Investigation of the Synoptic Situation Associated with Autumnal NonTC Heavy Precipitation in Xisha
Using daily rainfall data of Xisha Islands, reanalysis data of European Centre for MediumRange Weather Forecasts (ECMWF), typhoon data of Shanghai Typhoon Institute, the present study investigates the rainfall climatology of Xisha, and classifies its synoptic situation associated with the nonTC heavy precipitation for September-October. The method used is a new composite method, which combines REOF and Kmeans clustering. It is found that Xisha is dry from December to April of the following year, and wet from May to November. Monthly rainfall peaks at September and October. Three optimal categories are obtained, featuring southwest monsoon trough, the combined effect of the monsoon trough and cold, strong cold air, respectively. Among the three categories, the first one occurred mainly in September of the 1990s, while the third one, which appears mainly in October, has increased significantly since 1990s. Preliminary analysis shows that the monsoon trough was more active in 1990s and the cold air activities have become stronger since 1990s. This may be the major reason for the increasing of the total amount of the nonTC heavy rainfall.