Numerical Simulation of Precipitation Development in Supercooled Cumuli—Part II

WILLIAM R. COTTON Experimental Meteorology Laboratory, NOAA, Coral Gables, Fla.

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Abstract

A numerical model of supercooled cumuli is developed and discussed. Water substance in the model is idealized to be partitioned into the five phase components; namely, water vapor, liquid cloud water, liquid rainwater, frozen rainwater, and ice crystals. Continuity equations are developed that predict the distribution of water substance among the five phase components. The cloud dynamic framework consists of a simple one-dimensional Lagrangian model that includes the effects of entrainment. The model is able to operate either as a steady-state model or as a spherical vortex model.

The results of two case study experiments illustrated that the principle action of ice particles nucleated on sublimation nuclei, or by the freezing of cloud droplets in cumulus clouds containing moderate to heavy amounts of supercooled rainwater, is to promote the freezing of supercooled rainwater. On the other hand, clouds containing small amounts of supercooled rainwater are dynamically insensitive to moderate concentrations of ice crystals. In such clouds, extensive riming and vapor deposition growth of crystals in concentrations of several thousand per liter are required before they make significant contributions to the dynamic structure of the cloud.

Finally, it was found that the warm-cloud precipitation process can either invigorate or retard the dynamic behavior of a supercooled cloud, depending upon the height and magnitude of the precipitation process.

Abstract

A numerical model of supercooled cumuli is developed and discussed. Water substance in the model is idealized to be partitioned into the five phase components; namely, water vapor, liquid cloud water, liquid rainwater, frozen rainwater, and ice crystals. Continuity equations are developed that predict the distribution of water substance among the five phase components. The cloud dynamic framework consists of a simple one-dimensional Lagrangian model that includes the effects of entrainment. The model is able to operate either as a steady-state model or as a spherical vortex model.

The results of two case study experiments illustrated that the principle action of ice particles nucleated on sublimation nuclei, or by the freezing of cloud droplets in cumulus clouds containing moderate to heavy amounts of supercooled rainwater, is to promote the freezing of supercooled rainwater. On the other hand, clouds containing small amounts of supercooled rainwater are dynamically insensitive to moderate concentrations of ice crystals. In such clouds, extensive riming and vapor deposition growth of crystals in concentrations of several thousand per liter are required before they make significant contributions to the dynamic structure of the cloud.

Finally, it was found that the warm-cloud precipitation process can either invigorate or retard the dynamic behavior of a supercooled cloud, depending upon the height and magnitude of the precipitation process.

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