Multi‐frequency scatterometer measurements on water surfaces agitated by artificial and natural rain

Abstract

Images of rain events over the ocean acquired by a multi‐frequency/multi‐polarization Synthetic Aperture Radar (SAR) show different radar contrasts at different frequencies and polarizations. In order to better understand these effects, field and laboratory experiments were performed at different rain rates and wind speeds with scatterometers working at different radar frequencies, polarizations, and incidence angles. Our results show that the dominant scattering mechanism on a rain‐roughened water surface, observed at VV polarization, at all incidence angles is Bragg scattering from ring waves. At HH polarization the radar backscatter is caused by both ring waves and non‐propagating splash products, with the dominating effect depending on incidence angle. The reduction and enhancement of the surface roughness by ring waves and sub‐surface phenomena, respectively, result in a transition wavenumber between reduction of the radar backscattering and its enhancement of about 100 rad m⁻¹. We assume that this transition wavenumber depends on the drop‐size distribution of the rain. Taking into consideration the different dependencies of the radar backscatter at different frequencies and polarizations on rain rate, we suggest a method to estimate rain rates by calculating the ratio of the radar cross‐sections at L band, VV polarization and at C band, HV polarization. Provided an availability of SAR data at the respective frequency–polarization combinations, this method allows for investigating the nature of small‐scale (convective) rain events over the ocean.

Acknowledgements

The authors would like to thank Philipp A. Lange and Sönke Heyen who helped in preparing the measurements at the wind‐wave tank of the University of Hamburg, and Alexandra Bayer, Ute Hochbaum and Hanno von Knobloch, who helped in preparing the field measurements. The authors would also like to thank the department of RegDir. K. Schroh of the SBM (Sonderstelle des Bundes zur Bekämpfung von Meeresverschmutzungen) and H.‐J. Hoppe from the WSA (Wasser‐ und Schifffahrtsamt Cuxhaven) for their support in performing the field measurements at the tower of the WSA. This work was funded by the Federal Ministry of Education, Research, Science, and Technology (BMBF) under contract 01 LA 9819/2 (‘AURORa’).