Surface Turbulent Stress Derived from GPS Dropsondes

GPS Dropsonde wind profiles are used to determine the friction velocity and displacement height (vertical offset of the log-wind profile). The roughness length is also determined; however, it is a strong function of the surface boundary condition on speed. Uncertainties are estimated for each of these terms. These results are compared to friction velocities, roughness lengths and displacement heights determined from theory combined with observations of waves and high wind speeds in the North Atlantic. The friction velocities are compared for similar wind speeds in North Atlantic and hurricane conditions. The friction velocity for weak hurricane condition is much less than that for North Atlantic storms, suggesting that the weak hurricanes have a surface boundary condition on speed that is greater than expected, thereby reducing the wind shear and the stress. Similarly, the displacement height for weak hurricanes is greater than expected from the observed waves. These results indicate that sea spray, wave breaking, or some yet to be determined process acts as a stress-reducing interface between the wavecrests and the winds aloft. Such an interface has been modeled for very strong hurricanes, but was not anticipated for weak hurricanes.