Moist vortex resiliency in vertical shear flow

Prior studies of dry tropical-cyclone-like vortex resiliency have found that modification of the global static stability alters the ability of the vortex to resist external vertical shearing. In the context of a linear f-plane dynamical model in which external vertical shear is imposed as a time-invariant forcing, a progressive decrease in the global static stability yields a vortex evolution strongly impacted by an additional vertical shear across the vortex core associated with effective β gyres. This is an artificial problem given that static stability in the tropical cyclone (TC) environment is not observed to fluctuate significantly from typical values. In saturated regions of vertical ascent like the TC core, however, the static stability will be modified. This effect is introduced into a linearized, non-hydrostatic model of the vertically sheared TC through a simple shear-modulated diabatic heating.

Although the diabatic heating asymmetry introduced by asymmetric ascent yields potential vorticity production in the region of reduced static stability, the linear free alignment of the tilted, initially barotropic TC-like vortex is still governed by a baroclinic quasimode. A forced, damped harmonic oscillator paradigm of the sheared vortex evolution continues to be a meaningful one in this case. The zeroth-order impact of the evolving asymmetries on the azimuthal mean vortex is presented through quasilinear calculations based on the linear, sheared vortex simulations. These results are extended to baroclinic TCs, and their relevance examined when nonlinear terms are included using the dry dynamical core of the Weather Research and Forecast (WRF) model.