Spectral polarimetry for identifing and separating mixed biological scatterers

Ubiquitous presence of insects and birds in the Great Plains is detected routinely with weather radars. Whereas the former are mostly passive wind tracers, the latter have speeds comparable to, or larger than the environmental wind. Thus, during the bird migration season in spring and fall, standard estimates of Doppler shifts correspond to bird speeds and therefore are not suitable for meteorological interpretation. Nonetheless, judicious spectral analysis might identify and discriminate echoes from the two scatterer types; examples presented herein explore this possibility. Dominance of one or the other is established from the values of polarimetric variables. We report for the first time a case where the two types of biological scatteres are mixed within each resolution volume and over a large region. This case occurred in the evening of September 7, 2004, during the beginning of fall migration season. 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 differential reflectivity and copolar correlation coefficient have vastly different values at speeds corresponding to insects from those of birds, allowing the separation of the two scatterer types. Intrinsic polarimetric variables computed from spectral densities are much more reliable than the ones obtained with standard processing techniques. That is because the effects of noise can be reduced and mixing of contributions by different scatterers can be avoided. Histograms of differential reflectivity and copolar correlation coefficients computed in this manner separate almost perfectly the contributions by the two scattering species. We suspect that during bird migrations nocturnal insects are also present in the boundary layer at night, i.e., our observation is not an exception. On another similar occasion when we collected time series data the presence of mixed biological scatterers was confirmed.