The Baroclinic Moisture Flux

Abstract

Abstract As subsaturated air ascends sloping isentropic surfaces, adiabatic expansion results in cooling and relative moistening. This process is an effective way to precondition the atmosphere for efficient moist processes while bringing parcels to saturation, and thereafter acts to maintain saturation during condensation. The goal of this study is to develop a diagnostic quantity that highlights circulations and regions in which the process of parcel moistening by isentropic ascent is active. Among the many features that rely on this process for the generation of an important fraction of their energy are oceanic cyclones, transitioning tropical cyclones, warm conveyor belts, diabatic Rossby vortices, and predecessor rain events. The baroclinic moisture flux (BMF) is defined as moisture transport by the component of vertical motion associated with isentropic upgliding. In warm conveyor belt and diabatic Rossby vortex case studies, the BMF appears to be successful in identifying the portion of the circulation in which this process is actively bringing parcels to saturation to promote the formation of clouds and precipitation. On a broader scale, the climatological maxima of the BMF highlight regions in which parcel moistening by isentropic ascent is anticipated to have a nonnegligible impact on the atmospheric state either through the action of the mean flow or via the repeated occurrence of isolated large-BMF events. The process-centric foundation of the BMF makes it useful as a filtering or exploratory variable, with the potential for extension into predictive applications.