,-64℃的冷云盖是较强降水的指标性温度,不断东移的中尺度云团类似于"列车效应",带来持续降水,降水开始时间落后于中尺度云团生成时间约2~4 h。地面中尺度辐合线是触发此次强降水的重要中尺度系统,辐合线附近易触发对流,且对流降水沿着辐合线方向移动。低层正、高层负的垂直螺旋度,高温高湿的大气以及较高的位势不稳定为暴雨和强对流天气提供有利条件。在垂直上升运动区北侧有明显下沉运动补偿气流,使上升气流得以长时间维持。暴雨区位于925 hPa超低空急流核移动方向的左侧。