A model-based diagnostic study of "99.6" Meiyu front rainstorm

The long-lasting rainy season over East Asia in early summer is called Meiyu in China and Baiu in Japan. In June 1999, the precipitation in middle and low basin of Yangzi River is twice the average in normal years. A model simulation of "99.6" Meiyu Front rainstorm by nonhydrostatic mesoscale model MM5V3 is analyzed in an effort to study the mechanism of the Meiyu front. The diagnosis of Convective Momentum Transport (CMT) shows that, the budget residual X of the horizontal momentum has different effects in different periods of the low vortex with the shear line: On low levels, X strengthened the southwest flow to north on the occurring stage, on middle levels, X accelerated northwest flow behind East Asia Trough to north and also accelerated northwest flow ahead of East Asia Trough to the south. All these were in favor of deepening the East Asia Trough and mingling cold and warm air, and gave favorable conditions to produce rainstorm.