A PHYSICALLY-BASED SCHEME FOR THE TREATMENT OF STRATIFORM CLOUDS AND PRECIPITATION IN LARGE-SCALE MODELS .1. DESCRIPTION AND EVALUATION OF THE MICROPHYSICAL PROCESSES

A stratiform-cloud and precipitation scheme, incorporating prognostic variables for cloud liquid water and cloud ice, has been developed for the CSIRO global climate model (GCM). The scheme includes physically based treatments of key microphysical processes, turbulent mixing and semi-Lagrangian advection of cloud-water species and interactive cloud radiative properties. Objectives in the development of the scheme wer e to improve upon the physical realism of parametrizations used in ear lier schemes, whilst also trying to provide a scheme with moderate com putational overheads. The parametrized microphysical processes are eva luated in relation to observations and theory, and are compared to tre atments used in earlier schemes in a series of short GCM experiments. It is argued that the treatment of precipitation formation in warm, an d mixed-phase, stratiform clouds is more realistic in the present sche me than in earlier schemes, which used crude methods for the parametri zation of autoconversion, and did not treat key ice-processes in a con sistent way. In the present scheme, accretion processes are more impor tant, whereas autoconversion is less important than in earlier schemes . To determine whether the cloud scheme requires the use of a reduced (split) time-step, the sensitivity of the various terms to the time-st ep is evaluated in another series of short GCM experiments. It is show n that the various terms are not very sensitive to the rime-step, so t he scheme can be efficiently implemented without the use of a split ti me-step. Overall, analytical or time-centred treatments perform better than implicit or explicit schemes, especially in the calculation of t he precipitation of cloud ice, where only an accurate analytical treat ment is found to perform satisfactorily at large time-steps. As a prel iminary validation of the scheme, zonal mean fields from a six-year mo del run are presented for the month of July. The results generally agr ee well with observations; in particular, the modelled cloudiness and long-wave cloud-forcing fields are more realistic than those obtained with the standard version of the CSIRO GCM.