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 UniversityMontreal, QB

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Purpose

1. To identify synoptic-scale flow associated with extreme precipitation

2. To examine mesoscale modulations by existing orography

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Precipitation map from Hurricane Ike (2008). Precipitation appears more dependent on Valley location than actual cyclone track

Motivation

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dpdp

dr

gma

s

1

P

-Fred Sanders

•P : Precipitation Rate•g : gravity•ω: vertical ascent•(drs/dp)ma: Saturation mixing ratio along a moist adiabat•p:pressure

Quantification of expected precipitation inspired by:

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

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Methodology: Created a heavy precipitation dataset

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Calculated 90th Percentile

Threshold: 20mm

24hourly accumulated precipitation

Legend: Data-redProcess- Green Result- yellow

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Criteria for an event:

Hourly surface Obs.

Select the date with the heaviest 24hrly accumulated precipitation

CHECK: Are consecutive days related?

20mm in 1day or20mm in 2 consecutive days

Legend: Data-redProcess- Green Result- yellow

Step 1:

Step 2:

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Seasonality 221 precipitation

events (blue) Highest frequency of

precipitation events in 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 year11/5/2010 NROW XII

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Selected all cases of tropical storm influences over 30yr period regardless of intensity

Selected heaviest precipitation event for each year

Ranked heavy precipitation events within each year from largest-smallest

221 Events

45 Events

Composite Anomaly and Mean 500hPa Geopotential Height

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Source: the NOAA-ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/

ZONAL FLOW CATEGORY: EXAMPLE OF PARTITIONING TECHNIQUE AND COMPOSITING

Methodology:

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…

…

•Partitioning 45 cases based on 500mb flow pattern•Below are sample individual cases of the heaviest precipitation day within the zonal flow category

Those averages are used to create a composite plot, for example: Zonal Flow category containing 8 individual cases

•Within the zonal flow category, the heaviest precipitation day of each individual case are averaged over a 24hour period

Zonal flow (n=8): Amplified flow(n=13):

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500hPa composite heights (m; contoured) and absolute vorticity (10-5 s-1; shaded) averaged over the event day with the heaviest precipitation

500hPa Height and Vorticity

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)

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250hPa Wind maxima (kts; contoured) representative of individual cases within each category

250hPa Wind Maxima Variability

Zonal flow (n=8): Amplified flow (n=13):

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Amplified Flow Category

Zonal Flow Category

Precipitable water: Weighted climatology subtracted from composite precipitable water for each

case

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2

1

1 P

Pwdp

gPWTR

Where:•PWTR = precipitable water (kgm-2)•w(p) = mixing ratio at pressure p•Averaged over 1000hPa-300hPa

Amplified flow CategoryZonal flow Category

Total frontogenesis

Frontogenesis (10-1K 100km-13h-1): 975-900hPaAgeostrophic component Geostrophic component

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Zonal Flow Category

Amplified Flow Category

Mesoscale: Possible Impacts of the saint Lawrence valley

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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)

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Continued Research

For each group:Composite analysis: Moisture

and InstabilityCase studies

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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)

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ADDITIONAL SLIDE(S)

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Alberta-Clipper Type Category (n=4)

500mb Trough West of Montreal Category (n=7)

500mbTrough over Montreal Category (n=13)