An analysis of scatterometer returns from a water surface agitated by artificial rain: evidence that ring-waves are the main feature

Abstract

Both wind and rain roughen the sea surface, but whereas wind generates waves, rain generates craters, stalks and ring-waves. Average backscat tered power for scatterometer returns from water surfaces is closely related to small scale features on the water surface, so we use backscattered power from short wind-waves as a basis to evaluate the importance of ring-waves. Experi ments were conducted with a 13•5 GHz scatterometer (30° incidence angle, vertical polarization) in a wind-wave tank that is enhanced by a rain simulator. Rain intensities ranged from 3-30 mm h-1 and wind friction velocities were between 10 and 50 cm s • 1• The variance of surface elevation for small scale features �;m, i.e., ring-waves and short wind-waves, was computed for each case using data from a capacitance probe. Comparison of the data sets shows that the range of �;m for the rain cases is comparable to that from light to moderate wind cases-so ring-wave amplitudes are not negligible. Analysis of the radar data provides evidence that ring-waves are the dominant feature contributing to the backscattered power. Thus ring-waves need to be included in scatterometer numerical models that contain rain effects.

Estimating RCS of the sea surface perturbed by rain for rainfall rate retrieval

“…It has been shown that the crown and stalks phases, following raindrop splashes, are important features to be considered for analysing backscattering near grazing incidence angle, while ring waves are important for backscattering at incidence angles near nadir [3]. In our case, only ring waves have been considered, and been modeled as a random process with characteristics similar to those of the waves generated by wind.…”

“…calculated through the graph in [3] obtained for an artificial rain falling from lm height and using energetic consideration in order to find the relationship for raindrops falling at terminal velocity. Nevertheless, an accurate description of such relationship is not available in the literature, and for this reason, we have simply chosen hR as the parameter describing sea roughness and rainfall intensity, one can obtain the NRCS as a function of hR using the formula (4).…”

Improving rainfall measurement accuracy in spaceborne rain radar over sea

“…Indeed, hR is related to the rainfall rate R, nevertheless an accurate description of such relationship is not available in the literature. The relationship gwen in [4], for instance, is probably limited to that experimental case with that artificial rain. For this reason, we have simply chosen hR as the parameter describing sea roughness and rainfall intensity.…”

“…We wll consider a sea surface corrugated by w n d and by the effects of the raindrop splashes It has been shown that crown and stalks, arising from raindrop splashes, are important features to be considered for analysing backscattering near grazing incidence angle [9], while ring 0-7803-2567-2195 $4.00 0 1995 IEEE waves are important for backscattering at incidence angles near nadir [4], [5]. In our case, only ring waves have been considered; they have been modeled as a random process with characteristics similar to those of the waves generated by the wind, The treatment of the crown and stalk phases is more particular since these are impulsive effects, and are treated in [9], where the backscattering due to a single ring wave is computed in the time domain.…”

“…In such a case, volumetric backscattering from several cells above the sea surface is contaminated by the sea return [l]. Furthermore, it has been shown that the influence of the rainfall on the backscattering coefficient of the sea surface is not negligible [4], [5]. The estimation of the rain-perturbed surface backscattering coefficient can be exploited for ambiguity removal in the aforementioned algorithms, and also the surface referenced algorithm could get a substantial improvement.…”