Observations of Quasi-Symmetric Echo Patterns in Clear Air with the CSU–CHILL Polarimetric Radar.

On a few occasions during the summer and fall of 2002, and again in the fall of 2003, the Colorado State University (CSU)–University of Chicago–Illinois State Water Survey (CHILL) S-band polarimetric Doppler radar observed dumbbell-shaped radar echo patterns in precipitation-free air returns. Dumbbell shaped refers to two distinct and quasisymmetrical regions of echo surrounding the radar. These were horizontally widespread (thousands of square kilometers) layers, with the highest reflectivity factors (sometimes >20 dBZ) arranged approximately perpendicular to the direction of the mean wind. The echoes coincided with strongly positive differential reflectivity (ZDR) measurements (often >4 dB). Most interestingly, the echoes were elevated near the top of the boundary layer in the 2–3-km-AGL vertical range. Assuming a horizontally uniform layer of scatterers, the observations suggest that targets aloft are quasi prolate in shape and aligned horizontally along the direction of the mean wind. The echoes tended to occur on days when nocturnal inversions persisted into the following day, and solenoidal-like circulations (easterly upslope near the surface, and westerly flow aloft) existed. In some cases, the echoes exhibited diurnal behavior, with dumbbell-shaped echoes only occurring during the day and a more azimuthally uniform echo at night. On occasion, the echoes were coincident with the occurrence of widespread smoke from nearby forest fires. It is suggested that these echoes, which are rare for the CSU–CHILL coverage region, were caused by insects flying in a preferred direction, with the trigger for the migration being either the forest fires or oncoming winter. The local meteorological conditions likely affected the structure of these echoes.