The most advanced mesoscale WRF model in the world is used to simulate the genesis process of tropical cyclones.When the grid resolution is increased from 9 km to 3 km,the cumulus parameterization scheme in 3 km grid does not work,so the microphysical scheme is used to simulate the characteristics of convective-scale system in the 3 km domain.As a result,the formation process of tropical cyclone in the model becomes slow.If the 3 km domain is added after the low pressure disturbance has developed to a certain extent,the formation process tends to accelerate.This study is focused on the analysis of the phenomenon.Results show that the low-level (950-700 hPa) vertical shear of horizontal wind speed decreases by using only the microphysical scheme,which is not conducive to the development of convection.The decrease of low-level vertical shear is mainly due to the difference of vertical momentum advection between the two experiments.During the 6 h after adding the inner nest,the convective-scale (removing the area average component) vertical momentum advection at low-level is double on average and 5 times in maximum compared to control experiment.The increase of momentum mixing in the low-level is due to the increase of vertical velocity simulated by microphysical scheme.In addition,the CAPE (convective available potential energy) is rapidly consumed by using only the microphysical scheme.The decreases of low-level vertical shear and CAPE are not conducive to the development of convection near the disturbance center,resulting in the delay of genesis and development of tropical cyclone.This study demonstrates that there are still some problems in simulating the real development process of convection during the formation of tropical cyclone with the present microphysical schemes in WRF model.