Tropical Cyclone Motion and Evolution in Vertical Shear

Abstract

The motion and the evolution of tropical cyclone-like vortices in an environmental flow with vertical shear are investigated using a baroclinic primitive equation model. The study focuses on the fundamental dynamics of a baroclinic vortex in vertical shear, the influence of vortex structure, and the role of diabatic heating. The results show that the initial response of the vortex to the vertical shear is to tilt downshear. As soon as the tilt develops, the upper-level anticyclonic and lower-level cyclonic circulations begin to interact with each other. As a result of these interactions, the tilted axis of the vortex reaches a stable state after an initial adjustment, which varies with the structure of the vortex, its environmental flow shear, and the cumulus convective heating. The motion of an adiabatic vortex in vertical shear is controlled by both the steering of the environmental flow and vertical coupling mechanisms. Most of the vortices move with the environmental flow at about 650 hPa or with the layer mean between 350 and 900 hPa, but the stronger tropical cyclone vortices move with a relatively deeper layer mean flow. In addition to advection by the environmental flow, most vortices propagate to the left of the vertical shear due to downward penetration of the circulation associated with the upper-level anticyclonic potential vorticity (PV) anomalies that are displaced downshear. Diabatic and moist processes can substantially modify the adiabatic vortex motion by both the vertical transport of potential vorticity associated with diabatic heating and the development of convective asymmetries within the vortex core region. Diabatic heating can either substantially enhance the leftward motion tendency or result in a rightward motion relative to the vertical shear, depending on the vertical structure and intensity of the vortex and its environment. This occurs by transport of anticyclonic PV to the upper troposphere and cyclonic PV to the right of the vortex center relative to the vertical shear in the lower troposphere. A rightward motion tendency to the boundary layer flow is found to arise from enhanced heat fluxes from the ocean on the higher wind side of the vortex center. Cumulus convection is substantially enhanced on the downshear side of the vortex center due to the interaction between the vortex circulation and the vertical shear in the environmental flow. The asymmetric divergent flow associated with these convective asymmetries affects the vortex motion by deflecting the vortex to the region with maximum convection.