LAUSR

Abstract The initiation process of a rainstorm mesoscale convective system (MCS) that occurred in Beijing from 18 to 19 July 2016 was simulated using the Weather Research and Forecasting model, version 3.9, coupled with bulk or bin microphysics schemes. Overall, the rainstorm was simulated well, and thus the simulation was used to further assess the initiation process. Results showed that the convective cell was initiated and developed by the terrain convergence line and the forcing of latent heat release and the cold surface. The interaction between dynamic convergence and the latent heat forcing associated with the microphysical processes was studied separately with bulk and bin microphysics schemes to reveal the effects of the microphysical processes upon the initiation of the MCS. Further analysis of the mass and heat budgets of hydrometeors showed that the microphysical processes played a key role, both with the bulk and bin microphysics schemes, although considerable differences existed between their simulations. First, compared with the bulk scheme, the bin scheme displayed an earlier initiation and lower position of convection, which led to a lower mass center of rainwater, with more liquid water. Second, rainwater particles in the bin scheme simulation mainly came from the collection of cloud water and the melting of graupel, whereas the origin in the bulk scheme simulation was the melting of ice particles. The heat release of hydrometeors enhanced the environmental latent heat and further strengthened the circulation, which, combined with the cooling effects, caused stronger rainfall in the simulation with the bin scheme.