The Study Of Modulation Of Short Waves By The Long Waves

The Study of Modulation of Short Waves by the Long WavesAbstractWave is a normal phenomenon on the ocean and it affects the living of humandirectly.Usually it is the name of surface gravity wave concluding swell and windwaves which are relatively short.And it could be seen as a sum of waves of differentphases and amplitudes.Waves play an important role in ocean remote sensing and air-sea interaction.Ocean surface roughness of ocean is contributed mostly by intermediate and smallscale surface waves.The non-uniform flows caused by some phenomenon such asswells and fronts could modulate small-scale roughness.And this modulation is alsothe foundation of the microwave remote sensing to these phenomenon.Hydrodynamicmodulation transfer function is used to describe the effect of long waves to the shortwaves.The research about wave modulation is very important.During severaldecades a lot of improvements have been made and a lot of important facts are found.However the mechanism of wave modulation is not well understood yet.Experiments were conducted in the laboratory to observe the modulation of shortwind waves by mechanically generated monochromatic long waves.The surfaceelevations as well as the surface gradients are measured.Some alterations are made tothe Empirical Mode Decomposition (EMD) approach to avoid the blending ofIntrinsic Mode Function (IMF).In the altered mode decomposition approach,the IMFdecomposition is done in frequency space instead of in time domain.With this method,every surface elevation record is decomposed into a few IMFs.The high frequencyIMFs of the same ordinal number extracted from different wave records have almostidentical frequency ranges,therefore represent the surface undulations of a particularfrequency scale under different wave conditions.The squared elevations of each high frequency IMF are phase averaged to find its variation along the long wave profile.Itis found that the energy of high frequency waves are asymmetry on the surface oflong wave,with its maximum occurs at the upwind quadrants of the long wave crests.In view of the different phase shifts induced by various modulation mechanisms,together with the above observations,we credit the asymmetric distribution ofsmall-scale wave energy on long wave profile mainly to the wind sheltering effect oflong waves.The orbital velocity provide a background current to the short waves.And theDoppler effects should be noticed when the wave number spectrum are transferred tothe frequency spectrum.The advection of the wave number component by the orbitalcurrent of background waves produces a net downshift in the encounter frequency.After some theoretical analysis of recent modulation model and mechanisms,anew modulation model is made.This model connects wind stress to the spectrum ofshort waves.And calculation results from this model show that when a train of longwave travels with the wind,the maximums of short wave spectrum occurs at theupwind quadrants of the crests of the long wave.And the modulation of the shortwave changes little with the wave numbers.