Atmospheric Circulation Structures Associated with Freezing Rain in Quebec City, QC, and the St-Lawrence River Valley Sophie Splawinski, Hon. BSc. Atmospheric and Oceanic Sciences McGill University, Montreal, Quebec, Canada Contact: 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 ( ) Courtesy of PSU Dept. of Meteorology

Approach and Data A 30-year period ( ) 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 Categories Low Level Category Example n = 14 Mid Level Category Example n = 19

Partitioning Technique: The Categories Upper Level Category Example n = 5 East Coast Category Example n = 5

Partitioning Technique: The Categories Central 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 hPa thickness –850 hPa winds –Precipitable water –850 hPa temperature

Composites: 500hPa Analysis Low Level (n = 14) Mid Level (n = 19) Upper Level (n = 5) East Coast (n = 5) Central Plains (n = 5) YQB

Composites: MSLP, hPa thickness Low Level (n = 14) Mid Level (n = 19) Upper Level (n = 5) East Coast (n = 5) Central Plains (n = 5) YQB

Composites: 850hPa winds Low 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 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 Mid Level Upper Level East Coast Central Plains

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