Ocean Wind Fields from Satellite Active Microwave Sensors

Abstract

the Earth’s atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the This definition, obtained from 1, may introduce the Marine Atmospheric Boundary Layer (MABL) as the planetary boundary layer over the sea surface. In this layer, important exchanges of sensible and latent heat and momentum take place over a large spectrum of time and spatial scales, driving the sea waves, the drift ocean currents and the storage of CO2 by the sea due to the wind and the breaking waves. In this context, the leading quantity is the wind vector U. Its assessment is of paramount importance in the evaluation of the wind stress τ = Cd(Ta, Ts, Td) · |U| 2, (the drag coefficient Cd is a function depending, in a first approximation, on the air Ta, the sea Ts and the dew Td temperatures), and of the gas transfer velocity k = 2.8310−2 · |U|3 (Monahan, 2002), for instance. One of the major problems in understanding the dynamics of the wind in the surface layer, the bottom layer inside the MABL where the turbulent fluxes exhibit a variability smaller than 10%, is the difficulty to get experimental data at spatial scales from few meters to few kilometers. The satellite sensors discussed in this chapter measure the backscatter from the sea surface, providing maps directly related to the characteristics of the surface layer and to the wind blowing inside this layer. Satellite active microwave sensors are the only instruments able to provide information about the spatial structure of the wind in the marine surface layer over large areas.