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

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?

Fujita 1978Fujita 1978

Asymmetric Squall Line

CAPE 2400 j/kg

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

Descending Rear Inflow Jet

Elevated Rear Inflow Jet

Generally,weak shear

Strong shear

Role of Line-End VorticesRole of Line-End Vortices

Focuses and Intensifies

Rear-Inflow Jet

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?

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

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).

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

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?

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

Bryan and Weisman, 23rd SLS, 2006

20 m/s shear over 2.5 km

shaded: surface reflectivity, t = 3 hwhite contour: surface < 1 K

black contour: surface winds > 30 m s-1

shaded: surface reflectivity, t = 5 hwhite contour: surface < 1 K

black contour: surface winds > 30 m s-1

Line-averaged cross section, t = 3 hshaded: reflectivity (dBZ)

Line-averaged cross section, t = 3 hshaded: e (K)

white contour: buoyancy < -0.05 m s-2

system-relative flow vectors

Instantaneous cross section, t = 5 hshaded: reflectivity (dBZ)

Instantaneous cross section, t = 5 hshaded: e (K)

white contour: buoyancy < -0.05 m s-2

system-relative flow vectors

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

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

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)

from Atkins et al. (2004)from Atkins et al. (2004)

Tornadic Tornadic mesovorticesmesovortices

tend to be tend to be stronger at stronger at

levels below levels below 2.5 km AGL2.5 km AGL

•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

Rapid vortex strengthening Rapid vortex strengthening just prior to and during just prior to and during

tornadogenesistornadogenesis

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

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