On The Effect Of Rain On Sea Surface Waves: Experimental Study

Rainfall is a very common phenomenon in daily life;however,in the research of sea surface feature,the effect of rainfall can not be ignored.Especially on the retrieval of sea surface wind using altimeter data,the change of sea surface roughness during rain will affect the accuracy of wind retrieval.As a result of wind field and gravity,momentum is transferred between rain and sea water in air-sea boundary layer in horizontal and vertical directions,sea surface roughness,wind stress and other features can be altered by rainfall inevitably.In early studies,oceanographers figured that rainfall can damp gravity waves. Later studies found dual effect of rainfall on surface waves:damping in gravity wave range and growth in capillary and gravity-capillary wave range.However,the mechanism behind this phenomenon is still disputable.The author analyses the influence of rainfall on sea surface roughness,the growth of wind waves and swell and other features of sea surface waves using a group of control experiments carried in a wave tank in this thesis.Three types of waves are studied: mechanical waves of single frequency produced by a paddle,wind waves of several wind speeds,swells produced by clear away wind field from tank before rain section.Artificial rain is fallen in the middle of wave tank simulating natural conditions.The experiments are carried in two groups.Intensity of rain is set to be constant in the first group and the length of rain field is alterable,which is set to be 2m,4m and 6m.;in the other group,the length of rain field is set to be constant and rain intensity varies between 90,112.5 and 135 mm hr^-1.During the experiment,researchers record three velocity components of flow with high resolution ADV,displacement of water surface with wave gauge,and wind velocities on different height above water surface with automatic anemometer.Firstly,the author calculates the power spectra of all kinds of waves with and without rain according to the records of surface displacement.During the analysis of the change of spectra,the author finds that the shape of power spectra varies clearly under slow winds, however,when in high wind conditions,the variations are not obvious.In general,rainfall enhances the growth of high frequency waves and damps low frequency waves according to the spectra in slow wind conditions.Then the author analyses the total energy per square meter to observe the effect of rain as awhole.For mechanic waves,total energy increases during rain,and mean height of wave increases with the length of rain field.For swells, total energy decreases in rain conditions,and the degree of the decrease enhances with the increase of length of rain field and rain intensity.For wind waves,in high intensity rain condition,total energy increases under slow and medium winds and decreases under high winds;in relative small intensity condition,total energy decreases with rain intensity under high winds,but the variations under low winds are not large or clear.At last,the author analyses the effect of rain on sea surface roughness through calculation of mean square slope of sea surface,it indicates that roughness of swell decreases in rain conditions,the variation of roughness of windsea during rain in high wind conditions is small.