Structure and Dynamic Mechanisms of Roll Vortices in Hurricane Harvey during the Landfall

Abstract:

Roll vortices is a series of large-scale turbulence eddies and prevail in the hurricane boundary layer. In this study, the one-way nest WRF-LES model was used to explore the structure and generation mechanism of roll vortices in the hurricane boundary layer during Hurricane Harvey’s landfall from 0000 UTC 25 to 1800 UTC 27 August 2017. Simulation results indicated that roll vortices always align along the mean wind direction and prevail at a radius of 20 to 40 km from the hurricane center with a height of 100 to 3000 m. During the simulation period, the turbulence intensity of roll vortices is proportional to the hurricane intensity. Before and during the hurricane landfall, the strong inflow convergence leads to vertical perturbation and finally generate the roll vortices with suitable thermodynamic (Rf at around -0.2—0.2) and dynamic conditions (strong negative inflow wind shear). After the hurricane landfall, the decayed inflow weakens the inflow convergence and leads to the quick reduction of roll vortices’ strength. The analysis of vertical turbulence energy indicates that the atmospheric pressure perturbation, caused by air density perturbation, leads to strong roll vortices with a mean wavelength of about 1 km. While the buoyancy term is weak and negative that suppresses the roll vortices, the strong roll vortices are supported by the atmospheric pressure transport term in the turbulent kinetic energy (TKE) equation.