GRASSHOPPER RESPONSE TO RAPID VERTICAL DISPLACEMENTS WITHIN A CLEAR AIR BOUNDARY-LAYER AS OBSERVED BY DOPPLER RADAR

Dual-polarization radar recorded insect behavior when a gust flow interacted with a deep, dense cloud of large insects over North Dakota on 4 July 1987. These data reveal areas within the upper parts of the cloud with apparent active modified flight behavior. Disturbed areas were separated from adjacent undisturbed areas by only a few hundred meters. The boundary layer was not capped and circulations were free to rise above an apparent flight ceiling of almost-equal-to 1,800 m and where temperatures were in the range 12-15-degrees-C. Disturbances in some areas extended downward 1,000-1,200 m from the top of the cloud to levels where the temperatures were almost-equal-to 10-degrees-C warmer. Kinematic analysis of Doppler velocities revealed close correspondence between the areas of modified flight behavior and strong updrafts (and in a few instances, strong downdrafts). The data permit at least five explanations for the apparent modified flight behavior. These are passive sorting in updrafts, and active response to turbulence, rates of change of temperature, rates of change of pressure, and the presence of clouds.