Wind and waves in extreme hurricanes

Here we examine legacy hurricane sea state images to show that at high wind speeds, white caps remain constant and at still higher wind speeds, are joined, and increasingly dominated, by streaks of foam and spray. We then add analysis of hundreds of GPS dropsonde wind profiles, finding that at surface wind speeds of ~40 m/s the streaks merge into a white out, the roughness begins to decrease and a high-velocity surface layer begins to develop. The roughness reduces to virtually zero by ~80 m/s mean boundary layer wind speeds, rendering the surface aero-dynamically extremely smooth in the most intense part of extreme hurricanes. A preliminary assessment shows that cross-swell, dominant in large regions of hurricanes, allows the roughness to increase considerably before it reduces to the same low values. These findings suggest the need for sea-state and wind speed dependent roughness parameterizations in numerical prediction models. A new parameterization is formulated, which incorporates wind speed and direction wave spreading dependency.