Biological Scatterers as Indicators of Boundary Layer Structure

A variety of clear-air boundary layer phenomena are regularly visible on radar due to the presence of biological scatterers such as birds, bats, and insects. Whether observing fine lines, gust fronts, convective rolls, or simply the depth of the boundary layer, one may question to what extent the echoes from these organisms are representative of the underlying atmospheric structures and motions. For many it has generally been accepted that these creatures cannot be treated as passive tracers of wind fields, and thus must be rendered as clutter.

A developing paradigm in the study of airborne animals, aeroecology, is the understanding that, much like land surfaces or oceans, the atmosphere provides a diverse habitat in which organisms react to ever-changing spatial inhomogeneities. This may include actively seeking desirable temperatures or wind patterns, or avoiding enhanced turbulence. To ecologists, these interactions can provide insight into the flight, foraging, and migratory behaviors of these animals. To meteorologists, the presence or absence of certain organisms can provide a means to determine the location of atmospheric features of interest (e.g., boundary layer depth, specific isotherms, low level jets, or convective features).

To explore the relationship between atmospheric properties and the distribution of flying animals, a mobile, X-band radar was deployed at the site of a seasonal roost of insectivorous birds (Purple Martins, Progne subis) in Garland, TX. The radar operated in RHI mode for nine hours, monitoring the daytime foraging activities of the birds aloft. Complementing these data are the additional measurements from two nearby terminal Doppler weather radars (TDAL and TDFW) and a NEXRAD station (KFWS). These observations suggest that, while birds certainly are not passive tracers, many organisms may still be viewed as active indicators with the ability to provide valuable boundary layer information.