Tropical cyclone resiliency in vertical shear flow

This paper examines the resiliency of diabatic vortices in vertical shear flow through the addition of a parameterized heating to the equations of motion. The analysis is motivated by observations of mature tropical cyclones in weak to moderate shear environments, and extends a previous investigation of a vortex self-alignment mechanism intrinsic to the dry “adiabatic” dynamics. The primary results are derived from a linearized model of the vortex dynamics in which asymmetric heating is assumed proportional to the vertical motion and the vortex profile is both barotropic and dynamically stable. An effective reduction of static stability in the vortex core where the fractional cloudiness is typically greatest increases the intrinsic resonant damping rate and precession frequency of the vortex tilt asymmetry relative to the adiabatic solution. As a consequence, the shear-forced diabatic vortex evolves more rapidly towards a quasi-steady left-of-shear orientation. Unlike adiabatic simulations where the vortex tilt asymmetry approximately behaves as a forced, damped oscillator, the diabatic vortex in shear exhibits a pronounced shedding of spiral vortex Rossby waves during its evolution. The dynamical role of these shear-forced vortex Rossby waves in the resiliency of the vortex is clarified. The extension of the shear-forced results to initially baroclinic vortices more closely resembling tropical cyclones is also presented.