Work

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

Year: 2025

Type: article

Abstract: A new cirrus cloud parameterization for the ICOLMDZ atmospheric general circulation model (AGCM) is presented. It is built using prognostic equations for cloud fraction and cloud water vapor, 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 standard 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. As for the standard parameterization, ice fallspeed velocity is a major parameter for tuning the ice water content, but the new parameterization allows ice water content and cloud cover to be tuned separately.

Source: Journal of Advances in Modeling Earth Systems

DOI: 10.1029/2024ms004918

Authors: Audran Borella, Étienne Vignon, Olivier Boucher, Yann Meurdesoif, Laurent Fairhead