Sensitivity study of ice spectra shape parameter in the CAM

Abstract:

There are distinct simplifications and uncertainties in the representation of ice clouds in climate models. In this study, we examine the impacts of a given ice crystals size distribution parameter (i.e., spectra shape parameter, μ) with the Community Atmosphere Model version 5.3 (CAM5). The μ, which is set to zero in the default cloud microphysics scheme, is replaced by a nonzero value (e.g., 1, 3, and 5). Offline simulations show that the nonzero μ would make the ice particle size distribution more concentrated and then led to distinct impacts on the cloud microphysical processes such as ice fall speed, deposition growth, and auto-conversion of cloud ice to snow. Simulations with CAM5 model show that changing the μ from 0 (default) to 5 causes obviously changes in cloud ice water path, net cloud forcing, and precipitation. Furthermore, the anthropogenic aerosol indirect forcing through cirrus clouds is also obviously sensitive to the μ. In shorts, more attention needs to be paid to describe the μ in the cloud microphysics scheme used for climate models.