A New Prognostic Parameterization of Subgrid Ice Supersaturation and Cirrus Clouds in the ICOLMDZ AGCM

Abstract

A new cirrus cloud parameterization for the ICOLMDZ atmospheric general circulation model (AGCM) is presented. Prognostic equations for cloud fraction, ice water content and cloudy water vapor are introduced, and the processes that affect these quantities are parameterized. In particular, the tendency in ice crystal mass concentration due to changes in water phases is calculated, as is the macroscale mixing of cirrus clouds with their environment. The parameterization simulates ice supersaturation in clear sky, an ubiquitous metastable phenomenon in the upper troposphere that is necessary for the formation of in situ cirrus clouds. Our parameterization also allows for cirrus clouds to be sub- and supersaturated with respect to ice. It is evaluated on a case study of a warm conveyor belt cirrus cloud forming and dissipating above the Paris region. The representation of high cloud cover is improved with the new parameterization relative to the default version of the cloud scheme in comparison to observations, as is the representation of water vapor. In particular, the upper tropospheric dry bias from ICOLMDZ is corrected, although it is replaced by a wet bias in some cases. Global simulations show similar results. Contrarily to the default parameterization, ice water content and high cloud cover can be tuned separately, but the ice fall velocity parameter, that controls the intensity of ice autoconversion, remains the most determining parameter controlling the overall ice water content.