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Spectral polarimetry for identifing and separating mixed biological scatterers

Through a good part of spring and fall, strong echoes appear on radar screens starting at sunset and disappearing at sunrise. These have been attributed to migrating birds. We examine one such case of echoes in clear-air planetary boundary layer which occurred during fall of 2004 at the time of nocturnal bird migration. Most of the migratory bird movements occur at altitudes below 3 km with the bulk of action under 900 meters (npwrc.usgs.gov 2005). Bird flight velocity ranges from 8 to 22 m s (Gill 1994). Many insects also migrate at night. Microinsects are weak flyers, their deliberate or inadvertent motion is primarily wind driven. Insects ascend to altitudes of several hundred meters above the ground where they ride the airstreams crossing many kilometers in a single flight (Chapman et al., 2004). Although their speed can be as high as 8.3 m s (Riley 1999) typical values are 1 m s or less. Thus insects often behave as quasi passive wind tracers. It is this property that makes them suitable for wind measurements with Doppler radars. Thus we submit that lofted insects are excellent scatterers that enhance echoes needed for wind profiling, and that migrating and wandering birds are not passive wind tracers and typically fly at velocities different from those of the wind. We examine Doppler spectra and show that separation of birds from insects is possible if the antenna is pointing in (or near) the direction of the wind. Further spectral densities of dual polarization variables contain discernible signatures that offer a way of distinguishing between birds and insects and obtaining winds aloft.