Using Chaff with a Cloud Radar for Studying Entrainment Processes

The feasibility of using chaff to track motions associated with entrainment processes at the top and edges of cumulus clouds and at the top of the atmospheric mixed layer was examined using an airborne FMCW 95 GHz Doppler radar. The chaff used for this experiment was pre-cut metallic coated fibers that were dispersed from canisters carried in a mount attached beneath the wing of the CIRPAS Twin Otter research aircraft. The chaff was cut to 1/4 of the wavelength of the cloud radar (3/4 mm). The fibers have a terminal velocity of about 2 cm/s. The radar was mounted on the top of the aircraft fuselage in an upward-pointing mode and collected Doppler spectra every 0.3 seconds. The chaff experiments were designed to examine two types of entrainment processes. In one, chaff was dispersed near the top and the edge of small cumulus clouds. After the chaff was dispensed, the aircraft made penetrations of the cloud of interest at lower levels to observe the chaff clouds above with the radar. In the second approach, chaff was dispensed just above the top of the mixed layer with subsequent levels flown in the mixed layer to observe the time evolution of the chaff clouds. It was found that the Doppler spectra from the chaff had a substantially narrower distribution than those from clouds and precipitation. These spectra provide additional information about the chaff characteristics and the associated tracer motions. The possibilities for further extending and improving this technique for tracking air motions and studying entrainment circulations will be discussed.