Cold rain event analysis using 2-D video disdrometer, C-band polarimetric radar, X-band vertically pointing Doppler radar and POSS (Formerly Paper P10.5)

It has been shown in the recent past that the 2-D video disdrometer (2DVD) is capable of determining shapes and fall velocities of hydrometeors falling through its 10 cm by 10 cm sensor area. It has also been used in an intercomparison study with a C-band polarimetric radar and a UHF wind profiler during a seasonal event in a sub-tropical climate. Attenuation correction schemes were developed for the C-band polarimetric radar based on the data analysis of simultaneous 2DVD measurements.

In this study, we examine a mid-latitude cold rain event using a low-profile 2DVD, a C-band polarimetric radar and a vertically-pointing X-band Doppler radar as well as a few other co-sited instruments. The mid-latitude region in this case is a well-instrumented site belonging to Environment Canada, some 70 km north of Toronto, Ontario, where the 2DVD was installed in November 2006. On 2 consecutive days soon after the installation, a cold rain event passed through the 2DVD site. Co-located with the 2DVD were a vertically-pointing X-band Doppler radar and a precipitation occurrence sensor system (POSS). The C-band operational weather radar situated in King City ON, 30 km away from the 2DVD site, was also used to observe and analyze this event.

On the first day, the 2DVD data indicated that it was almost all liquid water hydrometeors (i.e. rain) at ground level. Furthermore, the C-band radar RHIs showed this event to be stratiform rain with a clear bright-band in the 1 -2 km region, indicated most clearly by the co-polar correlation coefficient. The fall velocity spectra from the 2DVD measurements show excellent agreement with the Gunn-Kinzer variation. The C-band radar reflectivity (Zh), differential reflectivity Zdr and the specific differential propagation phase (Kdp) at C band were computed using the 2DVD data and compared with the polarimetric radar data over the 2DVD site. Also derived were the rainfall rates from the C-band radar, POSS and the 2DVD. Good agreement was obtained in general.

The second day was more of a mixed phase hydrometeor event. A definite transition from a mixture of rain and melting snow to all snow was observed during its latter stage, confirmed by both the 2DVD and the POSS. The X-band radar also showed the bright-band reaching ground level around the time indicated by the 2DVD. With such high-quality ‘ground-truth', the C-band radar data Zh, Zdr, Kdp and the co-polar correlation coefficient over the 2DVD site were extracted to determine their characteristics in rain, snow and mixed phase hydrometeors. The classification scheme is further extended to the previous day which showed riming process taking place just above the melting region, evident from the vertically-pointing X-band Doppler radar measurements. Once again, Zh, Zdr, Kdp and the co-polar correlation coefficient characteristics from the C-band radar (RHI) data were derived. Together with the ground temperature measurements, they were used as input to a fuzzy-logic hydrometeor classification scheme (developed by Keenan et al for the Tropics) in order to test the validity. Results indicate that the limits of some input variables used in the scheme need to be adjusted to suit the cold (winter) rain events.