Departmental Seminar: The Baroclinic Moisture Flux by Dr. Ron McTaggart-Cowan

Dr. Ron McTaggart-Cowan, Research Scientist, Atmospheric Numerical Weather
Prediction Research, Environment and Climate Change Canada (ECCC)

Title:  The Baroclinic Moisture Flux

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 non-negligible 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.