Impacts of boundary layer parameterization schemes and air-sea coupling on WRF simulation of the East Asian summer monsoon

The planetary boundary layer (PBL) scheme in the regional climate model (RCM) has a significant impact on the interactions and exchanges of moisture, momentum, and energy between land, ocean, and atmosphere; however, its uncertainty will cause large systematic biases of RCM. Based on the four different PBL schemes (YSU, ACM2, Boulac, and MYJ) in Weather Research and Forecasting (WRF) model, the impacts of these schemes on the simulation of circulation and precipitation during the East Asian summer monsoon (EASM) are investigated. The simulated results of the two local turbulent kinetic energy (TKE) schemes, Boulac and MYJ, are more consistent with the observations than those in the two nonlocal closure schemes, YSU and ACM2. The former simulate more reasonable low-level southwesterly flow over East China and west pacific subtropical high (WPSH) than the latter. As to the modeling of summer monsoon precipitation, both the spatial distributions and temporal evolutions from Boulac and MYJ are also better than those in YSU and ACM2 schemes. In addition, through the comparison between YSU and Boulac experiments, the differences from the results of EASM simulation are more obvious over the oceanic area. In the experiments with the nonlocal schemes YSU and ACM2, the boundary layer mixing processes are much stronger, which lead to produce more sea surface latent heat flux and enhanced convection, and finally induce the overestimated precipitation and corresponding deviation of monsoon circulation. With the further study, it is found that the absence of air-sea interaction in WRF may amplify the biases caused by PBL scheme over the ocean. Consequently, there is a reduced latent heat flux over the sea surface and even more reasonable EASM simulation, if an ocean model coupled into WRF.