Planetary and Synoptic Analysis of Freezing Rain Events in ...
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 11th Northeast Regional Operational Workshop, Nov. 4-5th, 2009 Sponsored by Ouranos Motivation 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) Median annual hours of freezing rain from 1979 to 1990 (Fig. 2, Cortinas et al. 2004) 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: CycloneAnticyclone (Fig 5c, Rauber et al. 2001) 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 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) Freezing Rain Events in Montreal, 1979-2008 Hours of Freezing Rain per Event 35 163 synoptically independent freezing rain events 30 25 Most are short lived
Median = 3 hours Count 20 15 10 5 0 1 2
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Number of Hours Highest frequency in Dec, Jan, Feb
Severe events are defined as having 6 or more hours of freezing rain 46 severe events Annual Amount of Freezing Rain per Winter Season in Montreal, QC, 1979-2008 Total Hours of Freezing Rain (red) / Number of Events (blue) 80 12 70 10 60 8
50 40 6 30 4 20 2 10 0 1980 1985
1990 1995 Year 2000 2005 0 2010 Events Hours 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 Synoptic Partitioning
Events are partitioned according to the location of the longwave trough axis West Central East 500hPa heights (m; contoured) and absolute vorticity (10 -5 s-1; shaded) at the freezing rain onset time Synoptic Partitioning Western (blue) n=10 Central (green) n=17 Eastern (red) n=20
Absolute vorticity maxima for all severe events. Western Cases (n=10): MSLP (solid) and 1000500hPa thickness (dotted) mean composites Central Cases (n=17): MSLP (solid) and 1000500hPa thickness (dotted) mean composites Eastern Cases (n=20): MSLP (solid) and 1000500hPa thickness (dotted) mean composites Synoptic Analysis: Preliminary Results Western Cases Broad region of surface high pressure, and strong stationary cyclone near Greenland
Duration is 50% longer than both Central and Eastern cases Central Cases Long upper-level wavelength 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 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
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