Radar imaging of the seabed during the C-STAR experiment in 1996: on the determination of the relaxation rate parameter of short gravity waves due to current variations from in situ measurements and theory

During the field experiment in April 1996 within the “Coastal Sediment Transport Assessment using SAR imagery (C-STAR)” project of the Marine Science and Technology (MAST-III) programme of the European Commission (EC) an Air-Sea Interaction Drift Buoy (ASIB) system was equipped with special sensors and instruments to measure relevant hydrometeorological parameters. Air-sea interaction and other environmental data such as wind speed and direction, modulation characteristics of short wave energies, surface current velocity and direction, position, and water depth were derived in a Dutch coastal sea area covered by large sand waves. Using these in situ data the relaxation rate μ of short water waves due to current variations above submarine sand waves was calculated applying a first order weak hydrodynamic interaction theory; μ was calculated as a function of wave number k of short gravity waves in the range of P-, L-, C-, and X-band radar Bragg waves for three different wind speeds of U_w=0.8 m/s, U_w=3.8 m/s, and U_w=7.4 m/s, respectively. Several published parameterizations of μ showed that this parameter increases with k and U_w. However, the authors´ results show that μ increases also with U_w but decreases with k. This can possibly imply that the wind growth relaxation rate μ_w is not equivalent with μ of short gravity waves due to current variations above submarine sand waves as a function of k