The Use of Insects as Tracers for “Clear-Air” Boundary-Layer Studies by Doppler Radar

Abstract

Single- and multiple-Doppler radar systems are increasingly being used to monitor circulations within the clear-air boundary layer where the scatterers may be gradients of the refractive index or biota or a combination of both. When insects are the primary source of returned radar power, it must be assumed that the insects are either small and are being carried passively in the air, or are flying randomly so that the bulk velocity of all the insects contained within a pulse volume is zero relative to the air. This study presents dual-polarization radar observations of the interaction between a gust flow and a deep cloud of insects within a relatively unstable air mass over North Dakota on 4 July 1987. These data are unique in that they reveal several meteorological conditions for which the preceding assumption is not valid. The boundary layer was not capped, and circulations rose above an apparent threshold altitude above which these insects were not flying. Temperatures near the threshold altitude were in the range of 10°–15°C. The top of the insect layer remained near 1800 m AGL regardless of circulations that could have carried insects to higher altitudes. A biological hypothesis of flight response to strong updrafts was developed and tested with dual-polarization data. Localized decreases in differential reflectivity Z_DR, interpreted as the result of reorientation of insects in evasive flight were coincident with strong updrafts identified from the analyses of the Doppler velocities. This study shows that conditions exist for which the insects are not valid tracers of air motion. Therefore, care must be taken that combined insect and wind velocities are not taken as wind velocity alone.