Atmospheric Circulation Structures Associated with Freezing Rain in Quebec City, QC, and the...

Atmospheric Circulation Structures Associated with Freezing Rain in Quebec City, QC, and the

St-Lawrence River Valley

Sophie Splawinski, Hon. BSc. Atmospheric and Oceanic SciencesMcGill University, Montreal, Quebec, Canada

Contact: sophie.splawinski@mail.mcgill.ca

Co-Authors: Prof. John R. Gyakum, Dr. Eyad H. Atallah

Introduction

• An analysis of freezing rain (FZRA) over a 30-year period in Quebec City, QC.

• Severe events: partitioned into categories and shown to have distinct synoptic patterns.

• Composites • Result: unique thermodynamic and dynamic

signatures associated with each category.

Motivation• Freezing rain (FZRA)

effects on the general population

• Past Studies conducted in Eastern Canada: none associated with synoptic weather patterns

• Location of the St-Lawrence River Valley (SLRV) and effects of the valley

• Pressure-driven channeling• countercurrents

FZRA Occurrences over an 11-yr period (1979-1990)

Courtesy of PSU Dept. of Meteorology

Approach and Data• A 30-year period (1979-2008) is analyzed• YQB hourly surface obs

Individual Events

• 218 cases

• Duration: time ≥ 1hr

• A new event would be considered after 6hrs+ of consecutive non-FZRA reports.

Severe Events

• 48 cases

• Duration: time ≥ 6hr

• A new event would be considered after 6hrs+ of consecutive non-FZRA reports.

Data and Tools

Data:• Hourly surface observations at YQB• NARR

Tools:• GEMPAK• ARL Hysplit trajectory model

Partitioning Technique• Why were air parcel trajectories chosen as a method for categorization?

– Dynamic versus Instantaneous Approach

– Level: 3000m, nominal level associated with vertical motion• Also gave the best results

Partitioning Technique: The CategoriesLow Level Category Examplen = 14 Mid Level Category Example

n = 19

Partitioning Technique: The CategoriesUpper Level Category Example

n = 5

East Coast Category Example n = 5

Partitioning Technique: The CategoriesCentral Plains Category: Example

n = 5

Recap of all categories:

• 3000m height chosen.

• Based on specific layers at which air parcels begin their trajectories (t=-84h)

• 2º-by-2º matrix grid with 24 grid points and hence 24 trajectories

Composites

• 5 composite plots of the following:– 500 hPa height and temperature– MSLP, winds and 1000-500 hPa thickness– 850 hPa winds – Precipitable water– 850 hPa temperature

Composites: 500hPa AnalysisLow Level (n = 14) Mid Level (n = 19) Upper Level (n = 5)

East Coast (n = 5) Central Plains (n = 5)

YQB

Composites: MSLP, 1000-500hPa thicknessLow Level (n = 14) Mid Level (n = 19) Upper Level (n = 5)

East Coast (n = 5) Central Plains (n = 5)

YQB

Composites: 850hPa windsLow Level (n = 14) Mid Level (n = 19) Upper Level (n = 5)

East Coast (n = 5) Central Plains (n = 5)

YQB

Composites: Precipitable Water (and inset of 850hPa winds)Low Level (n = 14) Mid Level (n = 19) Upper Level (n = 5)

East Coast (n = 5) Central Plains (n = 5)

YQB

Composites: 850hPa Temperature (and inset of 850hPa winds)Low Level (n = 14) Mid Level (n = 19) Upper Level (n = 5)

East Coast (n = 5) Central Plains (n = 5)

YQB

Category Duration Comparisons

Percentage Thresholds for Duration of Freezing Rain Events per Category in Quebec City, QC (1979-2008)

50

14

68

16

40

0

80 8080

40

0

10

20

30

40

50

60

70

80

90

100

Percentage over 8hrs Percentage over 12hrs

Percentage

•Threshold determination (Cheng et al. (2004))

• “Processes occurring on the cold side of moving and stationary warm fronts and deep within East Coast air masses were most efficient at producing freezing precipitation per unit area.”. Rauber et al. (2001)

Category (hrs:) max min median mean % ≥ 8hr % ≥ 12hrs

Low Level 26 6 7.5 9 50 14

Mid Level 15 6 8 9 58 16

Upper Level 10 7 7 8 40 0

East Coast 23 7 13 15 80 80

Central Plains 15 7 10 11 80 40

Conclusion• 48 severe events

– Partitioned into 5 categories based on a dynamic, time-integrated approach using air parcel trajectories.

• Lower tropospheric dynamics:– Ascent and deformation zones– Intensity & duration: connected to spatial

distribution of atmospheric patterns– East Coast category associated with events of

longest duration – Better insight for meteorologists

Future Work• Continuing research in FZRA: 850hPa

analysis of LLJ• Connection between YUL and YQB

– Longer duration of FZRA in correlated events.

– Maintenance of cold air and the implications of wind channeling and the SLR and Champlain Valleys

Purple: Correlated Events

Green: Non-correlated Events

Blue: Severe Events (YUL)

Red: Non-Severe Events (YUL)

Acknowledgements

• NSERC

• Ouranos

• NCDC : for access to the NARR and NCEP Global Reanalysis

• NOAA ARL (Hysplit)

• Environment Canada