Daily to submonthly weather and climate characteristics of the summer 1998 extreme rainfall over the Yangtze River Basin

In this study, the daily to submonthly weather and regional climate characteristics associated with the extreme rainfall events over the middle to lower reaches of the Yangtze River Basin (YRB-ML) during the summer of 1998 are examined using the National Centers for Environmental Prediction/Department of Energy Reanalysis-2 data and a 54-day high-resolution (i.e., Δx = 4 km) simulation with a regional climate model (RCM). As verified against various observations, the RCM reproduces reasonably well the accumulated daily to biweekly rainfall events, their generation and subsequent evolution along the Meiyu front, and especially the rainfall events over the YRB-ML during the later stage of the 1998 floods. It is found that the early stage of the 1998 floods resembles in many aspects that occurred in normal years, except for a moister-than-climate state and stronger-than-normal low-level jets (LLJs) in Southeast China, whereas the later stage exhibits two abnormal height anomalies to the northeast and southwest of the YRB-ML, facilitating convergence of northeasterly and southwesterly flows along the Meiyu front. It is also found that the Meiyu front weakens in depth and intensity as it migrates northward across and then retreats to the YRB-ML, playing less important roles in rain production. Results show different daily to biweekly rainfall characteristics at the different stages, e.g., from eastward propagation to local generation of mesoscale convective systems and periodical rain production along the Meiyu front, most of which reach their maximum intensities over the YRB-ML during the morning to early afternoon hours. Results indicate that the northeasterly moisture supply enhances the production of steady heavy rain at the later stage of the 1998 floods. It is concluded that the West Pacific subtropical high and the Meiyu front control the general areas of rain production, the low-level moisture supply through LLJs and mesoscale disturbances determines whether or not sustaining flooding rains would occur, and what rainfall characteristics are at the daily to biweekly timescales.