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Bow Echoes: A Review of Conceptual Models

Bow Echoes: A Review of Conceptual Models. Matthew Dux NWS-Pleasant Hill/Kansas City March 16, 2007. WITH SPECIAL THANKS TO: GEORGE BRYAN, NOLAN ATKINS,STAN TRIER. Originally presented by: MORRIS WEISMAN (NCAR) @ The Midwest Bow Echo Workshop in Louisville, KY Feb 28-March 1, 2007.

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Bow Echoes: A Review of Conceptual Models

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  1. Bow Echoes: A Review of Conceptual Models Matthew Dux NWS-Pleasant Hill/Kansas City March 16, 2007 WITH SPECIAL THANKS TO: GEORGE BRYAN, NOLAN ATKINS,STAN TRIER Originally presented by: MORRIS WEISMAN (NCAR) @ TheMidwest Bow Echo Workshop in Louisville, KY Feb 28-March 1, 2007

  2. OUTLINE: Research Review: (modeling perspective) • Foundations of Bow Echoes • What is the role of deep-layer versus shallow shear? • Are Nocturnal Bow Echoes Different? • Difference between Non-Tornadic and Tornadic Mesovortices

  3. Fujita 1978

  4. Asymmetric Squall Line

  5. CAPE 2400 j/kg

  6. System 2D-Evolution System structure/evolution depends on the strength of the cold pool ( C ) versus ambient shear (dU) C/∆u << 1 C/∆u ~ 1 C/∆u > 1 “Optimal”condition for cold pool lifting

  7. Descending Rear Inflow Jet Generally,weak shear Elevated Rear Inflow Jet Strong shear

  8. Role of Line-End Vortices Focuses and Intensifies Rear-Inflow Jet

  9. Bow Echo Shear Environments: Idealized Modeling Perspective: “Organized” bow echoes require 15 - 20 m/s of shear in the lowest 2 - 5 km AGL Climatological Studies: Severe wind-producing systems (derechoes) require only 10 ms-1 of low-level shear, but exhibit deeper-layer shear as well So, is there a mismatch between the idealized simulations and the observations?

  10. What is the role of lower- versus upper-level shear? (Bryan… 2006 SLS)

  11. Cold Pool Strength ( C ) Average Surface Wind (m/s) Interpretation: upper-level shear is generally detrimental to the production of intense cold pools in these simulations Interpretation: • Surface winds are sensitive to lower level shear, especially for weak ΔU1. • Upper-level shear neither helps or hurts average surface U (owing to the detrimental effects on cold pool intensity).

  12. General Conclusions Area of Severe Winds • On average increasing low level shear (LLS) is 2-10 times more effective than increasing upper level shear • Forecasters are urged to look for areas where there is a local enhancement of LLS especially where the near surface winds are parallel with the LLS • Upper-level shear generally inhibits the development • of strong rear inflow jets

  13. Nocturnal Bow Echoes: Are they different? What is the source of instability that fuels these systems? What is the source of the cold pool? What is the source of the strong surface winds?

  14. From Jorgensen et al. 05 Non damaging bow echo at time of aircraft observation Low level stable layer – sounding launched 95 km SSW and 1.6 hrs prior to radar observations

  15. Bryan and Weisman, 23rd SLS, 2006 20 m/s shear over 2.5 km

  16. shaded: surface reflectivity, t = 3 h white contour: surface  < 1 K black contour: surface winds > 30 m s-1

  17. shaded: surface reflectivity, t = 5 h white contour: surface  < 1 K black contour: surface winds > 30 m s-1

  18. Line-averaged cross section, t = 3 h shaded: reflectivity (dBZ)

  19. Line-averaged cross section, t = 3 h shaded: e (K) white contour: buoyancy < -0.05 m s-2 system-relative flow vectors

  20. Instantaneous cross section, t = 5 h shaded: reflectivity (dBZ)

  21. Instantaneous cross section, t = 5 h shaded: e (K) white contour: buoyancy < -0.05 m s-2 system-relative flow vectors

  22. Nocturnal BE Summary • Convective system initially persists despite stable air at surface • Feeds off of air above the stable layer • Cold pool remains situated above stable layer • System bows and becomes severe when cold air reaches surface • However it takes a much longer time for this cold pool to fight through the stable layer • Mechanism for severe winds is the same as that shown in previous studies • Cold pool generation, RIJ, Line-end Vortices

  23. Damaging Versus Non-Damaging Mesovortices Originally Presented by: Nolan T. Atkins Lyndon State College, Lyndonville, VT @The Midwest Bow Echo Workshop in Louisville, KY

  24. Ideas: • A number of mesovortices can be present within a bow echo at any given time. • Can we distinguish between tornadic, damaging (straight line), and non-damaging mesovortices? • The most intense damage produced by a bow echo may be associated with damaging mesovortices (also shown by Wheatley et al. 2006)

  25. from Atkins et al. (2004)

  26. Tornadic mesovortices tend to be stronger at levels below  2.5 km AGL

  27. Tornadic mesovortices are longer lived • Many vortices can be present at a given time

  28. Rapid vortex strengthening just prior to and during tornadogenesis

  29. Tornadic Mesovortices: Preliminary Conclusions • Stronger (below 3 km) and longer-lived than non-tornadic counterparts • Deepen and strengthen (primarily low levels) prior to tornadogenesis • Vortex genesis occurs coincident and after genesis and descent of RIJ – and form along gust front where RIJ has enhanced convergence • 12 minutes from vortex genesis to tornadogenesis (need more cases) • Most intense damage produced by bow echoes may be associated with damaging mesovortices

  30. Thank you for your time and attention! • NOTE:Bow Echo Workshop DVD will be left in the operations area for review - Some of these presentations will be uploaded to our training/research page on the intranet

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