Rainfall Assimilation Using a New Four-Dimensional Variational Method: A Single-Point Observation Experiment

Accurate forecast of rainstorms associated with the mei-yu front has been animportant issue for the Chinese economy and society. In July 1998 a heavyrainstorm hit the Yangzi River valley and received widespread attention fromthe public because it caused catastrophic damage in China. Several numericalstudies have shown that many forecast models, including Pennsylvania StateUniversity National Center for Atmospheric Research's fifth-generationmesoscale model (MM5), failed to simulate the heavy precipitation over theYangzi River valley. This study demonstrates that with the optimal initialconditions from the dimension-reduced projection four-dimensionalvariational data assimilation (DRP-4DVar) system, MM5 can successfullyreproduce these observed rainfall amounts and can capture many importantmesoscale features, including the southwestward shear line and the low-leveljet stream. The study also indicates that the failure of previous forecastscan be mainly attributed to the lack of mesoscale details in the initialconditions of the models.