Observations of the directional spectrum relaxation of wavefields in slowly and rapidly turning winds

A so called “Exact Form” of the Maximum Likelihood Method is used to obtain directional spectrum
estimates from a 3-meter NDBC discus buoy, part of the Surface Wave Dynamics Experiment
(SWADE) field array. The method was used to look at the response of wavefields to turning winds with
various wind shift gradients. For slowly turning winds, the spectrum was found to smoothly adjust to
the wind changes. As the wind shift gradient increased, a portion of the spectral energy was found to
be decoupled from the wind while the bulk of the spectrum smoothly adjusted to the new wind direction.
Finally, for even larger wind shift gradients, the preexisting spectrum was completely decoupled
from the wind and decayed separately, as a new spectrum consistent with the wind direction was generated.
These observations confirm theoretical results previously obtained from a third generation
wave model. The relaxation of waves in turning winds was investigated using a simple relaxation model.
Correlation coefficients obtained in this study are higher than previously published studies, especially
for lower values of the wave age, and a relationship between the relaxation parameter and the wave age
is observed.