A parametrization of the ice water content observed in frontal and convective clouds

Abstract

The properties of the ice phase in a number of cloud types are investigated to improve the ice phase parametrization in atmospheric global-climate models. Frontal clouds over southern England and the sea areas around the British Isles, maritime convective clouds over the North Atlantic, and continental convective clouds over New Mexico and Montana in the USA are studied. Ice concentrations are seen to be several orders of magnitude higher than those which could be attributed to primary nucleation of ice nuclei at cloud-top temperatures. Thus secondary ice multiplication processes must be operating in each cloud type. Evidence suggests that the process of ice splinter production during riming, the Hallett–Mossop process which operates at temperatures around −6°C, is the dominant mechanism operating. The data analysed are parametrized as phase ratios, the fraction of cloud condensed water found in the liquid phase, and the variation of this phase ratio with temperature is examined. The greatest differences are observed between frontal and convective clouds, although smaller differences between continental and maritime clouds of the same type are also seen. In general, frontal clouds possess very high fractions of ice across a wide range of temperature. In contrast, convective clouds exhibit a wide range of phase ratio across the whole temperature range observed. These differences are attributed to the greater vertical wind velocities present in convective clouds. These parametrizations have been used in the UK Meteorological Office Global Climate Model. They are valid for clouds which span the Hallett-Mossop splinter-production temperature range.