The effects of environmental flow on the internal dynamics of tropical cyclones

Abstract

This dissertation focuses on two projects that examine the interaction between the internal dynamics of tropical cyclones and the large-scale environmental flow using a hierarchy of numerical model simulations. Diabatic heating from deep moist convection in the hurricane eyewall produces a towering annular structure of elevated potential vorticity (PV) called a hollow PV tower. For the first project, the three-dimensional rearrangement of hurricane-like hollow PV towers is examined in an idealized framework. For the adiabatic PV tower in the absence of environmental flow, barotropic instability causes air parcels with high PV to be mixed into the eye preferentially at lower levels, where unstable PV wave growth rates are the largest. When the diabatic forcing is included, diabatic PV production accompanies the inward mixing at low levels, and similarly diabatic PV destruction accompanies the outflow at upper-levels. The largest variation in PV is produced when the diabatic forcing is placed within the radius of maximum winds (RMW) due to its ability to efficiently extract kinetic energy from the specified heating source. For the adiabatic PV tower in vertical shear, the initial response of the vortex to the vertical shear is to tilt downshear and rotate cyclonically about the mid-level center. The cyclonic precession of the vortex around the center demonstrates the existence of an azimuthal wavenumber-1 quasimode that prevents the vertical alignment of the vortex. When the effects of diabatic forcing are included, the increase in inertial stability causes the resonant damping of the quasimode to become more efficient, leading to the emission of sheared vortex Rossby waves (VRWs) and vortex alignment. Generally, it is shown that the vortex response to vertical shear depends sensitively on the Rossby deformation radius, Rossby penetration depth, and the vortex beta Rossby number of the vortex. For the second project, we examine the development of shock-like structures in the tropical cyclone boundary layer for a stationary and slowly moving tropical cyclone. Using a two-dimensional slab boundary layer model and a three-dimensional boundary layer model, we show that both boundary layer models approximate the nonlinear viscous Burgers' equation in the tropical cyclone boundary layer. For the stationary tropical cyclone, radial inflow creates a circular shock near the surface while vertical mixing communicates the shock throughout the boundary layer. The peak Ekman pumping occurs at a height of 600 m, which is also the location of maximum turbulent transport, consistent with Hurricane Hugo (1989). For a moving TC, the asymmetry in the frictional drag causes an asymmetry in the boundary layer response. As the translation speed of the TC increases, the nonlinear asymmetric advective interactions amplify, leading to an anticyclonic spiral in the vertical velocity field and pronounced inflow in the right-front quadrant of the storm.