PRECIPITATION AND RADIATION MODELING IN A GENERAL-CIRCULATION MODEL -INTRODUCTION OF CLOUD MICROPHYSICAL PROCESSES

Cloud microphysical processes are introduced in the precipitation para meterization of a general circulation model (GCM). Three microphysical processes are included in this representation of warm cloud precipita tion: autoconversion of droplets, collection of droplets by falling ra indrops, and evaporation of raindrops falling in clear sky. The mean d roplet radius, tau, is calculated from the cloud water mixing ratio, w hich is computed in the model, and the cloud droplet number concentrat ion, N, which is prescribed. The autoconversion rate is set to zero fo r tau < tau(0), a prescribed threshold mean droplet radius. We investi gate the model sensitivity to tau(0) and to N, the cloud droplet conce ntration, which is linked to the concentration of cloud condensation n uclei and is likely to vary. We find that an increase in N leads to an increase in the amount of cloud water stored in the atmosphere. In fu rther experiments the mean droplet radius used in the parameterization of cloud optical properties is calculated in the same way as in the p recipitation parameterization in order to bring more consistency betwe en the different schemes. We again investigate the model. sensitivity to tau(0) and to N and we find that an increase in N significantly enh ances cloud albedo.