Impact of boundary layer processes on seasonal simulation of the East Asian summer monsoon using a Regional Climate Model

In this study, an improved planetary boundary layer (PBL) scheme (YSU scheme) is implemented in a regional climate model (SNURCM) to rectify the systematic bias of the model with the MRF PBL scheme, and the impact of new PBL processes on the simulation of precipitation in the East Asian summer monsoon (EASM) is investigated through regional climate simulations with the MRF and the YSU schemes for the summer of 1998 when extreme floods occurred over East Asia. Compared to the experiment with the MRF scheme that shows excessive monsoon precipitation, particularly over the ocean, the experiment with the YSU scheme improves the seasonal mean precipitation as well as associated large-scale circulations. The temporal progression of the monsoon precipitation and 500hPa geopotential height, and vertical structure are also improved by the revised scheme. The MRF scheme simulates more convective precipitation over the ocean than the YSU scheme, since excessive PBL mixing results in the positive feedback between convective precipitation and latent heat flux at the sea surface. The MRF scheme also simulates more non-convective precipitation over the ocean due to distorted large-scale circulations and excessive PBL mixing. In the experiment with the YSU scheme, the simulated precipitation over the ocean is in good agreement with the observation, since the positive feedback is relatively reduced and large-scale features are reasonably reproduced due to decreased PBL mixing. Excessive PBL mixing in the MRF scheme can also amplify the biases of precipitation over the ocean due to uncoupled air-sea interaction that can result in the imbalance of the energy budget at the sea surface. This implies that new YSU PBL processes can have crucial influences on the regional climate simulation without an ocean model. The results of additional experiments without the spectral nudging technique also reaffirm the impact of the YSU scheme, and further indicate that the development of a coupled regional climate model is required for more reasonable simulation of the EASM.