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The Impact of the Saint Lawrence Valley on Warm Season Precipitation Distribution, 1979-2008. Giselle C. Dookhie, John R. Gyakum, and Eyad H. Atallah Department of Atmospheric and Oceanic Sciences McGill University Montreal, QB. Purpose.
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The Impact of the Saint Lawrence Valley on Warm Season Precipitation Distribution, 1979-2008 Giselle C. Dookhie, John R. Gyakum, and Eyad H. Atallah Department of Atmospheric and Oceanic Sciences McGill University Montreal, QB NROW XII
Purpose • To identify synoptic-scale flow associated with extreme precipitation • To examine mesoscale modulations by existing orography NROW XII
Motivation Precipitation map from Hurricane Ike (2008). Precipitation appears more dependent on Valley location than actual cyclone track
Quantification of expected precipitation inspired by: -Fred Sanders • P : Precipitation Rate • g : gravity • ω: vertical ascent • (drs/dp)ma: Saturation mixing ratio along a moist adiabat • p:pressure NROW XII
Data • Environment Canada hourly surface observations and 24hrly accumulated precipitation amounts at Montreal/Pierre Elliott Trudeau International Airport, Quebec (YUL) for the period 1979-2008 • National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR) and Global Reanalysis • National Oceanic and Atmospheric Administration (NOAA) National Hurricane Center (NHC) Best Track Data NROW XII
Methodology: Created a heavy precipitation dataset 24hourly accumulated precipitation Calculated 90th Percentile Legend: Data-red Process- Green Result- yellow Threshold: 20mm NROW XII
Criteria for an event: Legend: Data-red Process- Green Result- yellow Step 1: 20mm in 1day or 20mm in 2 consecutive days Step 2: CHECK: Are consecutive days related? Select the date with the heaviest 24hrly accumulated precipitation Hourly surface Obs.
Seasonality • 221 precipitation events (blue) • Highest frequency of precipitation eventsin August, with the highest frequency of hurricane events in September • 11/12yrs that a tropical storm has an influence on a heavy precipitation event; hurricanes ranked within the top 2 events of each year NROW XII
221 Events Ranked heavy precipitation events within each year from largest-smallest Selected heaviest precipitation event for each year Selected all cases of tropical storm influences over 30yr period regardless of intensity 45 Events NROW XII
Composite Anomaly and Mean 500hPa Geopotential Height Source: the NOAA-ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/ NROW XII
Methodology: zonal flow Category: Example of partitioning technique and compositing NROW XII
Partitioning 45 cases based on 500mb flow pattern • Below are sample individual cases of the heaviest precipitation day within the zonal flow category • Within the zonal flow category, the heaviest precipitation day of each individual case are averaged over a 24hour period Those averages are used to create a composite plot, for example: Zonal Flow category containing 8 individual cases … … NROW XII
500hPa Height and Vorticity Zonal flow (n=8): Amplified flow(n=13): 500hPa composite heights (m; contoured) and absolute vorticity (10-5 s-1; shaded) averaged over the event day with the heaviest precipitation NROW XII
MSLP and 1000-500hPa thickness Zonal flow cases (8 individual cases): MSLP (solid) and 1000-500hPa thickness (dotted) Amplified flow cases (13 individual cases): MSLP (solid) and 1000-500hPa thickness (dotted) NROW XII
250hPa Wind Maxima Variability Zonal flow (n=8): Amplified flow (n=13): 250hPa Wind maxima (kts; contoured) representative of individual cases within each category NROW XII
Zonal Flow Category Amplified Flow Category NROW XII
Precipitable water: Weighted climatology subtracted from composite precipitable water for each case Zonal flow Category Amplified flow Category • Where: • PWTR = precipitable water (kgm-2) • w(p) = mixing ratio at pressure p • Averaged over 1000hPa-300hPa NROW XII
Frontogenesis (10-1K 100km-13h-1): 975-900hPa Total frontogenesis Ageostrophic component Geostrophic component Zonal Flow Category Amplified Flow Category NROW XII
Mesoscale: Possible Impacts of the saint Lawrence valley NROW XII
Conclusions • Structural differences: • Baroclinicity • Moisture supply O(1) from Gulf of Mexico and Atlantic, for Zonal flow and Amplified flow category respectively • Mesoscale Influences: • Frontogenesis along the axis of the Saint Lawrence Valley • Wind channeling (Northeasterly-Southwesterly) NROW XII
Continued Research • For each group: • Composite analysis: Moisture and Instability • Case studies NROW XII
References • Atallah, Eyad H. and John R. Gyakum. The Impact of the Saint Lawrence Valley on the Precipitation Distributions of Hurricanes Katrina and Rita. Nov. 2009. PowerPoint. • Carrera, M. L., J. R. Gyakum, and C. A. Lin, 2009: Observational study of wind channeling within the St. Lawrence River Valley. Journal of Applied Meteorology and Climatology, 48, 2341-2361. • Durnford, D. A., 2001-Thesis: An Analysis of Montreal’s Record-Breaking Heavy Rainfall Event of 8-9 November 1996, and a Comparison with its Best Analogue. • Environment Canada Climate Data Online (http://www.climate.weatheroffice.gc.ca/climateData/canada_e.html) • Fischer, A. P., 1998 –Thesis: A Synoptic Climatology of Montreal Precipitation. • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437–471. • Koch, S. E., M. DesJardins, and P. J. Kocin, 1983: An interactive Barnes objective map analysis scheme for use with satellite and conventional data. J. Climate Appl. Meteor., 22, 1487–1503. • Milrad, Shawn. “Re: EPI equation?” E-mail to Giselle Dookhie. 28 Oct. 2010. • National Hurricane Center (NHC) best-track archive dataset (http://www.nhc.noaa.gov) NROW XII
Additional Slide(s) NROW XII
Alberta-Clipper Type Category (n=4) 500mb Trough West of Montreal Category (n=7) 500mbTrough over Montreal Category (n=13) NROW XII