Characteristics of regional heavy rainfall in the pre-flood season in South China and prediction skill of NCEP S2S

Abstract

South China is one of the regions with the longest flood season and the highest frequency of floods in China. Understand the characteristics of heavy precipitation during the flood season and its potential mechanisms is crucial for predicting and preventing flood disasters in South China. In this study, we identified that the regional heavy precipitation during the first flood season in South China can be categorized into frontal heavy rainfall (FR) and warm-sector heavy rainfall (WR).The FR is mainly controlled by the southwest monsoon and the westerly jet in the northwest of the West Pacific Subtropical High (WPSH). The westerly wind brings enough water vapor to the front, creating an unstable environment that leads to heavy rainfall events. When precipitation occurs in warm regions, warm and humid air from the west side of the WPSH goes northward and westerly jet brings warm and humid air to the mainland to rise and condense, thus forming heavy rainfall.The extended period predictability of the National Centers for Environmental Prediction (NCEP) sub-seasonal to seasonal (S2S) forecast system for the two types of heavy rainfall decreases with the increase of the forecast lead time, and the prediction skill of NCEP S2S for FR is obviously better than that for WR. The poor prediction skill of NCEP S2S for WR is mainly due to the model inadequacies in predicting atmospheric circulation and water vapor transport anomalies corresponding to WR. When predicting WR, the model exhibits the easterly wind has controlled in the upper troposphere as early as ten days in advance. Meanwhile, the model struggles to predict the characteristics of the WPSH and the water vapor transport anomalies from the Bay of Bengal.This may be attributed to the absence of a distinct weather-scale boundary (front) for WR in the model, making it more difficult to forecast WR than FR.