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

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 1978

Asymmetric Squall Line

CAPE 2400 j/kg

System 2D-Evolution C/∆u << 1C/∆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 Vortices Focuses and Intensifies Rear-Inflow Jet

Bow Echo Shear Environments: Idealized Modeling Perspective: “Organized” bow echoes require m/s of shear in the lowest 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 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).

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 General Conclusions

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, m/s shear over 2.5 km

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

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

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

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

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

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

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

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

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)

from Atkins et al. (2004)

Tornadic mesovortices tend to be stronger at levels below  2.5 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 just prior to and during tornadogenesis

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

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