RADAR-OBSERVED FINE LINES IN THE OPTICALLY CLEAR BOUNDARY-LAYER - REFLECTIVITY CONTRIBUTIONS FROM AERIAL PLANKTON AND ITS PREDATORS

Sensitive Doppler radars regularly detect fine lines of enhanced refle ctivity in mesoscale boundary-layer convergence zones. Recent studies have concluded that these ''fine lines'' are attributable primarily to backscatter from concentrations of small, weakly flying insects (''ae rial plankton'') entrained in the convergence zones. Such concentratio ns are likely to be attractive to aerial predators that feed on small insects, raising the question of whether the presence of the predators themselves may contribute significantly to the radar-observed fine li nes. In this paper, we examine the relative contributions of aerial pl ankton and its predators to fine-line reflectivity, using field data f rom visual and radar studies together with a compilation of literature data on radar cross sections of birds and insects. Visual counts of b irds and dragonflies in convergence zones, together with simultaneous remote radar observations during the CaPE project in Florida, indicate d that aerial predators usually contributed little to fine-line reflec tivity (median contribution approximate to 2%). Assuming that the size distribution of insect targets was spatially invariant, the density o f insects composing the aerial plankton was inferred to be, on average , about one order of magnitude higher inside convergence zones than in nearby areas. These results suggest that clear-air radar reflectivity may be a useful measure of the quantity of aerial plankton in boundar y-layer convergence zones. This finding is relevant to biology because it indicates that remote sensing techniques can be usefully employed to document patterns and processes in the distribution of aerial plank ton. The results presented here also have relevance for operational me teorology, because most of the organisms comprising the plankton proba bly serve as passive tracers of horizontal air motions, and are theref ore ideal targets for remotely detecting wind patterns. In contrast, t he aerial predators move actively and rapidly, rendering them less use ful as tracers of wind fields in studies using Doppler radars. The inf luence of atmospheric structure on the ecology of aerial predators and their prey has received little attention, but we believe that sensiti ve radars with clear-air observational capabilities offer great potent ial as research platforms for future studies of aerial plankton and ae rial planktivory.