A PARAMETRIZATION OF THE ICE WATER-CONTENT OBSERVED IN FRONTAL AND CONVECTIVE CLOUDS

The properties of the ice phase in a number of cloud types are investi gated to improve the ice phase parametrization in atmospheric global-c limate models. Frontal clouds over southern England and the sea areas around the British Isles, maritime convective clouds over the North At lantic, and continental convective clouds over New Mexico and Montana in the USA are studied. Ice concentrations are seen to be several orde rs of magnitude higher than those which could be attributed to primary nucleation of ice nuclei at cloud-top temperatures. Thus secondary ic e multiplication processes must be operating in each cloud type. Evide nce suggests that the process of ice splinter production during riming , the Hallett-Mossop process which operates at temperatures around -6 degrees C, is the dominant mechanism operating. The data analysed are parametrized as phase ratios, the fraction of cloud condensed water fo und in the liquid phase, and the variation of this phase ratio with te mperature is examined. The greatest differences are observed between f rontal and convective clouds, although smaller differences between con tinental and maritime clouds of the same type are also seen. In genera l, frontal clouds possess very high fractions of ice across a wide ran ge of temperature. In contrast, convective clouds exhibit a wide range of phase ratio across the whole temperature range observed. These dif ferences are attributed to the greater vertical wind velocities presen t in convective clouds. These parametrizations have been used in the U K Meteorological Office Global Climate Model. They are valid for cloud s which span the Hallett-Mossop splinter-production temperature range.