Simulation study on the anomalous East Asian summer monsoon in 1998 using a variable-grid atmospheric general circulation model

In this paper,a variable-grid atmospheric general circulation model LMDZ4 developed by French National Center for Scientific Research(CNRS) Laboratoire de Meteorologie Dynamique(LMD) was used to simulate the 1998 East Asian summer precipitation.The performance of the variable-grid model in the simulation of summer precipitation over East Asia is examined.The results show that the summer flood of 1998 over East Asia is generally reasonably simulated by LMDZ4.Both the maximum center and the major rain-band,including the decreasing trend of the total precipitation from the southeast to the northwest,are reproduced to a certain degree.The strength of the model is that it reasonably reproduces two march and two retreat processes of the rain band,including both rainfall intensity and rain band coverage,and thus well reproduces the double-Meiyu over the Yangtze River Basin during the summer of 1998.However,the model still shows some weaknesses.A large bias is seen in the precipitation field over steep topography.Meanwhile,there are more low-level moisture transport convergences and stronger high-level circulation divergences over the Yangtze-Huai River Basin,North China and Sichuan Basin,which lead to more precipitation over there.Whereas,anti-cyclonic moisture transport biases and associated less precipitation are seen over the region south to 30°N.As for the double-Meiyu over the Yangtze River Basin(YRB) during 1998 summer,the duration period is shorter and the coverage is smaller than the observations for the two heavy precipitation events(especially for the second one).The model simulates more precipitation over the Yellow-Huai River Basin and South China(SC).Analysis shows that the simulated circulation biases well explain the short of model in simulating precipitation.The exceptional characteristics of LMDZ4 regional version lie in local zoom and the forecasting fields in the zoomed region recovering to reanalysis data at a 10-day timescale.The results from sensitive tests show that the recovery generally improves the simulation of precipitation over North China,but has bad effect on that over YRB and SC.Compared with the uniform-grid simulation,the local zoom at East Asia greatly increases the resolution and produces better simulation for the summer precipitation over East Asia.