Inertial stability, cumulus momentum transport, and the genesis of tropical plumes

Tropical plumes are identified in satellite data as elongated cloud bands originating from convective activity along the Intertropical Convergence Zone (ITCZ), often extending far into middle latitudes. Many previous studies consider tropical plumes as a product of quasigeostrophic forcing. We consider the view that tropical plumes are the upper branch of an enhanced thermally direct circulation, driven by latent heat release in deep cumulus convection, and by processes tied to low inertial stability. As low potential vorticity ( PV) ridges over and straddles the ITCZ, in the amplifying flow in advance of an intruding large-scale Rossby wave, plume genesis occurs.; As low PV advects across the ITCZ, the meridional inertial stability gradient equilibrates or reverses. Under these conditions, the work requirements of deep ITCZ convection to spread its outflow and force compensating subsidence ease. The diagnostic parameter “inertial available kinetic energy” (IAKE), developed as part of this study, reveals much reduced upper tropospheric inertial stability in conjunction with tropical plume genesis. With a vertical transport of easterly momentum by convection, IAKE becomes positive as convectively-generated (negative) PV lowers inertial stability poleward of the ITCZ as a convection-relative inertial instability forms. Westerly momentum transported vertically to cloud top, in contrast, results in the equatorward-direction the more favored direction for cumulonimbus outflow aloft.; The combination of cumulus momentum transport within an environment characterized by low inertial stability causes ITCZ convective outflow to locally and abruptly switch to intensified poleward flows as tropical plumes.; Working from these diagnostic conclusions, a numerical experiment to determine whether ambient inertial stability causes deep convection across the plume genesis region to physically align their internal flow structure is undertaken. This structural alignment results in convection vertically transporting the momentum to cloud top which produces the least convection-relative inertial stability in the direction of lowest environmental inertial stability. A population of cumulus, possessing a range of momentum transports, is shown to evolve into organized convective systems which transport the one momentum to cloud top that results in the least upper tropospheric inertial stability for their outflow.