Wave-wave interactions in a wind-wavetank as measured withmicrowave and optical systems

摘要

Microwave backscatter from the ocean surface has long been assumednto be proportional to the spectral density of windwaves at thenBragg-resonant wavelength. Although the spectral density ofngravity-capilary waves is primarily a function of wind forcing,ninteractions with longer waves are also an important factor in wave-wavenenergy transfer. Thus, the microwave backscatter should be affected bynthe presence of longer waves as well. Any effort to measure the effectsnof longer waves on the backscatter, usually referred to as thenmodulation transfer function, or mtf: should include both directnmeasurement of the backscatter and direct measurement of the short-wavenspectral density. The authors have used microwave and millimeter-wavenscatterometers at 10 GHz (X-band), 35 GHz (K_α band),nand 70 GHz (V band) to measure the backscatter. All three microwaveninstruments are CW Doppler systems. The X-band scatterometer offsets thenDoppler by 400 Hz so both the upwind and downwind peaks can recorded.nThe remaining systems are I&Q instruments, where the RF signalnmodulates a 60 MHz carrier wave, and the I&Q are retrieved from then60 MHz carrier in a separate section. The microwave signals are samplednat 3 kHz using a Data Translation A/D card in a PC, and the data arenstored on 8 mm EXABYTE type. The microwave and optical systems were allnaligned to view the same spot on the water, at a fetch of 16.5 meters