The role of turbulence in an intense Tropical Cyclone: Momentum diffusion, eddy viscosities, and mixing lengths

Improved representation of turbulent processes in numerical models of tropical cyclones (TCs) is expected to improve intensity forecasts. To this end, the authors use a large-eddy simulation (with 31-m horizontal grid spacing) of an idealized category 5 TC to understand the role of turbulent processes in the inner core of TCs and their role on the mean intensity. Azimuthally and temporally averaged budgets of the momentum fi elds show that TC turbulence acts to weaken the maximum tangential velocity, diminish the strength of radial inflow fl ow into the eye, and suppress the magnitude of the mean eye- wall updraft. Turbulent fl ux divergences in both the vertical and radial directions are shown to influence fl uence the TC mean wind fi eld, with the vertical being dominant in most of the inflowing fl owing boundary layer and the eyewall (analogous to traditional atmospheric boundary layer fl ows), while the radial becomes important only in the eyewall. The validity of the downgradient eddy viscosity hypothesis is largely confirmed fi rmed for mean velocity fi elds, except in narrow regions which generally correspond to weak gradients of the mean fi elds, as well as a narrow region in the eye. This study also provides guidance for values of effective eddy viscosities and vertical mixing length in the most turbulent regions of intense TCs, which have rarely been measured observationally. A generalized formulation of effective eddy viscosity (including the Reynolds normal stresses) is presented.