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

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)

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)

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

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 events1 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

0

5

10

15

20

25

30

35

Hours of Freezing Rain per Event

Number of Hours

Cou

nt

1980 1985 1990 1995 2000 2005 2010

0

10

20

30

40

50

60

70

80

0

2

4

6

8

10

12

Annual Amount of Freezing Rain per Winter Season in Montreal, QC, 1979-2008

EventsHours

Year

To

tal H

ou

rs o

f Fre

ezi

ng

Ra

in (

red

) / N

um

be

r o

f Eve

nts

(b

lue

)

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

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

West Central East

Absolute vorticity maxima for all severe events.

Western (blue) Central (green) Eastern (red) n=10 n=17 n=20

Synoptic Partitioning

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

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

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

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

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