Open Ocean Momentum Flux Measurements in Moderate to Strong Winds

Abstract

Measurements of the momentum flux were made by the Reynolds flux and dissipation methods on a deep water stable tower operated by the Bedford Institute of Oceanography, A modified Gill propeller-vane anemometer was used to measure the velocity. Drag coefficients from 196 Reynolds flux measurements agree well with those reported in Smith (1980) based on independent observations at the same site. Based on 192 runs, a comparison of the dissipation and Reynolds flux results shows excellent agreement on average, for wind speeds from 4 to 20 m s−1. The much more extensive dissipation data set (1086 h from the tower and 505 h from the weathership PAPA, CCGS Quadra) was used to investigate the dependence of the drag coefficient on wind speed, fetch and stability. The drag coefficient reduced to 10 m height and neutral conditions (CDN), is independent of stability and fetch (for fetch/height ≳800) but increases with wind speed above 10 m s−1. Some time series of the momentum flux and drag coefficient are shown to demonstrate additional sources of variation in the drag coefficient. CDN is observed to be smaller, on average. during rising winds than during failing winds or after a change in wind direction. Based on our results and many deep water results of others, we obtainwhere U10 is the wind speed at a height of 10 m. A method for calculating the stress from this CDN and observations of wind speed and surface minus air temperature at heights other than 10 m is also given.