西风带系统的异常活动对2003年淮河暴雨的作用机制研究

The Abnormal Activity of the Westerlies System and Its Impacts on 2003 Summer Heavy Rainfall over Huaihe Basins

利用NCEP/NCAR再分析资料,分析了2003年6—7月亚洲中高纬地区阻塞形势对淮河暴雨的作用。结果表明:亚洲北部阻塞高压的建立和维持,使得500 hPa高度上西风锋区南压,引导冷空气南下,造成中纬度地区冷暖空气和能量交换频繁。尤其在2003年6月20日后,阻塞高压活动加强,淮河流域北部维持为大气能量锋区,低层850 hPa副热带锋区也长期停滞在35°N附近。阻塞高压的异常活动通过对大气动量和热量的一次次向南频散,造成淮河流域连续发生暴雨天气过程。阻塞高压的强度及位置变化与前部低涡的活动密切相关,并通过西风带基本气流及长波系统的螺旋结构的变化,激发Rossby波的经向传播来影响副热带高压的南北进退,从而决定了雨带的位置。

The blocking activity and its impacts on heavy rainfall over Huaihe Basins during the period of Meiyu-season in 2003 is studied based on NCEP/NCAR daily reanalysis data and observation. The results show that the blocking activities are very remarkable and the evolution of blocking can be divided into three phases. When the blocking establishes and during its maintenance, it leads westerly front to move southwards as well as cold air consequently bursting out to south. As a result, cold and warm air mass and energy exchange frequently in midlatitude zone. Especially, the blocking in the west part of Baikal is getting more active, intensifies and moves southwards, thus, energy front and subtropical front maintain stably over the north of Huaihe Basins after 20th of June. Atmospheric momentum and heat diffuse to the south is led by the blocking, and atmospheric negative disturb momentum transfers to mean kinetic energy over the Huaihe Basins, so westerly jet is strengthened, which is favorable for the subtropical high＇s maintenance and persisting rainfall over this region. Statistical results indicate that the intensity and location of the blocking high are related closely to the low eddy occurred in front of it. The movement of the blocking system in zonal and meridional directions are in agreement with the latter, their correlation coefficients are 0.60 and 0.65 respectively based on the daily data. Their intensity variations are also in close relation, the blocking＇s strengthen and weaken are corresponding to low pressure eddy＇s deepening and weakening. The low eddy intensity variation and its horizontal movement have an impact on the position of the subtropical high ridge, the low eddy＇s strengthening/weakening leads to the subtropical high＇s southwards/northwards movement, similarly, the low eddy＇s movement to the west/east agrees with subtropical high＇s north/south shift. Additionally, results show that the blocking activity can trigger Rossby wave fluctuation in longitude direction so that to affect the subtropical high by changing westerly basic flow and helix configuration of long wave. Especially, during the beginning and the end of Meiyu, long wave pattern evolves gradually to northwest-southeast feature as the guidance in Asian mid and high latitudes. At the same time, basic flow changes remarkably companying the configuration of long wave, which leads the Rossby wave spreading to high-latitudes, atmosphere with high geopotential in lower and middle latitudes transmits to north companying the Rossby wave, thereby, subtropical high undergoes seasonal jump to north and rainfall belt shifts to north. On the contrary, rain belt moves several times southwardly during the period of Meiyu season, which are mainly because long wave pattern in mid-high latitudes evolves gradually to northeast-southwest festure or the basic flow changes reversely, and the Rossby wave spreads southwards, the atmosphere with low geopotential in mid-high latitudes transmits to south, forcing the subtropical high to .recede southwards.