1 / 35

McGill University

By Eyad Atallah and John Gyakum. The impact of the St. Lawrence Valley on the Precipitation Distributions of Hurricanes Katrina and Rita (2005). McGill University. Motivation. The St. Lawrence Valley strongly modulates the near surface winds.

loc
Download Presentation

McGill University

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. By Eyad Atallah and John Gyakum The impact of the St. Lawrence Valley on the Precipitation Distributions of Hurricanes Katrina and Rita (2005) McGill University

  2. 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.

  3. 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.

  4. 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

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

  6. Anecdotally, the answer is yes Precipitation map from Hurricane Ike (2008). Precipitation appears more dependent on Valley location than actual cyclone track

  7. So what about Katrina and Rita Rita Katrina

  8. Data • METAR surface observations • Balloon soundings • North American Regional Reanalysis (NARR)

  9. Data Shockingly, the NARR seems to actually capture this process Composite structures for NE wind events at YUL, n=20 and n=7 respectively

  10. Katrina

  11. Surface Observations Aug 31, 03Z

  12. Surface Observations Aug 31, 09Z

  13. Surface Observations Aug 31, 15Z

  14. Surface Observations Aug 31, 18Z

  15. Surface Observations Sep 01, 00Z

  16. YUL Meteogram Aug, 31

  17. MSLP and Surface Frontogenesis Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1 12Z/30

  18. MSLP and Surface Frontogenesis Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1 18Z/30

  19. MSLP and Surface Frontogenesis Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1 00Z/31

  20. MSLP and Surface Frontogenesis Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1 06Z/31

  21. MSLP and Surface Frontogenesis Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1 12Z/31

  22. MSLP and Surface Frontogenesis Fronto-genesis based on 30 m wind and potential temperature in .1K 100 km-1 3 h-1 18Z/31

  23. So how can such a shallow process impact precipitation? The key is stability

  24. Soundings from Maniwaki 00 Z 31 Aug 12 Z 31 Aug

  25. Cross Sections-Full Frontogenesis Blue lines represent Omega with dashed lines for ascent Solid black lines for theta-e Frontogenesis shaded 06Z / 31 AUG

  26. 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

  27. Cross Sections-Geo Frontogenesis The geostrophic frontogenesis is initially weak and not appropriately situated relative to the ascent 12Z / 31 AUG

  28. Cross sections-Full Frontogenesis The ascent eventually becomes troposphere deep by 15Z. 15Z / 31 AUG

  29. Rita

  30. YQB Meteogram Sep, 26

  31. Cross Sections-Full Frontogenesis Blue lines represent Omega with dashed lines for ascent Solid black lines for theta-e Frontogenesis shaded 06Z / 26 Sep

  32. Cross Sections-Full Frontogenesis Ascent is again triggered, however, omega profiles suggest more slant-wise ascent 12Z / 26 AUG

  33. Cross Sections-Full Frontogenesis The frontogenesis maximizes at about 15Z, along with the greatest ascent 15Z / 26 AUG

  34. Cross Sections-Geo Frontogenesis The geostrophic frontogenesis is almost completely absent. Furthermore, unlike Katrina, deep veering is almost non-existent. 15Z / 26 AUG

  35. 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.

More Related