P5.13 Detection of Birds and Insects Using Polarimetric Radar Observation

It has been known that polarimetric radar can discriminate biological and meteorological scatterers. Furthermore, Zrnic and Ryzhkov (1998) have shown that the polarimetric properties of small Rayleigh scatterers like insects and large non-Rayleigh scatterers like birds are significantly different. Vaughn (1985) has stated that prolate spherical water drops are a better model of birds and insects than spherical water drops (Riley, 1985). Usually the ratios of widthto-length are between 1:2 and 1:3 for birds and between 1:3 and 1:10 for insects (Vaughn, 1985). Due to the difference in shape and size, the polarimetric signatures of insects and birds are different. Birds have higher differential phase and lower differential reflectivity than insects. Thus, it is possible to develop an algorithm to distinguish between birds and insects using polarimetric variables. These advantages of polarimetric radar can significantly improve the current radar applications. First benefit is for quantitative precipitation estimation; non-meteorological echoes caused by birds and insects can be identified and removed to prevent contamination of rainfall estimates. Second, birds can contaminate Doppler radar velocity measurements (Jungbluth et al. 1996; Gauthreaux et al. 1998a,b). In the presence of migrating birds (mostly nighttime during the migrating seasons), radar measured velocities can be very different from the air velocities (projected along the radar beams) and the differences are typically in the order of 10 m/s (Gauthreaux et al. 1998b; Collins 2001; Bi et al. 2002). On the other hand, insects as passive tracers of air motions are usefull for wind measurements in most of cases. Third, for flight safety purpose, it might be possible to issue bird strike advisories (bird strikes to aircraft are a growing problem in world). A classification algorithm that enables identification of different types of meteorological and non-meteorological echo has been developed at NSSL using observations with the polarimetric prototype of the WSR-88D radar. Discrimination between insects and birds will be part of this algorithm. In this paper, several examples of birds and insects observed by the polarimatric KOUN radar are presented in sections 2 and 3. The discussions and conclusions are in section 4. 2. OBSERVATIONS OF TRANSITION BETWEEN INSECTS AND BIRDS