The Release of Latent Heat of Condensation in a Simple Precipitation Forecast Model

Abstract

A method is proposed for deriving a useful quantitative precipitation forecast from a physical model in which no account is taken of the release of latent heat of condensation. The approach is to calculate, on the basis of a simple theory, the ratio between the large-scale updraft speed when the latent heat is incorporated and the smaller updraft speed when it is not. The resulting ratio is then assumed to be equivalent to the ratio of the storm-average precipitation to the comparable smaller average computed from the thermodynamically dry model. An empirical test supports the assumption reasonably well for large winter storms in the central and eastern United States. For operational purposes it is suggested that the theoretical ratio be estimated statistically from the mean temperature of the layer from 1000 mb to 500 mb. Evidence is provided that this estimate can be made fairly successfully. Finally, some inferences are drawn concerning the role of cumulus convection in this type of storm.