Impact of the boundary layer processes on eyewall structure and mesovortices of hurricanes

In this study, the multiple nested state-of-the-art Weather Research and Forecast (WRF) model is used to investigate the impact of the boundary layer processes on hurricane eyewall structure and the formation of eyewall mesovortices. Hurricane Isabel (2003) was simulated by a series of numerical experiments with different boundary layer schemes including the Yonsei University, Mellor-Yamada-Janjic, Mellor-Yamada-Nakanishi-Niino 2.5 level, and Mellor-Yamada-Nakanishi-Niino 3 level schemes. The simulations show that the boundary layer scheme not only substantially affects the sub-grid scale (SGS) vertical transport of heat and moisture but also has an important bearing on vortex Rossby waves, mesovortices, and other resolved organized features in the vicinity of hurricane eyewall, suggesting that the impact of boundary layer scheme on hurricane intensity is realized through a combined effect from the direct parameterized SGS vertical turbulent mixing and from the transport induced by the resolved structures in the eyewall region. The analyses show that the boundary layer processes provide a mechanism to affect the barotropic instability for generation of eyewall mesovortices through changing eyewall potential vorticity (PV) concentration and generating vortex Rossby waves with different wavenumber. Our numerical experiments showed for certain external conditions the magnitude and vertical distribution of eddy viscosity are the key factors that determine the characteristics of eyewall disturbances (or vortex Rossby waves). Such a modulation on eyewall structure by eddy viscosity is realized through the complicated interaction among SGS vertical turbulent mixing, mesoscale structures, and barotropic instability.