Variations in friction velocity with wind speed and height for moderate-to-strong onshore winds based on measurements from a coastal tower

Abstract

Variations in friction velocity with wind speed and height are studied under moderate (≥9ms-1)-to-strong onshore wind conditions caused by three landfalling typhoons. Wind data are from a coastal 100-m tower equipped with 20-Hz ultrasonic anemometers at three heights. Results show that wind direction affects variations in friction velocity with wind speed. A leveling off or decrease in friction velocity occurs at a critical wind speed of ∼20 m s-1 under strong onshore wind conditions. Friction velocity does not always decrease with height in the surface layer under typhoon conditions. Thus, height-based corrections on friction velocities using the model from Anctil and Donelan may not be reliable. Surface-layer heights predicted by the model that are based on Ekman dynamics are verified by comparing with those determined by a proposed method that is based on the idea of mean boundary layer using wind-profile data from one of the landfalling typhoons. Friction velocity at the top of the surface layer is then estimated. Results show that friction velocity decreases by about 20% from its surface value and agrees well with previous results of Tennekes.