Observations of supercooled water and secondary ice generation by a vertically pointing X-band Doppler radar I. Zawadzki * , F. Fabry, W. Szyrmer J.S. Marshall Radar Observatory, McGill University, and Cooperative Center for Research in Mesometeorology (CCRM), Canada Accepted 27 July 2001 Abstract When a vertically pointing Doppler radar (VPDR) observes two groups of hydrometeors having distinct fall velocities, the observed Doppler spectra are bimodal. This bimodality has been repeatedly observed in snow. At altitudes higher than the level at which the bimodality appears, the reflectivity of the snow rapidly increases at the same time as its fall velocity reaches values beyond those expected from the aggregation of snow crystals. This is indicative of riming by supercooled cloud droplets. We show here that the spectral bimodality observed in the snow after indication of riming may be associated with secondary ice generation or with supercooled drizzle. Higher snow content favors the formation of secondary ice, while in situations of low snow content, the presence of the supercooled cloud may lead to the formation of drizzle. In both cases, the bimodality indicates the significant content of the supercooled cloud. This type of observation can be used operationally to detect the supercooled water coexisting with the snow. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Cloud physics; Radar meteorology; Microphysics; Freezing drizzle; Secondary ice crystals; Supercooled cloud water 1. Introduction Rimed snow is a common occurrence in winter storms when the prevailing temperature is relatively mild. Thus, the conditions for coexistence between the snow and the supercooled liquid cloud are often fulfilled in stratiform winter precipitation. Soundings 0169-8095/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII:S0169-8095(01)00124-7 * Corresponding author. Department of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street West, Montreal, QC, Canada H3A 2K6. E-mail address: isztar@radar.mcgill.ca (I. Zawadzki). www.elsevier.com/locate/atmos Atmospheric Research 59 – 60 (2001) 343 – 359