On the Generation of Roll Vortices due to the Inflection Point Instability in the Hurricane Boundary Layer

Roll vortices, or rolls, have been frequently observed in hurricane boundary layers. Because of their large vertical extent and strong magnitude, these features can significantly contribute to the momentum, sensible heat and moisture fluxes in hurricane boundary layer, and therefore may influence the overall hurricane structure and intensity. However, the effects of rolls are not represented in current hurricane models because of their small spatial scales. In order to parameterize the effects of rolls, we must develop an understanding of their formation mechanism. One of the primary mechanisms of their formation is the inflection point instability. We investigate this mechanism under various conditions in the hurricane boundary layer based on a high-resolution model (2D-LES) that explicitly resolves rolls. The results indicate that rolls are characterized by tilted streamlines in the vicinity of the inflection point of the mean flow profile. This tilting is critical for the rolls to extract kinetic energy from the mean flow. The spatial scale of rolls is determined by the characteristic depth of the hurricane boundary layer flow. The effects of stratification are also revealed. Stable stratification can either reduce or suppress the growth of rolls because of the negative work done by buoyancy. Stratification containing a well-mixed layer has less depressing effect and it provides favorable conditions for rolls to trigger internal gravity waves in the stably stratified layer. This work lays basis for parameterizing rolls in hurricane research and forecast models.