Improving the simulation of East Asian summer monsoon with mesoscale enhancement in an AGCM

Abstract

In this study, the parameterization of sea surface turbulent heat flux in National Center for Atmospheric Research Community Atmosphere Model version 5 (CAM5) is improved by including the contribution of mesoscale convective motion (i.e., mesoscale enhancement). The effect of mesoscale enhancement on the simulation of the East Asian summer monsoon circulation is further explored with SST-forced experiments that are conducted for the period of January 1979 to December 2015. Experiments suggest that the standard CAM5 exhibits evident biases in simulating the East Asian summer monsoon system. For example, the simulated westerlies at 850 hPa in low latitude of East Asia are too weak, the precipitations in the Bay of Bengal, the South China Sea, and the tropical western Pacific are underestimated, the simulated south Asia and western Pacific subtropical highs are too weak, and the upper and middle troposphere in the low latitudes of East Asia is too cold. By including the mesoscale enhancement in CAM5, these biases are clearly reduced. For example, the simulated summer mean precipitation in the Bay of Bengal is increased from 9.85 to 11.76 mm/day, which is closer to the observed value of 13.80 mm/day. Further analyses show that the improvements in the East Asian summer monsoon precipitations and circulations can be attributed to the enhancement of convections and the increase in water evaporation in the tropical western Pacific Ocean induced by mesoscale enhancement. Correspondingly, the latent heat due to vapor condensation in the atmosphere also increases, which reduces the cold bias in the mid-upper troposphere. Meanwhile, the geopotential height of the upper-middle-level atmosphere is elevated, which makes the simulated intensity of the south Asian and the western Pacific subtropical highs, two important components of the East Asian summer monsoon circulation, closer to the reality.