On the Lateral Entrainment Instability in the Inner Core Region of Tropical Cyclones

Turbulence is commonly regarded as a chaotic flow feature pertaining to the planetary boundary layer (PBL). In the inner core of a tropical cyclone (TC), however, turbulence can also be generated in the eyewall and rainbands above the PBL by cloud processes. The cloud induced turbulence eddies not only experience large lateral thermodynamic contrasts across the eyewall and rainbands to result in inter-connected vertical and horizontal turbulent mixing with comparable magnitudes but also entrain dry moat air with low equivalent potential temperature laterally into eyewall and rainband clouds. By analyzing in-situ aircraft measurements collected by the reconnaissance flights that penetrated the eyewalls and rainbands of Hurricanes Rita (2005), Patricia (2015), Harvey (2017), and Michael (2018), as well as numerical simulations of Hurricanes Patricia (2015) and Michael (2018), we show that the moat air entrained into the eyewall and rainbands meets the instability criterion, and therefore, sinks unstably as a convective downdraft. The resultant positive buoyancy fluxes are an important source for the turbulent kinetic energy (TKE) in the eyewall and rainband clouds. This mechanism of TKE generation via lateral entrainment instability needs to be included in the turbulent mixing schemes for a better representation of turbulent transport processes in numerical forecasts of TCs.