Studies of turbulence in the surface layer over water (Lough Neagh). III. Wave and drag properties of the sea‐surface in conditions of limited fetch

Abstract

AbstractWave and aerodynamic properties of the sea surface at Lough Neagh, Northern Ireland, are described with mean wind speed u (at a height of 10 m) varying from 4 to 8 m s−1 and fetch F varying from 8 to 22 km.The sea surface at Lough Neagh is characterized by wind induced waves of dominant period 2 to 4 s and a wave spectrum depending upon u and F, with a steep low frequency face (at frequency nf), sharp peak (at no) and high frequency region with ‘energy’ proportional to frequency to a variable power β in the range —4 to —5. The ‘constant’ of proportionality αg2 in the relation ϕ(n) = αg2n−β appropriate to the equilibrium range is variable between 6 and 12 (where unless stated to the contrary n in numerical expressions is in cycles s−1)‐ mean of 7·9 ‐with a mean β of —4·5, which compares to Phillips' overall mean value of αg2 of 7·2 with a mean β close to —5.Our data, and those from other sources, suggest the following relations, valid over a range of the dimensionless fetch gF/u2 from 102 to 105, And The first two are found to imply σh = 1·4 n−2o.The third relation is interpreted through identification of nf with the transition frequency separating a linear from an exponential wave growth.Our data are consistent with the idea of Kitaigorodskii that zo (u*) depends also upon the degree of wave development σ = gσh/u*2. At Lough Neagh σ* ∼ 10–40, implying zo is closely related to u* as is found to be the case using logarithmic wind profiles.There is evidence that Suggested by earlier Russian work for similar values of σ*.There is no evidence that wave power impresses itself upon the turbulence spectra of the air for values of height/wave amplitude ratio as low as 3·3:1.