Why Rolls are Prevalent in the Hurricane Boundary Layer

Recent remote sensing observations show that the hurricane boundary layer flow, although energetic, is not a region of homogeneous turbulence. In fact, the observations convincingly demonstrate that a large fraction of the turbulent flow in the regions away from the deep convective rainbands is highly organized into intense horizontal roll vortices that are approximately aligned with the mean wind and span the depth of the boundary layer. These observations show that rolls strongly increase the flux of momentum between the underlying surface and the main body of the storm compared to an equivalent hurricane boundary layer flow without rolls. The linear and nonlinear dynamics of hurricane boundary layer roll formation are outlined and it is shown why rolls are, in fact, the expected basic hurricane boundary layer state. The model presented here explains the hurricane roll features currently documented in field programs and makes predictions that can be tested in future experiments. The primary effects of rolls on the boundary layer fluxes are inherently nonlocal and nongradient and hence cannot be captured by standard downgradient turbulence parameterizations used in hurricane simulations. However, the nonlinear theory is the proper starting point for developing boundary layer parameterizations that include roll modification of the turbulent fluxes.