bow echoes: a review of conceptual models with special thanks to: george bryan, nolan atkins,stan...
TRANSCRIPT
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Bow Echoes:Bow Echoes: A Review of A Review of Conceptual ModelsConceptual Models
WITH SPECIAL THANKS TO:
GEORGE BRYAN, NOLAN ATKINS,STAN TRIER
Matthew DuxNWS-Pleasant Hill/Kansas City
March 16, 2007
Originally presented by: MORRIS WEISMAN (NCAR) @ The Midwest Bow Echo Workshop in
Louisville, KY Feb 28-March 1, 2007
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OUTLINE:OUTLINE:
Research Review: (modeling perspective)
Foundations of Bow Echoes
Are Nocturnal Bow Echoes Different?
Difference between Non-Tornadic and Tornadic Mesovortices
What is the role of deep-layer versus shallow shear?
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Fujita 1978Fujita 1978
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Asymmetric Squall Line
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CAPE 2400 j/kg
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System 2D-EvolutionSystem 2D-Evolution
C/∆u << 1 C/∆u ~ 1
System structure/evolution depends on the strength of the cold pool ( C ) versus ambient shear (dU)
C/∆u > 1
“Optimal” condition for cold
pool lifting
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Descending Rear Inflow Jet
Elevated Rear Inflow Jet
Generally,weak shear
Strong shear
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Role of Line-End VorticesRole of Line-End Vortices
Focuses and Intensifies
Rear-Inflow Jet
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Bow Echo Shear Environments: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?
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What is the role of lower- versus upper-level shear? (Bryan… 2006 SLS)
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Cold Pool Strength ( C ) Average Surface Wind (m/s)
Interpretation: upper-level shear isgenerally detrimental to the productionof intense cold pools in thesesimulations
Interpretation:• Surface winds are sensitive to lower levelshear, especially for weak ΔU1.
• Upper-level shear neither helps or hurts average surface U (owing to the detrimentaleffects on cold pool intensity).
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Area of Severe Winds On average increasing low level On average increasing low level shear (LLS) is 2-10 times more shear (LLS) is 2-10 times more effective than increasing upper effective than increasing upper level shearlevel shear
Forecasters are urged to look for Forecasters are urged to look for areas where there is a local areas where there is a local enhancement of LLS especially enhancement of LLS especially where the near surface winds are where the near surface winds are parallel with the LLSparallel with the LLS
Upper-level shear generally inhibits the developmentof strong rear inflow jets
General Conclusions
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Nocturnal Bow Echoes:Nocturnal Bow Echoes:
Are they different?
What is the source of the cold pool?
What is the source of the strong surface winds?
What is the source of instability that fuels these systems?
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Non damaging bow echo at time of aircraft observation
From Jorgensen et al. 05
Low level stable layer – sounding launched 95 km SSW and 1.6 hrs prior to radar observations
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Bryan and Weisman, 23rd SLS, 2006
20 m/s shear over 2.5 km
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shaded: surface reflectivity, t = 3 hwhite contour: surface < 1 K
black contour: surface winds > 30 m s-1
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shaded: surface reflectivity, t = 5 hwhite contour: surface < 1 K
black contour: surface winds > 30 m s-1
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Line-averaged cross section, t = 3 hshaded: reflectivity (dBZ)
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Line-averaged cross section, t = 3 hshaded: e (K)
white contour: buoyancy < -0.05 m s-2
system-relative flow vectors
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Instantaneous cross section, t = 5 hshaded: reflectivity (dBZ)
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Instantaneous cross section, t = 5 hshaded: e (K)
white contour: buoyancy < -0.05 m s-2
system-relative flow vectors
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Nocturnal BE SummaryNocturnal 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
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Damaging Versus Non-Damaging Damaging Versus Non-Damaging MesovorticesMesovortices
Originally Presented by:
Nolan T. AtkinsNolan T. AtkinsLyndon State College, Lyndonville, VTLyndon State College, Lyndonville, VT
@The Midwest Bow Echo Workshop in Louisville, KY@The Midwest Bow Echo Workshop in Louisville, KY
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Ideas:Ideas:
A number of mesovortices can be present within A number of mesovortices can be present within a bow echo at any given time. a bow echo at any given time.
Can we distinguish between tornadic, damaging Can we distinguish between tornadic, damaging (straight line), and non-damaging mesovortices?(straight line), and non-damaging mesovortices?
The most intense damage produced by a bow The most intense damage produced by a bow echo may be associated with damaging echo may be associated with damaging mesovortices (also shown by Wheatley et al. mesovortices (also shown by Wheatley et al. 2006)2006)
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from Atkins et al. (2004)from Atkins et al. (2004)
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Tornadic Tornadic mesovorticesmesovortices
tend to be tend to be stronger at stronger at
levels below levels below 2.5 km AGL2.5 km AGL
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•Tornadic mesovortices are longer livedTornadic mesovortices are longer lived•Many vortices can be present at a given timeMany vortices can be present at a given time
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Rapid vortex strengthening Rapid vortex strengthening just prior to and during just prior to and during
tornadogenesistornadogenesis
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Tornadic Mesovortices: Preliminary ConclusionsTornadic Mesovortices: Preliminary Conclusions
Stronger (below 3 km) and longer-lived than non-tornadic Stronger (below 3 km) and longer-lived than non-tornadic counterpartscounterparts
Deepen and strengthen (primarily low levels) prior to Deepen and strengthen (primarily low levels) prior to tornadogenesistornadogenesis
Vortex genesis occurs coincident and after genesis and Vortex genesis occurs coincident and after genesis and descent of RIJ – and form along gust front where RIJ has descent of RIJ – and form along gust front where RIJ has enhanced convergence enhanced convergence
12 minutes from vortex genesis to tornadogenesis (need 12 minutes from vortex genesis to tornadogenesis (need more cases)more cases)
Most intense damage produced by bow echoes may be Most intense damage produced by bow echoes may be associated with damaging mesovorticesassociated with damaging mesovortices
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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