Quantitative bird migration information from operational weather radars

Migration of birds can strongly bias wind measurements by windprofilers and weather radars, and detailed quality control is necessary to suppress contaminations related to the active flight of birds. While suppression of bird signatures in operational profiling sensors is becoming common practice, little effort has been made to obtain real-time information on bird migration itself. As part of an international project by the European Space agency (ESA) aimed at reducing collisions between aircrafts and birds, the Royal Netherlands Meteorological Institute (KNMI) has explored the potential of operational C-band Doppler weather radar as a bird migration sensor. A bird migration recognition algorithm has been developed, extracting bird density, speed and direction as a function of altitude. The weather radar data have been validated against simultaneous and co-located bird density measurements by a high precision bird radar, provided by the Swiss Ornithological Institute (SOI). This mobile tracking radar has been stationed next to weather radar sites in the Netherlands, Belgium and France during the peak bird migration season in autumn 2007 and spring 2008. The mobile tracking radar is capable of detecting and discriminating bird echoes with a high accuracy, making it an ideal reference for validating the weather radar observations.

We find that Doppler weather radar is highly successful in determining quantitative bird densities as a function of altitude. A quantitative correspondence in observed bird-densities is found between the weather radar and dedicated bird radar. With the established continent-wide networks of weather radars (e.g. OPERA, consisting of over 180 radars (http://www.knmi.nl/opera) and NEXRAD (http://radar.weather.gov), consisting of over 150 radars) almost all bird flyways across Europe and the United States could be monitored simultaneously all year round. Apart from highly improved bird strike warnings, such a network would yield invaluable information for scientific research on bird migration with respect to the influence of the spatial distribution of stopover sites, the weather regime and their variation in time.