Estimates for surface exchange coefficients and turbulence length scales using numerical simulations

Among the many uncertainties in numerical and theoretical models of hurricanes, perhaps the greatest uncertainty lies in the parameterizations of boundary-layer turbulence and air-sea exchange. In an attempt to reduce this uncertainty, I have conducted a very large set of idealized simulations of strong hurricanes (Category 4+). The simulations vary only in the specification of surface (10 m) exchange coefficients (Ck and Cd) and/or length scales in a turbulence parameterization. To determine which simulation, among the hundreds conducted, is most realistic, I have examined several simple yet well-observed metrics of hurricane intensity and structure, such as: maximum possible wind; minimum possible pressure; surface wind-pressure relationship; height of maximum wind; and surface inflow angle. An optimal set of values for exchange coefficients and turbulence length scales emerges from this analysis, and these values are reasonably consistent with recent observational and laboratory studies. Previous simulations that required Ck/Cd > 0.75 to produce Category 5 storms apparently used lower sea-surface temperatures (Ts about 26 C) and/or large horizontal diffusion; the relatively higher Ts (29 C) and low horizontal diffusion simulations in this study can produce Category 5 storms with Ck/Cd as low as 0.25.