SIMULATION STUDY ON CAUSES OF TROPICAL STORM FITOW (0114) HEAVY RAIN

With PSU/NCAR nonhydrostatic mesoscale model MM5, the rainfall process of tropical storm Fitow(0114) is simulated for 00:00 UTC 31 Aug. – 00:00 UTC 2 Sept. 2001. Mesoscale separation is performed on the results with the filtering scheme. Analyses show that the MM5 model well reproduced the position and intensity of heavy rain. Mesoscale characteristics of heavy rain were well represented in rainfall time scale, rainfall area, stream field and divergence at lower and upper levels. The interaction between inverted typhoon troughs and the mesoscale systems lead to heavy rain occurrence. The distribution of divergence fields at lower and upper levels can have a kind of indication for the rainfall. Heavy rains are closely associated with topography and land-sea distribution in South China. Weak instability is favorable to the generation of heavy rain.     

南海海温异常对华南气候影响的数值研究

采用NCEP／NCAR再分析的500hPa高度场、海温场(SST)、极冰资料，分析讨论了登陆广东省热带气旋活动异常与大气环流、极冰、SST、各种环流指数等影响因子之间的关系，得到登陆广东省热带气旋(TC)偏多、偏少年对应的大气环流、极冰、SST、各种环流指数的概念模型。分析结果表明：(1)亚欧大陆在秋、冬季中高纬度以经(纬)向环流为主，则次年登陆广东省的TC偏多(少)；(2)赤道东太平洋SST对热带气旋的影响最稳定和最明显，当1～4月赤道东太平洋SST偏暖(冷)时，当年登陆广东省的TC偏少(多)，次年登陆的TC偏多(少)；(3)夏、秋、冬季多(少)冰年，或者秋冬季连续多(少)冰年时，登陆热带气旋多为正常偏少(多)。     

CLIMATIC CHARACTERISTICS OF THE ONSET OF SOUTH CHINA SEA SUMMER MONSOON II. INTER-DECADAL VARIATION

By using the 40-year NCEP (1958-1997) grid point reanalysis meteorological data, we analyzed the inter-decadal variation on the climatic characteristics of the onset of South China Sea summer monsoon. The results are as follows. (1) There was great difference on the onset date of the SCS summer monsoon between the first two decades and the last two decades. It was late on the 6th pentad of May for the first two decades and was on the 4th and 5th pentad of May for the next two decades. (2) Except for the third decade (1978-1987), the establishment of the monsoon rainfall was one to two pentads earlier than the onset of the summer monsoon in all other three decades. (3) The onset of the SCS monsoon is the result of the abrupt development and eastward advancement of the southwesterly monsoon over the Bay of Bengal. The four-decade analysis shows that there were abrupt development of the southwesterly monsoon over the Bay of Bengal between the 3rd and 4th pentad of May, but there was great difference between its eastward movement and its onset intensity. These may have important effect to the earlier or later onset of the SCS summer monsoon. (4) During the onset of the SCS summer monsoon, there were great difference in the upper and lower circulation feature between the first two and the next two decades. At the lower troposphere of the first two decades, the Indian-Burma trough was stronger and the center of the subtropical high was located more eastward. At the upper troposphere, the northward movement of the center of subtropical high was large and located more northward after it landed on the Indo-China Peninsula. After comparison, we can see that the circulation feature of the last two decades was favorable to the establishment and development of the SCS summer monsoon.     

南海夏季风建立的气候特征Ⅱ—年代际变化

用NCEP 40年再分析格点气象资料对南海夏季风建立的气候特征的年限变化进行了分析，得到：（1）南海夏季风的建立时间前20年与后20年明显不同，前20年较晚为5月第6候，后20年则分别为5月的第4候和第5候，（2）除了第3个10年（1978－1987年）外，其余3个10年南海夏季风雨季的建立都比夏委风建立要早1至2候。（3）南海夏季风建立是孟加拉湾西南季风爆发性发展和东进的结果，4个10年孟加拉湾西南季风都在5月第3候及5月第4候有一个爆发性的发展过程，但它的东进速度及爆发强度却很不相同，这对南海夏季风建立时间的早晚有重要影响。（4）南海夏季风建立期间，前20年与后20年高低空环流形势相差很大，前20年与后20年相比对流层低层印缅槽较强而副热带高压中心位置偏东，对流层高层副热带高压中心移上中南半岛后北移距离较大，位置偏北，经过比较可以看出对于南海夏季风的建立和发展来看后20年的环流形势较为有利。     

华南暴雨中尺度对流系统的形成及湿位涡分析

利用MM5模式对发生在1998年5月23～24日华南暴雨和中尺度对流系统(Mesoscale Convective System,简称MCS)模拟的模式输出资料,根据湿位涡守恒原理和倾斜涡度发展理论分析了暴雨和MCS形成和发展的原因.结果表明,暴雨和MCS发生在倾斜湿等熵面具有弱对流稳定性的下陷区,沿湿等熵面下滑的冷空气与倾斜上升并具有较强对流有效位能的暖湿空气在下陷区会合的过程中经历了对流稳定性减小的过程,导致暴雨和MCS发生发展区域有气旋性的涡旋发展.对流发展区域的上空满足条件对称不稳定发生的条件,MCS中上升气流呈倾斜状态.由于湿等熵面倾斜,在暴雨和MCS的发展过程中,水平风垂直切变和湿斜压度的增大也有利于涡旋的发展,使暴雨和MCS得以维持.最后,给出了华南地区湿等熵面上暴雨和MCS发生发展的一个物理概念模型.