A new metric for estimating the influence of evaporation and moisture-flux convergence upon seasonal precipitation rates

The objective of this paper is to introduce a diagnostic metric—termed the local-convergence ratio—that can be used to quantify the enhancement/reduction of the atmospheric hydrologic cycle, and precipitation in particular, resulting from land-surface/atmosphere interactions. 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 we propose a new metric, based upon the atmospheric moisture tendency equation, that 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 we compare traditional estimates of the flux-based moisture-recycling ratio with the newly-introduced local-convergence ratio at specific sites across the globe. Further we analyze the contribution of evaporation and atmospheric moisture-flux convergence to model-based estimates of climatological precipitation over the North American continent. As one example we find that “hot-spots” of evaporation-driven land-atmosphere coupling counter-intuitively depend upon the presence of relatively large moisture-flux convergence from outside the region. Implications of this result, along with others, will be discussed.