The influence of different parameterizations on diurnal cycle of land precipitation in CAS-ESM

Abstract

The impacts of physical parameterizations on the diurnal cycle of precipitation (DCP) over land are investigated by comparing long-term simulations of the Chinese Academy of Sciences Earth System Model (CAS-ESM). Significant improvements in the DCP occur when the model replaces the Community Atmosphere Model version 4 (CAM4) physics package with the CAM5 package at both global and regional scales. Globally, the reduced amplitudes and delayed phases of DCP with the CAM5 package can be explained by the diurnal variations in convective available potential energy (CAPE). Regionally, the improvements in the DCP are caused by the decreased daytime convergence at 850 hPa, smaller vertically integrated water vapor transport, weakened vertical velocity, and smaller stratus cloud liquid water amount simulated by the CAM5 package. Besides, CAS-ESM with the CAM5 package produces the decreased amplitudes and delayed phases of the diurnal variation of the convective cloud, which is closely related to the planetary boundary layer (PBL) processes. CAS-ESM with the CAM5 package exhibits a significantly lower PBL height than with the CAM4 package for both direct model outputs and estimates with the bulk Richardson number method. The peaks of lifting condensation level (LCL) deficit (difference between PBL height and LCL height) occur later (∼1–3 h) and are lower with the CAM5 package than with the CAM4 package. Consequently, CAS-ESM with the CAM5 package decreases the probability of cloud formation and produces more realistic DCP. This study underscores the importance of PBL schemes to the simulated convective clouds and DCP.