Adaptive technique to extract the intrinsic insects' backscatter differential phase from polarimetric spectra

Biological scatterers such as insects and birds are ubiquitously present in the planetary boundary layer (PBL). Often small insects are good wind tracers hence can provide information about wind in clear air conditions. But, both the type and amounts of insects and birds exhibit diurnal changes that beg for detail explanation. We examine the change in polarimetric variables of biological scatterers over a six hour period from early evening through sunset to midnight which occurred on Sep 8, 2004. We apply a special technique on the polarimetric spectral densities for estimating the backscatter differential phase δ inherent to insects. The dependence of δ in azimuth is gratifyingly smooth compared to the dependence of δ obtained by standard procedures. Nonetheless, the values in the east and west semicircles of radar coverage defy simplistic explanation. It might be that the difference in solar fluxes at the time of sunset is responsible for the non symmetric signature of δ. That is, it could be that the nocturnal insects are emerging on the east side while the diurnal insects are still present and descending on the west side. Other scenarios might be at play as well. The changes are surprisingly rapid so that observations at ten minute intervals are needed to capture the continuity of evolution at the time of sunset. Even after sunset and well into the night the biological inhabitants of the PBL continually present evolving and somewhat inhomogeneous fields of polarimetric variables. The technique might be useful for discriminating some insects and is definitely suited for separating insects from birds. Adaptive selection in the frequency domain allows to avoid/suppress signatures of migrating birds or other scatterers and to extract signatures of wind-blown insects.