Progress in the study of coherent structures in the hurricane boundary layer

Recently published observational and theoretical studies have shown that roll vortices are a common and persistent feature of hurricane and tropical cyclone boundary layer mean flow. The roll circulation is dominated by alternating bands of higher- and lower-speed near-surface flow roughly parallel to the mean azimuthal winds above the PBL. These wind speed variations are on the order of +/- 5 to 10 m/s over lateral (roughly radial) distances of 300 to 2000 m. Associated with this azimuthal flow modulation is a somewhat weaker overturning circulation in which the enhanced (reduced) azimuthal flow is correlated with the downdraft (updraft) branches of the overturning flow. Thus, the roll circulation directly enhances the transfer of momentum across the boundary layer. Since all conventional turbulence parameterizations used in numerical model PBL parameterizations assume homogeneous turbulence, it is crucial to improve our understanding of these coherent structures and their effects on the fluxes of momentum, heat and water vapor across the hurricane PBL. For example, the observations and theoretical studies estimate that the inhomogeneous roll circulation enhances the averaged surface stress and the momentum flux in the mid-PBL by no less than a factor of two over conventional estimates. We have previously shown that, despite the potentially enormous surface heat fluxes in tropical cyclones, tropical cyclone PBL rolls are the result of a primarily shear-driven instability of the mean flow that quickly establishes a new equilibrium composed of a modified mean flow with an embedded three-dimensional roll circulation. We present recent advances in our development of a nonlinear hurricane boundary layer theory that captures both the nonlinear mean flow dynamics and the nonlinear effects of the mean flow perturbations, such as rolls. If time permits, we will also present preliminary investigations into the characteristics of smaller-scale, transient linear features that also form in such highly sheared boundary layer flows.