The earliest stages of wind wave generation in the open sea

Abstract

We have explored the earliest stages of wind wave generation in the open sea, from the first initial wavelets appearing on an otherwise flat surface or low smooth undulations till the practically fully developed conditions for the very low range of wind speeds we have considered. We suggest the minimal wind speed for the appearance of the first wavelets to be close to 1.8 ms−1. The peculiar conditions associated to the development of coastal sea breezes allow us to consider the local waves as generated under time-limited conditions. The 2D spectra measured during these very early stages provide the first evidence of an active Phillips process generation in the field. After appearing in these very early stages, wavelets quickly disappear as soon as the developing wind waves take a leading role. We suggest that this process is due to the strong spatial gradients in the surface orbital velocity, which impedes the instability mechanism at the base of their formation, while at a later stage of development, these gradients decrease and wavelets reappear. In a decadal perspective, the progressive decrease of the intensity of the sea breezes in the northern Adriatic Sea, where we have carried out our measurements, is associated to the steadily milder winters, and therefore not sufficiently cold local sea temperatures in early summer.