THE IMPACT OF THE ST. LAWRENCE VALLEY ON THE

PRECIPITATION DISTRIBUTIONS OF

HURRICANES KATRINA AND RITA (2005)

McGill University

By Eyad Atallah and John Gyakum

Motivation The St. Lawrence

Valley strongly modulates the near surface winds.

The results are a mostly bi-modal wind distribution, with winds either from the west-southwest or from the Northeast.

Motivation This wind channelling

(see Carrera et al. 2009) can have a significant impact on the sensible weather.

Historically, this has been thought to mostly be important for precipitation type.

Motivation However, the wind channelling can also serve as a

focus for precipitation, through enhanced frontogenesis, when cyclones approach the Valley from the southwest.

L

Since this is a relatively shallow phenomenon, can it really significantly impact precipitation distribution?

Question

Anecdotally, the answer is yes

Precipitation map from Hurricane Ike (2008).

Precipitation appears more dependent on Valley location than actual cyclone track

So what about Katrina and Rita

Katrina

Rita

Data METAR surface observations Balloon soundings North American Regional Reanalysis

(NARR)

DataShockingly, the NARR seems to actually capture this process

Composite structures for NE wind events at YUL, n=20 and n=7

respectively

Katrina

Surface Observations Aug 31, 03Z

Surface Observations Aug 31, 09Z

Surface Observations Aug 31, 15Z

Surface Observations Aug 31, 18Z

Surface Observations Sep 01, 00Z

YUL Meteogram Aug, 31

MSLP and Surface Frontogenesis

Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1

12Z/30

MSLP and Surface Frontogenesis

Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1

18Z/30

MSLP and Surface Frontogenesis

Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1

00Z/31

MSLP and Surface Frontogenesis

Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1

06Z/31

MSLP and Surface Frontogenesis

Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1

12Z/31

MSLP and Surface Frontogenesis

Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1

18Z/31

The key is stability

So how can such a shallow process impact precipitation?

Soundings from Maniwaki

00 Z 31 Aug 12 Z 31 Aug

Cross Sections-Full Frontogenesis

Blue lines represent Omega with dashed lines for ascent

Solid black lines for theta-e

Frontogenesis shaded

06Z / 31 AUG

Cross Sections-Full Frontogenesis

As the potentially unstable air approaches the St. Lawrence Valley, ascent is triggered in the lowest 500 hPa.

12Z / 31 AUG

Cross Sections-Geo Frontogenesis

The geostrophic frontogenesis is initially weak and not appropriately situated relative to the ascent

12Z / 31 AUG

Cross sections-Full Frontogenesis

The ascent eventually becomes troposphere deep by 15Z.

15Z / 31 AUG

Rita

YQB Meteogram Sep, 26

Cross Sections-Full Frontogenesis

Blue lines represent Omega with dashed lines for ascent

Solid black lines for theta-e

Frontogenesis shaded

06Z / 26 Sep

Cross Sections-Full Frontogenesis

Ascent is again triggered, however, omega profiles suggest more slant-wise ascent

12Z / 26 AUG

Cross Sections-Full Frontogenesis

The frontogenesis maximizes at about 15Z, along with the greatest ascent

15Z / 26 AUG

Cross Sections-Geo Frontogenesis

The geostrophic frontogenesis is almost completely absent. Furthermore, unlike Katrina, deep veering is almost non-existent.

15Z / 26 AUG

Conclusions The St. Lawrence Valley can impact not only

precipitation type but location and intensity. While forcing for ascent is shallow, stability

profiles result in deep ascent. An argument can be made that this this

process is most efficient in the warm season, and especially related to extratropical transition because of the inherently moist/unstable air masses involved.