Worldwide Measurements of Directional Wave Spreading

Abstract

The directional spreading of waves is important for both theoretical and practical reasons. Enough measurements have now been made to draw conclusions about the behavior of wave spreading at sites in different climatic regimes. The measurements presented here of the directional spreading of fetch-limited waves agree in general with those of M. A. Donelan et al., but additional evidence is also found to support the conclusion of I. R. Young et al. that the spreading function is bimodal at high frequencies. The spreading factor ? is defined to be the square root of the in-line variance ratio defined by R. E. Haring and J. C. Heideman. This spreading factor gives an integrated measure of the degree of directional spreading in the wave spectrum and predicts the reduction in the in-line particle velocities under waves due to direction spreading. The value of ? is 1 for unidirectional waves and 0.707 for omnidirectional waves. For fetch-limited conditions, ? is essentially constant at 0.906. Results from the Exact-NL wave model agree reasonably well with this value, but an operational third-generation model produces directional spreading that is broader than the observations. The statistics of ? are calculated from thousands of hours of measurements from many sites around the world. The median value of ? is 0.880 for low latitude monsoon conditions and 0.867 for tropical cyclones. For extratropical storms, ? decreases with latitude. Regressions on the statistics for high waves give ? = 0.944 for latitude 36° and ? = 0.869 for latitude 72°. The latitudinal dependence of ? is caused by the facts that waves are generally more broadly spread at sites close to the center of a storm and that storm tracks are concentrated at high latitudes.