Airborne radar observations of the flight behavior of small insects in the atmospheric convective boundary layer

The vertical flight behavior of insects in the convective boundary layer (CBL) is examined by means of profiling airborne Doppler radar data collected in the central Great Plains in late spring. On fair-weather days, the CBL grows from the ground up in morning hours and matures at a depth of 1,000-1,500 in shortly after midday. It is well mixed by thermals bubbling up from near the surface. Nevertheless the CBL is dominated, over its entire depth, by well-defined regions of high insect concentrations, here referred to as insect plumes. This is inferred from radar, whose echoes in the CBL are largely caused by microinsects (< 10 mm diameter). This study focuses on the vertical motion of the radar scatterers relative to the vertical air motion, in natural conditions. It is shown that insect plumes tend to be collocated with updrafts in the CBL and that microinsects tend to fall or fly down against the updrafts at an average speed of 0.5 +/- 0.2 m/s. This estimate is based on a comparison of the close-range radar velocities, some 100 m above and below the aircraft, with the vertical air velocity measured at flight level. We hypothesize that the gregarious behavior of small insects in the CBL is explained by their tendency to oppose updrafts at a rate that is surprisingly proportional to the updraft strength. This finding is also strong evidence for the biotic nature of the echo plumes. This hypothesis is tested elsewhere by means of a simple numerical simulation.