Numerical modeling of boundary layer roll vortices in high wind conditions

A number of recent observational, numerical and theoretical papers have conclusively demonstrated that roll vortices are prevalent in hurricane boundary layers. These coherent structures represent a potentially important contribution to the vertical transport of momentum and enthalpy that is not currently included in hurricane models. The roll-induced fluxes are inherently nonlocal and non-gradient and hence cannot be captured by standard turbulence parameterizations. The lack of roll-induced fluxes in existing hurricane models has potentially major implications for simulating or forecasting hurricane intensity. Hurricane track forecasts have improved significantly in recent years as the result of a concerted effort to understand the processes that control the track and better data assimilation techniques. However, hurricane intensity forecasts have not made comparable improvements. Evidently there are physical processes that are not yet well represented in the numerical models. Since hurricanes are heat engines whose fuel source is the water evaporated from the sea surface the identification of mechanisms that may have the potential to make first-order changes in the BL flux parameterizations is an important component of the NOAA's Hurricane Forecast Improvement Project (HFIP) program that is dedicated to improving intensity forecasts.

A two-dimensional atmospheric boundary layer model, which explicitly calculates two-way interaction between the mean flow (hydrostatic) and convective motion (non-hydrostatic) is used to examine the impact of the Coriolis force and latent heat release in clouds on formation and evolution of roll vortices. The model was validated against 3D LES model simulations reported in the literature and through simulations of the classical Benard convection. Numerical experiments were conducted with the same initial and boundary conditions, but with and without Coriolis force and latent heat release for low (15 m/s) and high (45 m/s) wind conditions. The convective kinetic energy budget is analyzed to investigate the contribution of shear production and buoyancy work to convective kinetic energy in each experiment.