1. Planetary and Synoptic Analysis of Freezing Rain Events in Montreal, Quebec Gina M. Ressler, Eyad H. Atallah, and John R. Gyakum Department of Atmospheric and Oceanic Sciences, McGill University The 11 th Northeast Regional Operational Workshop, Nov. 4-5th, 2009 Sponsored by Ouranos

2. Motivation Median annual hours of freezing rain from 1979 to 1990 (Fig. 2, Cortinas et al. 2004) Freezing rain is a major environmental hazard Especially common along the St. Lawrence river valley Severe events can have a devastating effect on people, commerce, and property (1998 Ice Storm)

3. Motivation Most research has focused on: climatology (Stuart and Isaac 1999; Cortinas et al. 2004) case studies (Higuchi et al. 2000; Gyakum and Roebber 2001) statistical methods for prediction (Cheng et al. 2004) Fewer synoptic analyses (Cortinas 2000; Rauber et al. 2001) Surface analysis illustrating Rauber's Pattern C: Cyclone- Anticyclone (Fig 5c, Rauber et al. 2001)

4. Objectives Construct a complete list of events Characterize the relevant planetary and synoptic features of a Montreal freezing rain event Synoptic archetypes Event causation, duration, and severity Montreal, Dec. 2008

5. Data Environment Canada hourly surface observations at Montreal, Quebec (YUL) for the period 1979-2008 National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR)

6. Freezing Rain Events in Montreal, 1979-2008 163 synoptically independent freezing rain events Highest frequency in Dec, Jan, Feb Most are short lived Median = 3 hours Severe events are defined as having 6 or more hours of freezing rain 46 severe events

8. Composite Anomalies for Severe (6hr+) Events The sea level pressure composite is consistent with Rauber et al.'s (2001) Pattern C and Cheng et al.'s (2004) Type 2, both describing a surface Cyclone-Anticyclone couplet

9. Synoptic Partitioning 500hPa heights (m; contoured) and absolute vorticity (10 -5 s -1 ; shaded) at the freezing rain onset time Events are partitioned according to the location of the long- wave trough axis WestCentralEast

10. Absolute vorticity maxima for all severe events. Western (blue)Central (green)Eastern (red) n=10 n=17 n=20 Synoptic Partitioning

11. Western Cases (n=10): MSLP (solid) and 1000- 500hPa thickness (dotted) mean composites

12. Central Cases (n=17): MSLP (solid) and 1000- 500hPa thickness (dotted) mean composites

13. Eastern Cases (n=20): MSLP (solid) and 1000- 500hPa thickness (dotted) mean composites

14. Synoptic Analysis: Preliminary Results Western Cases Long upper-level wavelength Broad region of surface high pressure, and strong stationary cyclone near Greenland Duration is 50% longer than both Central and Eastern cases Central Cases Similar to Western cases; weaker anticyclone (Greenland cyclone) Eastern Cases Short upper-level wavelength, negatively tilted Strong surface cyclone; two separate anticyclones Deformation zone; more frontogenetical

15. Continued Research Anomaly plots for each synoptic group; statistical significance For each group, investigate: Temperature and precipitation evolution Frontogenesis Moisture sources and air mass origins Spatial extent (ie. along the St. Lawrence river valley) Case studies for each synoptic group