A unified parameterization of clouds and turbulence using CLUBB and subcolumns in the Community Atmosphere Model

K Thayer-Calder,P A Bogenschutz,A Gettelman,H Morrison,E Raut,C Craig,V E Larson,Z Guo,M C Wyant,S Goldhaber,J K Weber,B M Griffin,J Höft,M Wang,C.-C Chen,S J Ghan

doi:10.5194/gmd-8-3801-2015

Abstract

Abstract. Most global climate models parameterize separate cloud types using separate parameterizations. This approach has several disadvantages, including obscure interactions between parameterizations and inaccurate triggering of cumulus parameterizations. Alternatively, a unified cloud parameterization uses one equation set to represent all cloud types. Such cloud types include stratiform liquid and ice cloud, shallow convective cloud, and deep convective cloud. Vital to the success of a unified parameterization is a general interface between clouds and microphysics. One such interface involves drawing Monte Carlo samples of subgrid variability of temperature, water vapor, cloud liquid, and cloud ice, and feeding the sample points into a microphysics scheme. This study evaluates a unified cloud parameterization and a Monte Carlo microphysics interface that has been implemented in the Community Atmosphere Model (CAM) version 5.3. Model computational expense is estimated, and sensitivity to the number of subcolumns is investigated. Results describing the mean climate and tropical variability from global simulations are presented. The new model shows a degradation in precipitation skill but improvements in shortwave cloud forcing, liquid water path, long-wave cloud forcing, precipitable water, and tropical wave simulation.

Highlights

Most climate models today use separate parameterizations to model separate cloud types, such as stratiform clouds, shallow cumuli, and deep cumuli
This study evaluates a unified cloud parameterization and a Monte Carlo microphysics interface that has been implemented in the Community Atmosphere Model (CAM) version 5.3
The use of multiple, separate cloud parameterizations leads to unnecessary complexity

Summary

Introduction

Most climate models today use separate parameterizations to model separate cloud types, such as stratiform clouds, shallow cumuli, and deep cumuli. Each parameterization uses its own separate equation set. The resulting suite of parameterizations is intended, collectively, to represent the full range of subgrid-scale clouds included in the climate model. While the use of separate parameterizations for separate cloud regimes offers several advantages, it suffers disadvantages. The use of multiple, separate cloud parameterizations leads to unnecessary complexity. Some of the complexity is of a practical sort: it is hard to understand a suite of parameterizations written by different authors that use differing coding conventions and assumptions.

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