Spectral density of polarimetric variables separating biological scatterers in the VAD display

Echoes in clear air from two types of biological scatterers mixed within the resolution volumes over a large region, observed with S-band dual polarization radar, are presented. This case occurred in the evening of 7 September 2004, at the beginning of the fall migrating season of song birds (passerines). Polarimetric spectral analyses are used for distinguishing birds and insects in multimodal spectra. Spatial continuity of spectral peaks shows clear separation of insect (wind) speeds from bird speeds. Spectral densities of polarimetric variables exhibit vastly different values at speeds corresponding to insects than from those of birds, allowing the separation of the two scatterer types. Therefore, the statistics of the intrinsic polarimetric variables computed from spectral densities are unbiased and closer to the ensemble statistics of the echo type than the ones obtained with standard processing techniques. A novel display of the spectral densities versus azimuth, termed spectral velocity-azimuth display (SVAD), is constructed for easier viewing and interpretation of the velocity field. Analyses of the SVADs reveal the mean velocities and the directions of the two types of scatterers.