Effects of Convective Microphysics Parameterization on Large-scale Cloud Hydrological Cycle and Radiative Budget in the NCAR CAM5

 

Author

Guang Zhang — University of California, San Diego

Category

CAPI Deep Convective Clouds

Description

A two-moment microphysics scheme for deep convection was previously implemented in the NCAR Community Atmosphere Model version 5 (CAM5) by Song et al. (2012). The new scheme improved hydrometeor profiles in deep convective clouds and increased deep convective detrainment, reducing the negative biases in low and mid-level cloud fraction and liquid water path compared to observations. In this work, we examine the impacts of this improved microphysical representation on regional scale water and radiation budgets. As a primary source of cloud water for stratiform clouds is detrainment from deep and shallow convection, the enhanced detrainment leads to larger stratiform cloud fractions, higher cloud water content, and more stratiform precipitation over the ocean, particularly in the subtropics where convective frequency is also increased. This leads to increased net cloud radiative forcing. The inclusion of convective microphysics significantly improves the vertical distribution of clouds. In the western Pacific warm pool region, it shows a trimodal distribution of clouds corresponding to shallow, congestus and deep convection, in agreement with previous observations. In the subtropics, low-level cloud amount is increased drastically. Over land regions, cloud amounts are reduced due to lower relative humidity, leading to weaker cloud forcing and increased OLR. Comparing the water budgets to cloud resolving model simulations shows improvement in the partitioning between convective and stratiform precipitation, though the deep convection is still too active in CAM5. The addition of convective microphysics leads to an overall improvement in the regional cloud water budgets.