Ensemble hindcast experiments for the flood period over China in 1998 by use of the CCSR/NIES atmospheric general circulation model

Sets of numerical hindcast experiments were carried out to study the excessive rain that happened over China in 1998 by using an atmospheric general circulation model. The monthly sea surface temperatures for 1998 were prescribed as the model boundary conditions. The initial atmospheric conditions for each of the 30 member simulations were obtained from the daily reanalysis data for 00 UTC from April 1 to April 30, 1998. The initial conditions for snow mass, soil temperature, and soil wetness were prescribed as those of the model climatology. The ensemble averages of the 30 member hindcast experiments captured the positive rainfall anomaly occurred over China in the summer of 1998, with 5 degrees of northward shift. The observed patterns of summer geopotential anomalies were qualitatively reproduced as well. It was revealed that initial atmospheric anomalies in April have apparent impacts on the simulated flow patterns over Eurasia and the North Pacific, and rainfall anomalies over China during the summer of 1998. However, the overall results suggest that tropical sea surface temperature anomaly played a key role in the heavy rainfall over China in 1998.