Evaluation of a New Mixed-Phase Cloud Microphysics Parameterization with SCAM, CAPT Forecasts and M-PACE Observations

Liu, X., University of Wyoming

General Circulation and Single Column Models/Parameterizations

Cloud Modeling

Liu, X, S Xie, and SJ Ghan. 2007. "Evaluation of a new mixed-Phase cloud microphysics parameterization with the NCAR single column climate model (SCAM) and ARM M-PACE observations." Geophysical Research Letters 34, L23712, doi:10.1029/2007GL031446.

Xie, S, J Boyle, SA Klein, X Liu and S Ghan. 2008. "Simulations of arctic mixed-phase clouds in forecasts with CAM3 and AM2 for M-PACE." Journal of Geophysical Research, in press.

In recent decades, the Arctic has been experiencing warming twice as large as the global averages. However, the uncertainty in climate model simulations and future projections is much larger in the Arctic than throughout the rest of the globe. Arctic clouds play a central role in the Arctic climate feedbacks and radiative budget, but are not well represented in current climate models.

Most global climate models generally prescribe the partitioning of condensed water into liquid droplets and ice crystals in mixed-phase clouds according to a temperature-dependent function, which affects modeled cloud phase, cloud lifetime, and radiative properties. This study evaluates a new physically-based mixed-phase cloud microphysics parameterization (for ice nucleation and water vapor deposition) against the ARM Mixed-phase Arctic Cloud Experiment (M-PACE) observations using the NCAR Community Atmospheric Model Version 3 (CAM3) running in the single column mode (SCAM) and in the CCPP-ARM Parameterization Testbed (CAPT) forecasts.

Both SCAM and CAPT tests show that the new physically-based cloud microphysical scheme produces a more realistic simulation of the cloud phase structure and the partitioning of condensed water into liquid droplets against observations during the M-PACE than the standard CAM with an oversimplified cloud microphysics scheme. CAM3 in the CAPT forecasts significantly underestimates the observed boundary layer mixed-phase cloud fraction. The simulation of the boundary layer mixed-phase clouds and their microphysical properties is considerably improved in CAM3 when the new scheme is used. The new scheme also leads to an improved simulation of the surface and top of the atmosphere longwave radiative fluxes.

In most current climate models, conventional parameterizations for the partitioning of condensed water into liquid droplets and ice crystals in mixed-phase clouds (i.e., assuming larger cloud liquid amount at higher temperatures) are not applicable to the M-PACE observations of Arctic clouds, which showed the opposite trend, i.e., liquid dominated near the tops of the clouds with precipitating ice near the cloud base. In order to realistically simulate the vertical cloud structure in mixed-phase clouds for large-scale models, explicit treatment of ice nucleation and liquid conversion to ice via the Bergeron-Findeisen process should be considered. This can also impact cloud radiation budgets.