Precipitation Formation in a Convective Storm

Abstract

A simple and computationally efficient method is described for estimating hydrometeor size distributions within a convective storm. The method requires air motion measurements (from Doppler radar in this case, but it could be used with a dynamic model), and specification of the cloud water field and the mechanism by which the hydrometeors originate. The cloud water field that corresponds to the wind field used is estimated by calculating condensation and depletion rates along air parcel trajectories. It is assumed that the storm is in steady state and that hydrometeors grow only by accretion. The technique is applied to one of the storms documented in the Cooperative Convective Precipitation Experiment (CCOPE), assuming that hydrometeors originate by primary ice nucleation alone. The distribution of hydrometeor sizes that is obtained is very unrealistic, in such a way that one or more other sources must have dominated hydrometeor formation. Since the trajectory analysis indicated that the source had to be at temperatures above 0°C, it must have been either coalescence or some melting process. Alternatively, there could have been a strong transport of small hydrometeors on scales unresolved by the Doppler radar. The analysis scheme is a useful tool for learning about precipitation mechanisms from held data, and it will be more useful if it can be extended to time-varying cases without becoming too unwieldy.