Microphysical characteristics of winter storms using ground-based polarimetric radars and aircraft observations

Winter storms in south central Canada are characterized by deep cloud systems, widespread precipitation and a variety of precipitation types. The Alliance Icing Research Study (AIRS), whose primary objectives were concerned with the study of wintertime aircraft icing regions, provided a great opportunity to investigate the microphysical properties of these storms. The AIRS field project was conducted during the winter of 1999/2000 near Montreal, Canada.

The main observation platforms used in this study are 1) the McGill University S-band scanning Doppler radar using the slant-45 polarization scheme and located about 20 km west of Montreal, 2) the McMaster University scanning portable X-band Doppler radar using an alternate H-V polarization scheme and located at Mirabel airport about 30 km NNW of the S-band radar, and 3) the NASA Twin Otter and National Research Council of Canada Convair 580 cloud physics research aircraft. The aircraft focused their in-situ data collection in the vicinity of Mirabel. The S-band radar carried out 24-angle volume scans every 5 min throughout the project. The X-band radar focused data collection on the region in the vicinity of the aircraft in a series of stares, RHIs and sector volume scans. When the aircraft were not in the vicinity of Mirabel, data collection was concentrated to the southwest in the area of best dual Doppler coverage. Other instrumentation located at Mirabel included the McGill University vertically pointing X-band Doppler radar (VPR), an upper air station, and a suite of microwave radiometers.

Comparisons of the polarimetric data collected by the S-band and X-band radar systems were made under a wide variety of cloud conditions. The in-situ aircraft data were used as particle type verification. The resultant spectra also served as input to theoretical calculations to explain the discrepancies in polarization signatures of the two radars. The VPR, radiometers and upper air system provided additional supporting information. The results are a refinement in radar-based particle identification techniques for winter storm systems. Selected cases from the AIRS field project will be presented demonstrating the ability to identify regions of the storm where important microphysical processes such as coalescence, aggregation, riming, melting and secondary ice production are dominant.