Momentum fluxes and wind gradients in the marine boundary layer : a multi-platform study

During five autumn weeks, measurements of turbulent fluxes were obtained in the Baltic Sea at three levels on a 30-m tower and two levels on an ASIS buoy 4 km from the tower together with profiles of wind and temperature. Wave data and SST were obtained from ASIS. In the mean, momentum fluxes measured on the tower and on ASIS during onshore winds agree closely. Dimensionless wind gradients [phi]m(z/L) for (i) stable conditions are linear in z/L (L is the Obukhov length); (ii) unstable, growing sea conditions are much smaller than predicted by `standard' equations, due to an indirect effect of the boundary layer height. Individual wind profiles extrapolated from ASIS to tower by integration of [phi]m(z/L) deviate by about 0.5 m s–1 from measured values, but corresponding mean profiles agree well for all levels from 1.18 m to 30 m. This random variation in the wind field is shown to be related to inherent dynamics of the atmospheric surface layer.