Characteristics and Predictive Focus of Torrential Rain Along the Outer Boundary of West Pacific Subtropical High in Hebei Province
Composite analyses of 69 torrential rain cases along the outer boundary of West Pacific subtropical high (WPSH) are carried out, based on the upper-level and surface observations, NCEP/NCAR reanalysis and the rainfall data of Beijing, Tianjin and Hebei Province. The results show that: (1) The frequency of torrential rain events increases obviously from the northwest to the southeast of Hebei Province, accompanied by three high-frequency areas, which are located in Qinhuangdao and Tangshan at the south foot of Yanshan Mountain, Xingtai at the east foot of Taihang Mountain and Cangzhou and Hengshui in Hebei Plain, respectively. There are two low-frequency areas, located in the mountains of the northwest of Baoding and the area in the north of Zhangjiakou and the north of Chengde respectively. (2) The 69 torrential rain cases can be divided into three groups according to the flow patterns of WPSH, which are called blocking, quasi-east-west banding and northeast-southwest banding WPSH, respectively. Some typical flow patterns at 500 hPa and the frequency of rainfall over 50 mm and 100 mm in each group are plotted by composite analysis, and spatial distributions of the elements including the atmospheric precipitable water vapor, CAPE, vertical velocity at 700 hPa, pseudo-equivalent potential temperature and K index etc. are given out. (3) Based on the statistics, the mean and extreme values of some physical quantities of the 69 torrential rain cases are counted, which can be considered as the important clues to forecast the amount and location of heavy rainfall, and to estimate the extremity of torrential rain in weather forecasting operation. (4) More attentions should be paid to the location and intensity of the middle/low-level convergence in addition to the location, intensity and flow pattern of WPSH and the upper trough in the operational forecasting of the torrential rain along the outer boundary of WPSH. Severe rainfalls are prone to occurr in the areas along the 584 or 586 dagpm line at 500 hPa, the shear or the center of the vortex at 700/850 hPa, as well as the inverted trough or the center of the surface low pressure.