On the Accuracy of Ocean Winds and Wind Stress – An Emperical Assesment

Abstract

In this paper we shall consider the accuracy of in situ measurements of wind and wind stress over the ocean, and also the contrasting characteristics of different wind stress parameterisations. There is much scatter in the drag coefficient or roughness length measurements for winds below 10 m/s. While this scatter may be caused by sampling limitations and other measurement errors, there is increasing evidence that swell waves may modify the effective surface roughness. However, at these lower wind speeds, the resulting uncertainty in the wind stress is very small, only a few percent of the magnitude of the wind stress at 20 m/s. At wind speeds between 10 to 20 m/s the measurements from the open ocean are less scattered with both eddy correlation and inertial dissipation wind stress estimates giving similar values. At these higher wind speeds the instrumentation on meteorological buoys is relatively low compared to the height of the dominant waves. However we shall present data for wind velocity fluctuations and buoy motion which demonstrate that meteorological buoys can be used to adequately determine the wind velocity, and hence the wind stress, even in high wave conditions. At wind speeds above 20 m/s, different parameterizations predict significantly different wind stress values. At these higher wind speeds we suggest that, compared to the behaviour at lower wind speeds, the sea surface roughness will increase less rapidly with increasing wind speed. Unfortunately the available wind stress data are few, particularly for winds above 25 m/s, and insufficient to test this prediction.