On the application of HF ocean radar to the observation of temporal and spatial changes in wind direction

Abstract

The application of a 30-MHz narrow-beam ground-wave ocean radar to the observation of wind directions is described. It is found that the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\cos^{s} (\theta/2)</tex> model for wind-wave directions does not apply in a specific case of shallow water where swell waves are behaving nonlinearly. To experimentally extract unambiguous wind directions from this model requires sampling three different beam angles simultaneously. In practice some time and space stationarity is assumed. Detailed analysis in time and space reveals structure in the transition of the cold front from sea to land which, although unexpected, agrees with coastline observations where they are available. The nature of the structure is only briefly discussed. The response of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda = 5</tex> -m wind waves to the frontal change was two orders of magnitude faster than time constants for similar events previously modeled using pitch-and-roll buoy data. This discrepancy needs to be reconciled before lower frequency radars can be used without ground truth for wind-direction changes.