Experimental investigation of microwave signal scattered by the breaking waves

The work is concerned with the study of the breaking surface wave impact on the scattered radar signal in laboratory conditions using optical methods for analyzing the state of the water surface.

The experiments were carried out on the reconstructed TSWiWaT wind wave flume of the IAP RAS. The channel is 12 m long, the channel cross-section varies from 0.7 x 0.7 m at the entrance to 0.7 x 0.9 m in the working section at a distance of 9 m. The airflow speed on the axis is 3-35 m/s, which corresponds to the values of the wind speed U10 of 11-50 m/s.

At the beginning of the channel, a wavemaker was installed, operating in a pulsed mode and generating a train of three long waves every 15 seconds. In front of the area under study, an inclined plate was installed under the water, simulating shallow water and stimulating the breaking of waves in the zone of optical and radar measurements. In parallel, wind waves were generated. Due to the design features of the experimental setup, the distance from the beginning of the channel to the inclined plate in the case of optical measurements was 884 cm, and for radar measurements - 781 cm.

Radar measurements were carried out using a Doppler scatterometer operating at a wavelength of 3.2 cm, with the ability to simultaneously receive two direct and two cross-polarizations (VV, HH, VH, HV). The dimensions of the observation window on the water surface varied depending on the selected incidence angles (30, 40, 50 degrees). Optical measurements were carried out independently of radar measurements using three cameras with a shooting frequency of 50 Hz. Using a specially developed algorithm based on threshold processing of the image brightness, the time dependences of the fraction of breakers on the area of the investigated water surface during the passage of a train of three waves were calculated.  Due to the different configuration of the experiments, the data of radar and optical measurements are separated in time, their synchronization was performed using correlation analysis.  Comparison of the data made it possible to find that, on cross-polarization, the received power monotonically increases with an increase in the fraction of breakers, while on direct polarization, the change in power remains within the values observed during collapses of wind waves. Further comparison of the values of the radar cross-section of the water surface and the relative area of the wave breaking will make it possible to determine the influence of the breaking on the formation of the scattered signal.

This work was supported by the Russian Science Foundation (RSF) project No. 21-17-00214.