E. Linacre and B. Geerts
Evaporation from the sea can be greatly enhanced by spray formation, which occurs in a ‘moderate gale’ or worse, when winds exceed 15 m/s (1). For instance, in one case a surface wind of 20 m/s was measured at a height of 2m; the air had a relative humidity of 80% at 20° C, and the sea surface was at 22° C. In this case the evaporation rate was measured to be 5.4 mm/d (equivalent to 150 W/m2), whereas the Dalton equation given in Note 4.E [i.e. Eo = 0.5 (es - e) mm/d] yields only 3.8 mm/d. The latter really refers to a fairly smooth water surface, whilst spray creates considerable additional area for evaporation.
Spray produces a broad spectrum of drop sizes. The larger droplets tend to fall back into the ocean shortly after becoming airborne. The smaller droplets may evaporate entirely. The smaller the droplets, the greater the surface area of a given amount of water, and hence the more rapid the evaporation. Indeed, when water is sprayed onto the plate of an overhead projector, the smaller drops can be observed to disappear first.
Spray evaporation is important in the formation and maintenance of tropical cyclones. As the wind increases beyond 15 m/s, the evaporation rate and latent heat flux increase dramatically, and this latent heat is transported into the eyewall cloud, where it is released. The expansion caused by such heat release reduces the surface pressure of the cyclone (Note 1.G), and a deeper cyclone implies a stronger circulation. Stronger winds in turn imply more latent heat flux from the underlying ocean. This feedback mechanism is essential to the air-sea interaction theory that explains tropical cyclogenesis (2).