A numerical study of roll vortices in the hurricane boundary layer

Recent satellite, Doppler radar and aircraft-based observations indicate roll vortices are prevalent in hurricane boundary layer. Because of the large vertical extent and strong intensity, these features can have a significant influence on the vertical momentum, sensible heat and moisture fluxes in hurricanes and therefore may have a potential for influencing the hurricane intensity. However, the roll vortices are not explicitly represented in existing hurricane models because of their small spatial scales. In order to parameterize the effect of roll vortices in hurricane models we must know the favorable conditions for their formation and their impact on the boundary layer structure.

In this study, we couple a two-dimensional LES (2D-LES) model with an axisymmetric hurricane Boundary Layer (AHBL) model to investigate the favorable conditions for roll vortices formation, as well as their characteristics. The AHBL model is initialized with the composite dataset from 790 GPS dropsondes collected in 13 hurricanes (Zhang, 2011). The 2D-LES model is non-hydrostatic and has a very fine spatial resolution, capable of modeling atmospheric large eddies. The two models are coupled and explicitly solve the two-way interaction between the large-scale hurricane boundary layer flow and small-scale convective motions. The 2D-LES model is placed at different distances from the storm center and uses the mean-flow vertical temperature, moisture, and wind profiles provided by the AHBL model. At the same time, it provides the fluxes induced by roll-vortices to the AHBL model and therefore allows the roll vortices to modify the large-scale flow. The locations where roll can be generated, as well as the corresponding roll vortices length and velocity scales are analyzed based on the simulation results.