|Paper title||SeaWinds Normalized Radar Cross Section noise characterization for improved coastal processing|
|Form of presentation||Poster|
Most of the human world population lives along the coast. Therefore, their lives are heavily affected by the meteorological phenomena that characterize these areas. In that sense, coastal winds play a relevant role. Indeed, for example, the presence of sea breezes, katabatic flows and orographic winds in general, can strongly impact local micro climates and, for example, wind energy potential. Furthermore, they play a fundamental role in the generation of local ocean currents and in the dispersion of air pollutants.
As such, accurate and highly sampled coastal winds observations are of paramount importance to modern societies. In order to pursue this objective, scatterometer-derived wind vectors are potentially very useful, but first the excessively land contaminated radar footprints should be removed, while the low contaminated ones should be corrected by means of a land contribution ratio (LCR) based Normalized Radar Cross Section (NRCS) correction scheme. In addition, the NRCS noise should be carefully characterized in order to properly weight the backscatter NRCS measurements contributing to the wind field retrieval.
An assessment of the noise (Kp) affecting the NRCS measurements of the Seawinds scatterometer (onboard QuikSCAT) is carried out in this study. An empirical method is used to derive Kp (Kp'), which is then compared to the median of the Kp values (Kp'') provided in the Level 1B Full Resolution (L1B) file with orbit number 40651, dated 10 of April 2007, and the main differences are discussed. A sensitivity analysis aiming at assessing the presence of any dependencies with respect to (w.r.t.) different wind regimes, the kind of scattering surface, the scatterometer view and the polarization of the signal is carried out. In addition, the presence of any biases is assessed and discussed. Finally, a theoretical NRCS distribution model is proposed and validated against real measurements.
The main outcomes of this study demonstrate that H-Pol measurements are noisier than those V-Pol, for similar wind speed regimes. In addition, the noise decreases with increasing NRCS values, in line with the expectations. Furthermore, Kp' may largely differ from Kp'', especially for the peripheral measurements, with differences up to 20%. In particular, the Kp values provided for the outermost slices seem to be understimated, especially for what concerns the H-Pol measurements with indices 6 and 7. In addition, the Kp' values estimated over the sea surface are lower than those estimated at all scattering surface types. This trend is not seen for Kp'', for which the differences are almost absent. Furthermore, some inter slice biases up to 0.8 dB are present for H-Pol acquisitions while these only are up to 0.3 dB for V-Pol ones, in both cases increasing with the relative distance between the slices, in line with the general Geophysical Model Function (GMF) sensitivity as a function of incidence angle. These biases have a non-flat trend w.r.t. the acquisition azimuth angle for both polarizations. These small variations may be due to changes in the wind speed and direction distribution for each bin.
The theoretical NRCS distribution proves to be effective. It can be used both for simulation studies and for checking the accuracy of the NRCS noise.