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Effects of Wind Direction on Variations in Friction Velocity With Wind Speed Under Conditions of Strong Onshore Wind
Author(s) -
Fang Pingzhi,
Zhao Bingke,
Zeng Zhihua,
Yu Hui,
Lei Xiaotu,
Tan Jianguo
Publication year - 2018
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1029/2017jd028010
Subject(s) - wind speed , wind gradient , roughness length , wind profile power law , wind shear , wind direction , log wind profile , maximum sustained wind , drag , meteorology , shear velocity , environmental science , geology , atmospheric sciences , mechanics , physics , turbulence
The effects of wind direction on variations in friction velocity with wind speed are studied under moderate (≥9 m/s) to strong (≥22 m/s) onshore wind conditions using 20‐Hz ultrasonic wind data from a coastal tower at three different heights. The effects of different averaging time intervals of 20, 10, 2, and 1 min on the variations are also investigated. Three typhoons passed by the tower during the 150 hr of observations. Regardless of wind direction, friction velocity increases with increasing wind speed, and linear regression shows that the rate of increase is ~50% less than previously reported. However, a leveling‐off or decrease in friction velocity with increasing wind speed is found under strong wind conditions using a bin‐averaged method. Wind direction affects the variations in friction velocity with wind speed. Friction velocity increases with increasing wind speed at a similar growth rate to that of previously published results, with a leveling‐off or decrease at wind speeds higher than 22 m/s when the wind blows roughly normal to the shoreline. Form drag induced by sea waves with longer wavelengths is suggested to be responsible for the effects of wind direction on the variations in friction velocity with wind speed. An averaging time interval of 1 min yields representative variations in friction velocity with wind speed using the observations described in this study.