Ocean Wind Fields from Satellite Active Microwave Sensors [chapter]

S. Zecchetto
2010 Geoscience and Remote Sensing New Achievements  
The Marine Atmospheric Boundary Layer "In 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 surface". 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,
more » ... g the sea waves, the drift ocean currents and the storage of CO 2 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 = C d (T a , T s , T d ) · |U| 2 , (the drag coefficient C d is a function depending, in a first approximation, on the air T a , the sea T s and the dew T d 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. Satellite active microwave sensors The active microwave sensors (Campbell, 2002; CCRS, 2009; Elachi, 1988) are radars operating in the microwave region (1 to 30 GHz in frequency, 1 to 30 cm in wavelength) at different polarizations and incidence angles. Over the sea, the radar return depends, besides the geometry of the radar illumination, from the degree of development of the sea surface roughness (Valenzuela, 1978) , composed by centimeter sea waves produced by the wind. Since the wind field has its own spatial pattern, which depends on its strength, on the thermodynamic characteristics at the air-sea interface and on the interaction between the wind 1
doi:10.5772/9111 fatcat:4sqssss6drb7jpdyk3x3m5mauu