Precipitation Mechanisms in a Shallow Convective Cloud Model

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  • 1 Cloud Physics Observatory, Department of Meteorology, University of Hawaii, Hilo 96720
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Abstract

Precipitation mechanisms in shallow convective clouds are studied using an axisymmetric cloud model. Clouds are classified into continental and maritime clouds, and further subdivided into warm and cool clouds. Different microphysical factors were selected in the model to represent these different cloud systems. In maritime clouds condensation and collection processes are sufficient to develop precipitation, while in continental clouds graupel formation appears to be a necessary step. In the latter case recirculation of ice crystals is required to initiate riming so that it takes a longer time for rainfall initiation than in the maritime case. In maritime clouds inclusion of the ice phase does not change the rainfall pattern, although both raindrops and graupel contribute equally to precipitation.

The possibility of cloud modification is studied by. increasing the ice nuclei concentration. In continental clouds an increase in ice nuclei concentration of 100 times more than the natural ice nuclei concentration causes activation of a higher number of ice nuclei at warmer temperatures, so that ice crystals can grow large enough to initiate riming during a single upward journey in the cloud without requiring recirculation. In this case it rains heavier and earlier than in the case of normal ice nuclei concentration.

Abstract

Precipitation mechanisms in shallow convective clouds are studied using an axisymmetric cloud model. Clouds are classified into continental and maritime clouds, and further subdivided into warm and cool clouds. Different microphysical factors were selected in the model to represent these different cloud systems. In maritime clouds condensation and collection processes are sufficient to develop precipitation, while in continental clouds graupel formation appears to be a necessary step. In the latter case recirculation of ice crystals is required to initiate riming so that it takes a longer time for rainfall initiation than in the maritime case. In maritime clouds inclusion of the ice phase does not change the rainfall pattern, although both raindrops and graupel contribute equally to precipitation.

The possibility of cloud modification is studied by. increasing the ice nuclei concentration. In continental clouds an increase in ice nuclei concentration of 100 times more than the natural ice nuclei concentration causes activation of a higher number of ice nuclei at warmer temperatures, so that ice crystals can grow large enough to initiate riming during a single upward journey in the cloud without requiring recirculation. In this case it rains heavier and earlier than in the case of normal ice nuclei concentration.

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