Comparison of a simple logarithmic and equivalent neutral wind approaches for converting buoy-measured wind speed to the standard height: special emphasis to North Indian Ocean

Abstract

The difference between the transferred wind speed to 10-m height based on the equivalent neutral wind approach (U n) and the logarithmic approach (U log) is studied using in situ observations from the Indian, Pacific, and Atlantic Oceans, with special emphasis given to the North Indian Ocean. The study included U n − U log variations with pressure, relative humidity, wind speed, air temperature, and sea surface temperature (SST). U n − U log variation with respect to air temperature (T a) reveals that U n − U log is out of phase with air temperature. Further analysis found that U n − U log is in phase with SST (T s) − T a and varies between −1.0 and 1.0 m/s over the North Indian Ocean, while for the rest of the Oceans, it is between −0.3 and 0.8 m/s. This higher magnitude of U n − U log over the North Indian Ocean is due to the higher range of T s − T a (−4 to 6 °C) in the North Indian Ocean. Associated physical processes suggested that the roughness length and friction velocity dependence on the air–sea temperature difference contributes to the U n − U log difference. The study is further extended to evaluate the behavior of U n − U log under cyclonic conditions (winds between 15 and 30 m/s), and it was found that the magnitude of Un − U log varies 0.5–1.5 m/s under the cyclonic wind conditions. The increasing difference with the wind speed is due to the increase in the momentum transfer coefficient with wind speed, which modifies the friction velocity significantly, resulting in U n higher than U log. Thus, under higher wind conditions, U n − U log can contribute up to half the retrieval error (5 % of the wind speed magnitude) to the satellite validation exercise.