Coastal zone surface stress with stable stratification

Summertime eddy correlation measurements from an offshore tower are analyzed to investigate the dependence of the friction velocity u* for stable conditions on the mean wind speed V, air-sea difference of virtual potential temperature δθ_v, and nonstationary submeso motions. The quantity δθ_v sometimes exceeds 3°C, usually because of the advection of warm air from land over cooler water at this site. Thin stable boundary layers result. Unexpectedly, u* (V)does not depend systematically on the stratification δθ_v even for weak winds. For weak winds, u* (V)increases systematically with increasing submeso variations of the wind. The u* (V) relationship for a given V is greater in nonstationary conditions. Additionally, this study examines u* (V) as a function of wind direction. The u* (V) relationship appears to be affected by swell direction for weak winds and advection from land for short fetches.