Mixed-phase clouds: from theory to application in a global model

A lack of liquid water in mixed-phase clouds has been implicated as a possible cause of model outgoing short-wave biases, and associated SST biases, over the Southern Ocean. A physically based method for parametrizing the role of sub-grid scale turbulence in the production and maintenance of supercooled liquid water in mixed-phase clouds is presented. The approach used is to simplify the dynamics of supersaturation fluctuations to a stochastic differential equation that can be solved analytically to provide expressions for the mixed-phase cloud fraction and mean liquid water content. We demonstrate that the approach captures the properties of decameter-resolution Large Eddy Simulations of a turbulent mixed-phase environment and show how it can be implemented in a GCM and the effects that this has on Southern Ocean biases and on Arctic stratus.