Steady-state supersaturation distributions for clouds under turbulent forcing

Abstract

The supersaturation equation for a vertically moving adiabatic cloud parcel is analysed. The effects of turbulent updrafts are incorporated in the shape of a stochastic Lagrangian model, with spatial and time correlations expressed in terms of turbulent kinetic energy. Using the Fokker-Planck equation, the steady-state probability distributions of supersaturation are analytically computed for a number of approximations involving the timescale separation between updraft fluctuations and phase-relaxation, multiplicative noise simplification and droplet or ice particle size fluctuations. While the analytical results are presented in general for single-phase clouds, the calculated distributions are used to compute mixed-phase cloud properties -- mixed fraction and mean liquid water content in an initially icy cloud -- and are argued to be useful for generalising and constructing new parametrisation schemes.