Effects of ice formation on convective cloud development

Abstract

A study is performed of the importance of the ice-phase formation in convective cloud development. With this purpose, a one-dimensional and time-independent cloud model is applied to radiosonde data, corresponding to different atmospheric instability conditions, and modifications are introduced in the microphysics of the drop freezing processes. It is shown that raindrop freezing, occurring at middle levels, can contribute markedly to the cloud development in moderate atmospheric instability conditions. An example is considered in which the updraft speed above the freezing level and the altitude of the cloud top are substantially decreased when this process is suppressed. The effects of modifying the raindrop freezing rate as a function of the temperature are simulated by changing the value of the preexponential coefficient in the Bigg's equation. It is found that the cloud parameters are not very sensitive to such variations, but that, in moderate instability conditions, a significant enhancement of the updraft speed at middle levels can occur when the raindrop freezing rate is increased by more than one order of magnitude. The cloud development in high instability conditions is much less sensitive to these variations. Vertical plots are also obtained for the concentration of solid and liquid cloud particles formed during the process, and some possible effects of cloud seeding, represented, for instance, by an increase of the raindrop freezing rate, are discussed.