On the action of rain in calming the sea and generating sea surface roughness

It is well known among sailors that rain tends to calm long sea waves. Additionally to the wave damping process, rain produces short scale roughness which becomes visible especially in low wind situations. The variation of sea surface roughness due to rain influences radar backscattering. Thus it has to be taken into account when utilizing radar technique for ocean remote sensing. To analyse the action of raindrops on the sea surface, drop impacts have been photographed with high speed cameras. Laboratory investigations show that the mixing behaviour between the drop fluid and the sea surface is closely related to the hydrodynamic process of drop impact. Although for all observed drop classes spherical cavities were formed during the first milliseconds after impact, further development of the cavity differed significantly. Additionally to the laboratory investigations in-situ measurements of roughness enhancement by rain have been performed. The experiment was conducted in the harbour of Kiel using a drifting buoy system. Among other sensors the buoy is equipped with a resistance wire gauge for short wave measurement and a newly developed rain sensor. Due to drop impact processes the rain induced roughness covers a broad frequency band from 3 Hz to 30 Hz, wavelengths between 17 cm and 0.7 cm, respectively. Therefore the backscatter of X-, C- and L-band radar wavelengths, which correspond to this range, are mostly affected by rain induced sea surface roughness variations.