The Water Balance of Orographic Clouds

Abstract

A significant question bearing on the prediction of orographic precipitation and the seeding of orographic clouds is what fraction of the water condensed over an orographic barrier falls on the barrier as precipitation. This has been treated in a rather inadequate manner to date, largely because of lack of basic data. Through the use of abundant storm-sounding data taken upwind of two Southern California orographic barriers and data from the corresponding mountain recording raingage networks, comparisons of computed condensation and observed precipitation have been made for a number of winter storms over a four-year period. The results indicate that approximately one quarter of the orographically produced condensate fell as precipitation on the watersheds. A breakdown into air mass stability on the basis of the inflow rawinsonde data showed that, for similar orographic flow conditions, more precipitation was produced by unstable air masses than by stable air masses.