Using a Doppler light detection and ranging (lidar) system to characterize an atmospheric thermal providing lift for soaring raptors

ABSTRACT.   Raptors and other large birds in soaring flight take advantage of upward drafts of air called thermals to maintain altitude with minimal flapping. I used a Doppler light detection and ranging (lidar) system to characterize a thermal in which raptors were soaring. Doppler lidar allows imaging of wind fields to reveal the structure of updrafts and downdrafts in a thermal. The thermal I monitored was in the form of a horizontal convective roll created at a transition from clear sky to partly cloudy sky, and gave both lift and lateral motion to the soaring birds. The thermal was 700 m high with a vertical wind speed that peaked at 3 m/s, so raptors could have soared to and maintained that altitude as the horizontal wind moved the thermal. My results suggest that imaging wind fields with Doppler lidar can be a useful tool for studying thermals and how they are used by soaring birds. An effective combination for further study of bird flight interaction with wind phenomena would be to add lidar measurements to an established means of tracking bird flight by radio or GPS transmitters, aircraft tracking, or radar.