Large-scale Environmental Characteristics of Heavy Rainfall Before and After Tropical Cyclone Landing in South China

In order to develop some operational forecasting methods in the period of 24 hours for precipitation before and after tropical cyclones landing in South China, the analysis of the relationship between the environmental factors and the rain field is made, using NCEP/NCAR reanalysis data, satellite cloud images and 38 daily precipitation data of 7 tropical cyclones. The regions with daily rainfall amount over 10 mm are selected, especially those areas exceeded 50 mm rainfall caused by TC which are divided into three types: zonal, meridional and NE—SW orientated kinds. The detail analysis on the main meteorological factors related to the heavy rainfall is done by means of diagnosis and statistical methods. The relationship between streamline field and rainfall distribution, the effect of different subtropical high intensity and position, the function of the jet and wind shear at low-level on rainfall zones and the function of the all moisture flux convergence and monsoon cloud surge and so on are investigated. The result indicates that before and after tropical cyclones landing in South China, the rainfall distribution and intensity in 24 hours relate closely to meteorological factors such as streamline field at upper level, the subtropical high at middle level, the low-level jet, the low-level wind shear, the moisture flux convergence, the monsoon cloud surge and so on. On the other hand, the different allocation of these factors leads to different types of rainfall region and different precipitation intensity. The main characteristics of the zonal model can be described as the follows: the subtropical high's position at middle level is more westward, low-level wind shear connects to the center of tropical cyclone, and an easterly jet or southeasterly jet is around tropical cyclone; southwest monsoon cloud surge is northeastward; a divergent center on the west or the south to the tropical cyclone at the upper level streamline field. And the main pointsof meridional model are like these: subtropical high position at middle level is more eastward and its western edge is with a square figure; a wind sheer line is in the direction of N—S or NE—SW through the tropical cyclone center, and a northwestward or northward low-level jet is at eastern side; southwest monsoon cloud surge moves to tropical cyclone; a middle-level trough in westerlies or a subtropical jet is with an N—S axis next to the tropical cyclone at western side. As to the NE—SW orientated type, the subtropical high position is more eastward and its intensity is weaker; a trough in the westerlies at middle level moves towards tropical cyclone, the low-level wind shear line through the center of tropical cyclone is in the direction of NE—SW, a northeastward low-level jet is on the southeastern side. The conceptual models according to the above relationship between the large-scale environmental characteristics and rain field are summarized. Also, some physical causes of the heavy rainfall formation in each are explained. The result may provide a referable method for the very short-range rainfall prediction before and after tropical cyclone's landfall.