Wednesday, 6 August 2003
Observations of clear-air dumbbell-shaped echo patterns with the CSU-CHILL polarimetric radar
On a few occasions during the summer and fall of 2002, the CSU-CHILL S-band polarimetric Doppler radar scanned temporally persistent smoke layers associated with nearby forest fires. As opposed to previous radar studies of narrow smoke plumes (the few that exist), these were apparently horizontally widespread (1000s of sq. km) layers filled with ash particles that persisted far (10s and in one case 100s of km) from the fire sources. While optically thin, these layers often showed substantial radar returns, sometimes peaking above 20 dBZ in reflectivity factor. The most striking feature of the radar echoes was that, in all cases, they formed a dumbell-shaped pattern, with the highest reflectivity factors arranged approximately perpendicular to the direction of the mean wind as estimated by Doppler radar methods. These regions of higher reflectivity factor coincided with strongly positive differential reflectivity measurements. Because of the widespread nature of the optically visible smoke layers, the dumbell-shaped radar pattern is suggestive of large ash particles that are quasi-prolate in shape and aligned horizontally along the direction of the mean wind. This would reduce the radar cross-section of the particles while scanning along the wind axis, and maximize it while scanning perpendicular to the wind axis. On one of the days (17 October), coincident smoke plume penetrations by the NCAR C-130 as part of the IDEAS2 project support the inference of prolate-shaped ash particles. Aerodynamic modeling and microwave scattering modeling of ash particles are used to demonstrate how the observed radar pattern could develop. Alternative hypotheses for the radar observations are explored as well. This work provides a demonstration of the ability of the CSU-CHILL radar to infer basic information on size, shape, and orientation of smoke and ash particles.