MOISTURE SUPPLY AND GROWTH OF STRATIFORM PRECIPITATION

Abstract

The steady-state water-budget theory is developed for stratiform precipitation. The variation of cloud liquid water with altitude in a layer depends on the rate at which it is produced by the updraft and depleted by the precipitating particles. Calculations representing this balance are made for a model cloud which follows the wet adiabat and has a parabolic distribution of vertical velocities and an exponential particle-size distribution. In the rain portion of the cloud, accretion is the only significant growth process; production exceeds consumption, so that liquid water is stored as cloud with a maximum content of 0.2 to 0.3 g m?3 at the +2C level. Within the melting layer the growth rate increases, due to the combined effects of condensation and accretion, and cloud liquid water is sharply depleted. In a narrow layer just above the 0C level, production again exceeds consumption, and a secondary maximum of cloud liquid water occurs at about the ?2C level. At colder temperatures, the rate of growth by sublimation increases and cloud liquid water vanishes at about ?4C for a surface precipitation rate of 2 mm hr?1, and above ?10C for 10 mm hr?1. Thus, at low precipitation rates, icing is confined to a narrow layer just above the 0C level, while at higher rates it may occur at much colder temperatures in combination with snow. The stratiform precipitation process is shown to be extremely efficient with these implications: (1) cloud storage need not be considered in quantitative calculation of precipitation rates; (2) artificial seeding only serves to impose a small perturbation on the precipitation rate with no increase in the net amount, although in a moving system it may be possible to increase rainfall by 5 to 10 per cent at a given locality at the expense of the precipitation downstream.