An Empirical Study on the Parameterization of Precipitation in a Model of the Time Mean Atmosphere

Abstract

An empirical study based on three years (1981?83) of monthly mean data revealed that colocated anomalies in precipitation (?PP) and vertical motion at 500 mb (??) are moderately well correlated over the United States,in winter. The ?PP data are spatial averages in 344 Climate Divisions while the ?, are derived from initialized fields of the ECMWF and NMC NWP models. To a first-order approximation the deficit of rain associated with anomalous downward motion is just as large as the surplus of rain associated with anomalous upward motion. Therefore, it should be possible to generate some of the latent heat of condensation in a linear model for the time mean atmosphere by expressing ?PP linearly in ?? Monthly mean ? of the ECMWF and NMC are highly correlated with each other and relate about equally well to rainfall over the United States. The empirical constant a in the relation PP=a&omega turns out to be about ? ?0.6 mm day?1/(10?2N m?2s?1, which is on the same order of magnitude as the theoretical amount of precipitation produced by diabatic ascent of magnitude 10?22 S2s?1. Attempts to empirically extract the role of atmospheric moisture in the relation between ?PP and ?? were made by comparing summer to winter, high latitudes to lower latitudes and the United States to India but the results are at best modest. Implementation of parameterized latent had sources and sinks in a linear steady state anomaly model for the time mean atmosphere is equivalent to reducing its static stability by a sizable amount. This leads to increased response to a prescribed forcing.