Turbulent Transport in the Inner-Core of Tropical Cyclones and Its Role in Storm Intensification

Asymmetric turbulent eddy processes play a vital role in the evolution of the primary and secondary circulations of a tropical cyclone (TC). Because numerical models use the discrete grids, the turbulence with a continuous spectrum must be split into two parts: model resolved and sub-grid-scale (SGS) components. The latter must be determined parametrically by a turbulent mixing scheme, which is a major source of uncertainty in numerical predictions of TCs. In a TC environment, turbulence not only exists in the boundary layer, but also can be generated above the boundary layer in eyewall and rainband clouds. Thus, the treatment of turbulent mixing must go beyond the conventional scope of the boundary layer since no physical interface separates the turbulence generated by the boundary layer processes and cloud processes aloft. To better represent the turbulent processes in the TC inner core, in this dissertation study, a new “moist” turbulent kinetic energy (TKE) scheme that includes the effects of buoyancy induced by multi-phase water (vapor, liquid, and solid) has been developed and implemented in the Hurricane Weather Research & Forecasting (HWRF) model used for operational forecasts of TCs. The new TKE scheme successfully reproduces the large TKE above the boundary layer in the eyewall consistent with radar observations. The simulations show that the TC intensification is sensitive to the sloping curvature of mixing length. This study reveals positive feedback among the various processes underlying the TC intensification and provides a clear physical picture of multiple roles that the turbulent processes play in TC intensification. To understand the dynamics of TC intensification, a generalized mathematical framework that can be used to diagnose both balanced and unbalanced dynamical processes in TC intensification is developed in this dissertation study. The diagnostic results using this new tool show that the radial SGS turbulent forcing associated with the super-gradient wind produces a positive tendency to accelerate the vortex, whereas the forcing associated with the sub-gradient wind tends to lower the height of peak tangential wind.