ARM Data Is for the Birds
Scientists use lidar and radar data to study bird migration patterns.
Birds are well attuned to weather and climate conditions, annually migrating south to warmer climates in the autumn and back north to their breeding grounds in the spring. Their aerial movements are influenced by atmospheric flows spanning a range of spatiotemporal scales. A recent study looked at how some large-scale atmospheric motions such as the Great Plains low-level jet (LLJ) impact this behavior. The LLJ is a prominent atmospheric flow in the central United States that consists of enhanced winds in the lowest few kilometers of the atmosphere following sunset and persisting into early morning. The LLJ provides significant tailwinds during spring migration and strong headwinds during autumn migration, but how these tailwinds affect migratory behavior is not clear. Using lidar and radar data from the Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility site in Oklahoma, scientists explored how nightly and seasonal LLJ variability influences aerial migration.
Scientists suspected that the LLJ plays a prominent role in defining migratory routes, but the flight strategies birds use with respect to these winds had not been examined. An investigation of the altitudinal selection behavior of aerial migrants (e.g., birds, bats, and insects) over Oklahoma during two spring and two autumn migration seasons shed some light on these flight strategies. The scientists found that in general migrating organisms choose to fly within the jet in spring, often concentrating in the favorable wind speed maximum. In autumn, most typically fly below the jet, although some will rapidly climb to reach altitudes above the inhibiting winds. The migration intensity was relatively constant throughout the spring due to predominantly favorable southerly jet winds. Conversely, autumn migrants were more apt to delay departure to wait for the relatively infrequent northerly winds.
The ARM Southern Great Plains site in Oklahoma is located in the “central flyway,” one of the three main North American flyways for seasonal bird migrations. The Great Plains LLJ is a prominent atmospheric flow spanning the latitudinal extent of the central United States from Mexico to Canada. This rapid stream of meridional winds has likely played a lasting role in nocturnal migration across the central United States. To explore interactions between the LLJ and migrating birds, two independent data streams were needed: wind measurements and animal distributions. Wind retrievals from Doppler lidar are generally unaffected by scattering from birds or insects, yielding true atmospheric motions. While designed for measuring cloud particles, the Ka-band ARM zenith radar (KAZR) also observes scattering signals from flying organisms such as birds, bats, and insects. The team used KAZR radar reflectivity below 2 km as a proxy for animal density. Since the specifications of the radar were designed for clouds, not birds, definitely delineating between birds and insects in the signal was not possible, so the study refers to these organisms generally as “migrants” although birds likely dominate the signal (due to their much larger scattering cross section). In this study, the ARM Doppler lidar and KAZR measurements were combined over two years to examine whether the choice of migration flight altitude is influenced by the LLJ. In particular, the study examined whether the seasonal shift in migration direction results in seasonal differences in altitude selection; whether the nightly variability in jet speed, direction, and altitude affect the overall abundance of migrating organisms in the airspace; and whether there are seasonal differences in migratory decisionmaking with respect to LLJ conditions. The scientists found that the altitude at which migrants choose to fly does appear to depend on the presence, height, and favorability of LLJ winds. Specifically, seasonally favorable jets promote enhanced migration activity through the depth of the favorable wind layers. Unfavorable jets motivated avoidance behaviors such as flying at an altitude that minimizes the impact of the southerly jet winds or delaying migration until more favorable conditions occur. Overall, the results suggest that aerial migrants in the U.S. Great Plains region exploit the LLJ for maximal wind assistance.
University of Oklahoma
The lidar and Ka-band radar data were obtained from the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a U.S. Department of Energy Office of Science user facility sponsored by the Office of Biological and Environmental Research. This work was partially supported by National Science Foundation grant EF- 1340921.
C. E. Wainwright, P. M. Stepanian, and K. G. Horton, “The role of the U.S. Great Plains low-level jet in nocturnal migrant behavior.” International Journal of Biometeorology 60(10), 1531-42 (2016). DOI:10.1007/s00484-016-1144-9