Effects of Atmospheric Stability and Wind Fetch on Microwave Sea Echoes

The influences of atmospheric stability and wind fetch on microwave scattering from sea surfaces are presented. The equivalent neutral wind speed (ENWS) and the friction velocity (FV) above sea surface are evaluated by the similarity theory of Monin and Obukhov in combination with the Charnock relation. The numerical simulations show that atmospheric stability and wind fetch would make effects on ENWS and, therefore, further on the scattering coefficients (NRCS). And the NRCS decreases with increasing air-sea temperature difference (ΔT=T_a-T_s) because the ENWS is lower than actual wind speed in stable atmospheric condition, and vice versa. Moreover, the ENWS/FV and the NRCS decrease with increasing wind fetch under unstable atmospheric condition. However, in stable atmospheres, the ENWS/FV and the NRCS are almost insensitive to wind fetch. In this paper, the response of NRCS to ΔT is also discussed. Compared with FV, the variations of the angle spread function with ΔT can be neglected because of its minor contribution.