Bird Migration Echoes Observed by Polarimetric Radar

Weather surveillance radars observe clear-air radar echoes (CAE); aerial biota and irregularities in atmospheric refractive index caused the echoes. Using weak returns of atmospheric CAE, the radars have been applied to wind findings in a short radar range in a fair weather, in particular S-band radars. Long wavelength radars perform good to observe air motion, because a detectable turbulent scale depends on the wavelength and large-scale turbulences cause strong refractive index perturbations. C-band radars occasionally observe atmospheric CAE at a cloud edge and the thin layer between the atmospheric boundary layer (ABL) and the free atmosphere, where are characterized by a strong moisture (refractive index) gradient. Sometimes accurate wind estimates are difficult, because biological echoes could contaminate the atmospheric echoes. Conversely, these biological echoes have been applied to monitoring bird migrations and conservations [1]. Polarimetric radars are a powerful tool to discriminate biological echoes using dual polarized returns; the radar measures the differential reflectivity Zdr and the copolar correlation coefficient ρhv between horizontal and vertical polarized returns, and other parameters [2]. Zdr represents a mean axis ratio of scatterers; spherical scatterers are characterized by a value of zero dB and horizontally (vertically) long scatterers generate positive (negative) signatures. Atmospheric CAE mostly generate signatures of |Zdr | ≤ 1.5 dB. In this paper, we classify the signatures of Zdr exceeding (below) 1.5 dB as P-type (N-type) and |Zdr | ≤ 1.5 dB as A-type. ρhv expresses a stability and regularity of scattering cross-sections; atmospheric echoes indicate almost unity (stable and uniform cross-sections),