Representation of Clouds in Large-Scale Models

Abstract

A prognostic scheme for stratiform and convective clouds is developed for large-scale models. The time evolution of clouds is defined through the large-scale budget equations for cloud water content and cloud air (which is converted into a prognostic equation for fractional cloud cover). The scheme considers the formation of clouds in connection with large-scale ascent diabatic cooling, boundary-layer turbulence, and horizontal transport of cloud water from convective updrafts. Clouds dissipate through adiabatic and diabatic heating, turbulent mixing of cloud air with unsaturated environmental air, and depletion of cloud water by precipitation. The scheme differs from conventional schemes in its approach, which is fully prognostic and model consistent, and in the larger degree of complexity as the formation of anvil and circus clouds originating by cumulus updrafts and boundary-layer clouds is included. The scheme has been tested in the European Centre for Medium-Range Weather Forecasts (ECMWF) global forecast model and compared with the ECMWF operational cloud scheme. The results show that realistic cloud fields are produced when compared to observed values of cloud cover and cloud water content. The representation of cloud process in connection with anvil clouds is shown to have a strong effect on the hydrological cycle and the maintenance of the tropospheric water vapor content.