Plume Dispersion in the Convective Boundary Layer. Part I: CONDORS Field Experiment and Example Measurements

Abstract

Project CONDORS (CONvective Diffusion Observed by Remote Sensors) measured the dispersion of a nonbuoyant plume in the highly convective boundary layer. Laboratory and numerical models have predicted vertical profiles of a passive tracer that are significantly non-Gaussian in shape in convective conditions. However, adequate field data to verify the model results were not available before CONDORS. A lidar measured the three-dimensional distribution of oil fog, and a radar similarly observed a plume of microwave-scattering chaff. The instrumentation, experimental methodology, and data reduction techniques are described. Analysis is still in progress but preliminary results support most of the simulations, including the deviations of vertical profiles from Gaussian model predictions. The experiment centered around the 300 m instrumented tower of the Boulder Atmospheric Observatory. The parameters in convective scaling theory were determined, and extensive information on wind statistics was gathered. Mixed-layer height was found by a consensus of determinations from the vertical extent of remotely sensed tracers, from on-site radiosonde profiles, and from lidar-observed gradients of the haze. During the first phase in late summer of 1982, oil fog and chaff were released simultaneously either from the surface or from an elevated position on the tower. During the main phase a year later, the oil fog was usually released from the surface and chaff separately from the tower, and two conservative gas tracers were also sampled on a single arc at the surface. The use of multiple tracers permitted an evaluation of their performance. Careful processing of the remotely sensed tracer data successfully removed interfering effects, such as attenuation of the lidar radiation and ground clutter in the radar signal. Although these tracers were not conserved, valid profile shapes were obtained, and local downwind flux of measured concentrations was successfully applied in place of the release rate. However, any interpretation of the chaff data must consider the effect of gravitational settling. The three types of tracers complemented one another in revealing the dispersive behavior of plumes in convective conditions.