Some numerical experiments are carried out with the improved MM4 model by incorporating with different lateral boundary conditions and topographic forcing as well as the different parameterization schemes of the planetary boundary layer.This helps us to understand the reliability of the model and to verify whether the model is able to simulate the Mei-yu front systems together with the storm events when the Fritsch-Chappell cumulus parameterization scheme is incorporated.A comparison is made between the simulation with Fritsch-Chappell cumulus parameterization and that with Kuo-Anthes scheme.It is found that the forecasting skill of geopotential height and rainfall is signifcantly improved in the improved MM4 model.The improved version of the model is very sensitive to the lateral boundary condition and the topography other than the parameterization scheme of the planetary boundary layer.It should be very careful if the lateral boundary condition or topographic forcing in the model is changed.