Partial lateral forcing experiments reveal how multi-scale processes induce devastating rainfall: a new application of regional modeling

Abstract

The 1998 extremely heavy rainfall events over East Asia are investigated through partial lateral forcing (PLF) experiments with the Weather Research and Forecasting model to determine the impacts of the synoptic (SY), intra-seasonal (IS), and inter-annual (IA) forcing across the lateral boundary on the extreme climate anomalies. The large-scale lateral boundary forcing was derived from an ensemble reanalysis dataset and decomposed into climatological, SY, IS, and IA components. The PLF experiments show that the IS forcing not only triggers the monsoon onset and drives two northward propagation events of the subtropical front but also has dominant contributions to the two heaviest rainfall events over the Yangtze River Basin (YRB) and South China, suggesting the critical role of the intra-seasonal variability in the devastating 1998 floods. Previous studies perceived that the northward propagating IS oscillation from the tropics regulates the extreme heavy rainfall of East Asia summer monsoon in 1998. However, we find that the IS forcing from the mid-latitude plays a more important role than the forcing from the tropics in generating the two extreme rainfall events in 1998. During the first extreme event in June, the IS forcing across the western boundary is the major cause of the northward advance of the subtropical front and the heavy rainfall over the YRB and South China, with the IS forcing across the northern boundary providing the second largest contribution. During the second extreme event (July 15–August 5), the IS forcing from the eastern boundary plays a dominant role in driving the southward retreat and northward advance of the subtropical front, causing another heavy rainfall over the YRB and South China. The western and northern IS forcing also has large contributions to the second extreme event. We have estimated the contributions to the seasonal anomalous rainfall by the three types of forcing. The SY forcing tends to have a moderate effect on the YRB rainfall but significant reduction of the rainfall in South China. The IS forcing has dominant contributions to the seasonal-mean rainfall anomalies over all three sub-regions of China (North China, the YRB, and South China). The IA forcing mainly enhances the rainfall in South China but reduces the precipitation in the YRB slightly. This study portends a promising application of regional climate models to identify key factors causing extreme climate events. The PLF methodology can be used to study a broad range of climate phenomena and to understand the effects of variety of dynamic and physical processes in climate variability and predictability.