Spatial scales of surface wind observations and analysed wind fields over the North Atlantic Ocean

Abstract

It is well known that spatial scales of oceanic eddies are smaller than scales of atmospheric eddies. Since the spectral distribution of kinetic energy of atmospheric eddies may influence the properties of wind driven oceanic eddies, an excellent resolution of small scale variability of wind fields used as input fields of coupled models of atmosphere and ocean is necessary. Analysis of spatial scales of atmospheric fields is done in terms of spectral energy densities. These are determined in two different ways: directly from objectively analysed fields or by using spatial correlation functions of direct observations averaged for 20 km × 20 km boxes. In the spectral range of wavelengths of less than 1000 km spectral energy densities of analysed fields have lost about 15 to 50% of the variance compared to direct observations. A considerable part of this loss of the variance depends on smoothing done by interpolation schemes themselves. Concerning problems of air-sea interaction care should be taken also to avoid that systematic errors of analysed wind fields lead to systematic errors in turbulent exchange. It is shown that high observed wind speeds are considerably underestimated in analysed fields of numerical models of weather prediction.