New findings on drag coefficient behavior in tropical cyclones

A total of 1270 GPS dropsonde wind profiles 200 km of the tropical cyclone center were divided into MBL wind speed groups of 20-29, 30-39, 40-49, 50-59, 60-69, and 70-79 m/s. Roughness length, friction velocity, drag coefficient, and 10 m level neutral stability wind speed (U10) were diagnosed in each MBL group using the profile method. The drag coefficient increases linearly with U10 reaching a maximum of 2 x 10-3 at a wind speed of 41 m s-1, thereafter it decreases with increasing wind speed to a minimum of 0.6 x 10-3 at U10 of 61 m/s. When broken down into radial groups of R<=30 km and R> 30 km, the profiles within 30 km of the center show Cd values that display little variation of Cd with wind speed and smaller values of Cd near 1 x 10-3, consistent with Donelan's continuous breaking hypothesis. Profiles outside of 30 km show the behavior described earlier (initial increase followed by a decrease). After dividing the R> 30 km profiles into storm relative azimuth sectors, the increase-then-decrease behavior (with much larger Cd values) was found to be confined to the front left sector where the winds flow across the primary swells. These results have bearing on the current wave-roughness interactions parameterizations being considered for mesoscale numerical forecast models.