| description abstract | Rain gauge systems are required to measure rainfall data on buoys at oceanic sites that are not suited for conventional rain sensors. A piezoelectrical rain gauge has been developed for use on buoys, to provide rain measurements just above the sea surface. Based on the piezoelectric effect, each drop is measured separately with regard to its size, which is derived from the momentum transfer function of the sensor. Thus, the characteristics of rain?for example, the momentum flux, the drop intensity, and the drop size distribution?can be determined. In addition to laboratory investigations, showing the principal applicability of spherical piezoelectrical ceramics for rain detection, and the calibration, a field experiment had been conducted, comparing the new rain sensor results with those of other conventionally used rain gauges (Joss?Waldvogel disdrometer, FM?CW Doppler radar, and a ship rain gauge). The results are in good agreement. Such devices are needed to obtain a better understanding of rain-induced sea surface processes like the damping of sea waves, the short-scale sea surface roughness enhancement, and the generation of ambient noise, all of which affect radar and acoustic remote sensing. Roughness variations of the sea surface due to rain have been measured by a drifting buoy system. Using the piezoelectrical rain gauge, simultaneous in situ measurements of rain and rain-related sea surface variations have been performed. Results show a strong increase in the energy density spectra in the frequency band from 3 to 30 Hz, representing wavelengths between 17 and 0.7 cm, respectively. Therefore, the backscatter of X-, C-, and L-band radar wavelengths, which correspond to this range, are strongly affected by rain-induced sea surface roughness variations, especially in low-wind situations. | |
