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 ofenhanced reflectivity in mesoscale boundary-layer convergence zones. Recent studies have concludedthat these ‘fine lines’ are attributable primarily to backscatter fromconcentrations of small, weakly flying insects (‘aerial plankton’)entrained in the convergence zones. Such concentrations are likely tobe attractive to aerial predators that feed on small insects, raising thequestion of whether the presence of the predators themselves maycontribute significantly to the radar-observed fine lines.In this paper, we examine the relative contributions of aerialplankton and its predators to fine-line reflectivity, using field datafrom visual and radar studies together with a compilation of literaturedata on radar cross sections of birds and insects. Visual counts ofbirds and dragonflies in convergence zones, together withsimultaneous remote radar observations during the CaPE project inFlorida, indicated that aerial predators usually contributed little tofine-line reflectivity (median contribution ≈ 2%). Assuming thatthe size distribution of insect targets was spatially invariant, thedensity of insects composing the aerial plankton was inferred to be,on average, about one order of magnitude higher inside convergencezones than in nearby areas.These results suggest that clear-air radar reflectivity may be auseful measure of the quantity of aerial plankton in boundary-layerconvergence zones. This finding is relevant to biology because itindicates that remote sensing techniques can be usefully employed todocument patterns and processes in the distribution of aerial plankton.The results presented here also have relevance for operationalmeteorology, because most of the organisms comprising the planktonprobably serve as passive tracers of horizontal air motions, and aretherefore ideal targets for remotely detecting wind patterns. Incontrast, the aerial predators move actively and rapidly, renderingthem less useful as tracers of wind fields in studies using Dopplerradars. The influence of atmospheric structure on the ecology ofaerial predators and their prey has received little attention, but webelieve that sensitive radars with clear-air observational capabilitiesoffer great potential as research platforms for future studies of aerialplankton and aerial planktivory.