A new metric for estimating the influence of evaporation on seasonal precipitation rates

Abstract

The objective of this paper is to introduce a diagnostic metric - termed the local-convergence ratio - that can be used to quantify the contribution of evaporation (and transpiration) to the atmospheric hydrologic cycle, and precipitation in particular, over a given region. Previous research into regional moisture (or precipitation) recycling has produced numerous methods for estimating the contributions of "local" (i.e., evaporated) moisture to climatological precipitation and its variations. In general, these metrics quantify the evaporative contribution to the mass of precipitable water within an atmospheric column by comparing the vertically integrated atmospheric fluxes of moisture across a region with the fluxes via evaporation. Here a new metric is proposed, based on the atmospheric moisture tendency equation, which quantifies the evaporative contribution to the rate of precipitation by comparing evaporative convergence into the column with large-scale moisture-flux convergence. Using self-consistent, model-derived estimates of the moisture-flux fields and the atmospheric moisture tendency terms, the authors compare estimates of the flux-based moisture-recycling ratio with the newly introduced local-convergence ratio. Differences between the two ratios indicate that they can be considered complementary, but independent, descriptors of the atmospheric hydroclimatology for a given region. © 2008 American Meteorological Society.