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

Microwave backscatter from the ocean surface has long been assumed to be proportional to the spectral density of windwaves at the Bragg-resonant wavelength. Although the spectral density of gravity-capilary waves is primarily a function of wind forcing, interactions with longer waves are also an important factor in wave-wave energy transfer. Thus, the microwave backscatter should be affected by the presence of longer waves as well. Any effort to measure the effects of longer waves on the backscatter, usually referred to as the modulation transfer function, or mtf: should include both direct measurement of the backscatter and direct measurement of the short-wave spectral density. The authors have used microwave and millimeter-wave scatterometers at 10 GHz (X-band), 35 GHz (K_α band), and 70 GHz (V band) to measure the backscatter. All three microwave instruments are CW Doppler systems. The X-band scatterometer offsets the Doppler by 400 Hz so both the upwind and downwind peaks can recorded. The remaining systems are I&Q instruments, where the RF signal modulates a 60 MHz carrier wave, and the I&Q are retrieved from the 60 MHz carrier in a separate section. The microwave signals are sampled at 3 kHz using a Data Translation A/D card in a PC, and the data are stored on 8 mm EXABYTE type. The microwave and optical systems were all aligned to view the same spot on the water, at a fetch of 16.5 meters