Directional Response of Wind Waves to a Large Wind Shift

Abstract

The directional response of a fully arisen sea to a ?90° wind shift is studied using a combination of airborne radar and in situ directional wave observations. The observations were made in February 1991 as a part of the Surface Wave Dynamics Experiment. Radar and buoy mean wave directions in several frequency bands are polynomial-smoothed in fetch and duration coordinates and analyzed for the directional relaxation parameter b by using finite differences of the gridded, smoothed data in a one-dimensional advection equation for the mean wave direction. The analysis is carried out using several different sets of buoy wind and wave data in an event window of 40 h in duration by 200 km in fetch (100?300 km offshore). For the most well-populated and reliable inverse wave age class in the study, 1.2 ? U/c < 1.6, the authors find b = 3.3(±0.1) ? 10?5. The data do not support any inference as to possible wave age dependence other than, perhaps, the null hypothesis, b = const (U/c). Frequencies near the spectral peak do not respond according to the relaxation model, and misleading values of b may result from a standard analysis of the data. Wave?current interactions are a potential source of bias. Reflected waves in the study area may be biasing the present result low by as much as 20%.