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Deviation of Wind Stress From Wind Direction Under Low Wind Conditions
Author(s) -
Chen Sheng,
Qiao Fangli,
Huang Chuan Jiang,
Zhao Biao
Publication year - 2018
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1029/2018jc014137
Subject(s) - wind stress , wind gradient , wind direction , wind power , wind profile power law , maximum sustained wind , meteorology , wind speed , environmental science , wind shear , atmospheric sciences , geology , physics , engineering , electrical engineering
The deviation of the wind stress vector from the wind direction at the air‐sea interface under low wind conditions was investigated based on direct eddy covariance flux measurements taken at a coastal tower in the northern South China Sea. The wind stress deviates significantly from the mean wind direction under low wind conditions, with the deviation angle sometimes exceeding 90°, indicating upward momentum transfer from the ocean to the atmosphere. Negative downwind drag coefficient values begin to occur at a wind speed of approximately 4 m/s. Our results show that ocean swells and nonstationary airflow play critical roles in wind stress. Prominent peaks at the dominant swell frequency in the vertical velocity spectra are observed at a height of 17 m over the mean sea surface, implying that swell‐induced perturbations can reach a height of at least 17 m, and the wave boundary layer can extend more than 10 m above the sea surface. The results of our analysis indicate that at the observation height, the influence of nonstationarity in the wind field is more significant than that of swell‐induced motions on the deviation of wind stress. After the removal of nonstationary motions, the deviation angles of the wind stress from the wind direction are generally reduced and vary substantially at low wind speeds.