Parameterizing the Vertical Motion of Small Insects in the Convective Boundary Layer

In warm weather, the daytime convective boundary layer is full of small insects. For several decades small day-flying insects such as aphids have been considered as passive tracers in the boundary layer. However, Geerts and Miao (2005, Environmental Entomology, 34, 361-377) compared the vertical motion recorded by an aircraft-mounted W-band radar and a collocated gust probe, finding that small insects resisted ascent in updrafts and that this opposition increased proportionally to the strength of the updraft.

Here we have extended this experiment using two months of summertime CBL data from the Ka-band cloud radar and Doppler lidar collocated at the Department of Energy’s Atmospheric Radiation Measurement Southern Great Plains site in Lamont, Oklahoma. Since the cloud radar data contains the effects of insect contamination and the Doppler lidar data does not, we are able to isolate the insects’ vertical motion and compare it to the surrounding air motion. Although there is considerable scatter in the data our main findings mirror those of Geerts and Miao, with the insects opposing updrafts at a rate proportional to the updraft strength. Conversely this opposition is not seen in downdrafts.

Our findings fit with a modified Lagrangian stochastic model of insect dispersion in the daytime convective boundary layer in which a term has been included to account for the flight behavior of small aphid-sized insects. Within this framework the airborne dispersal of small insects - many of which are serious agricultural pests - can be predicted more reliably on the basis of insect aerial density profiles.

These results indicate that even small insects cannot be considered as passive tracers in the vertical direction. Their responses to the surrounding airflow will have a noticeable impact on long-term statistics of vertical motion in the CBL derived from the millimeter-wavelength radars and caution should be exercised when interpreting insect-contaminated vertical motion data.