A Case Study: The Omaha Derecho

June 27, 2008

By Steve Gronstal

Outline

• What caught my attention?• What is a Gravity Wave? • Gravity wave Lifespan. • CISK.• Synoptic Enhancements.• Evidence of the Gravity Wave. • Synopsis.• References.

Little Warning

A small scale storm.

Brutal Winds and Extreme Rainfall

The Aftermath:

The Push

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What is a Gravity Wave?

(A.K.A a Buoyancy Wave)

• A wave where Buoyancy acts to oscillate an air parcel

Wave Covariance & Quadrature

-5.0

-2.5

0

2.5

5.0

0 2 4 6 8 10

variation correspondingto bold dashed line

u, w and p

Horizontal distance (arbitrary units)

Ver

tical

dis

tanc

e (a

rbitr

ary

units

)

Why Wave Covariance?

• Without it, the Pressure Gradient Force would actually work against the wind vector, thereby dissipating the wave’s energy.

Why Wave Quadrature?

• Omega = 0 at the lowest temperature.• Thereafter, Omega < 0 == upwards motion. • This adiabatically warms the parcel.

Life Span of a Gravity Wave

• Typically they die out unless:

– A sufficiently thick duct traps energy from dissipating into the upper atmosphere.

– A continual supply of energy acts against dissipation.

An Atmospheric Duct

• Must have a statically stable layer

• Real Valued Brunt-Vasala Frequency (N)

*G d

Ndz

CISK

• Convective Instability of the Second Kind.

• A propagating line of convection that cyclically enhances more convection.

How CISK Propagates a Gravity Wave

• Gravity Wave creates a localized low with upward motion

• Condensation & Convection creates surface divergence.

• Surface Divergence enhances a new localized low.

Synoptic Enhancement

Synoptic Enhancement

Synoptic Enhancement

Evidence of a Gravity Wave

• Pressure Perturbation

• Wave Covariance

• Wave Quadrature

• Wave Ducting

The Pressure Change

1) A dip followed by an increase in Pressure

2) 0.2 in Hg = 5 mb!

Wave Covariance

• Wind changes and pressure changes happen simultaneously.

Wave Quadrature

• Temperature changes exhibit a 90º phase shift from Pressure changes.

The Duct

The duct aught to be about ¼ the vertical wavelength

Why Call it a Derecho?

• A gravity wave with extreme downdrafts

• Winds oscillate always pointing towards the local low pressure area.

Synopsis of Events

• South Dakota Thunderstorm induced a gravity wave.

• This gravity wave triggered more convection (CISK), which helped propagate it.

• The Synoptic Adjustments helped release even more energy into the wave.

• These events provided the energy needed to cause $10,000,000 dollars in damage!

Overall Effect

• 70-90 mph winds with gusts up to 115 mph. • 0.50–0.86 inches of rain within 15-25 min. • Hail ranged from peas to marbles to golf balls.

References & Thank You’s

• Thank you to my advisor Arlen Huggins for his support and direction throughout my time at UNR.

• Thank you to my instructor Mike Kaplan for his efforts and lessons imparting the fundamentals of Synoptic Meteorology.

• References: – Hendee, David, “Thunderstorm lands heavy blow; many still without power”, Omaha.com, published June 28, 2008.– Geller, Marvin A.; Liu, Hanli; Richter, Jadwiga H. “Gravity Waves in Weather, Climate, and Atmospheric Chemistry: Issues and Challenges

for the Community”, White paper from the June 2006 Gravity Wave Retreat, The Institute for Integrative and Multidisciplinary Earth Studies (TIIMES).

– McNoldy, Brian. “Gravity Waves: What's the Attraction?”, for Multi-community Environmental Storm Observatory, www.mcwar.org/articles/cafe/gw/gravwaves.html.

– Bauck, B.H., 1992: An Unusually Strong Gravity Wave over Western Washington. Wea. Forecasting, 7, 389–397.– Vaughan, Online paper, “Mathematical treatment of gravity waves.”– Bosart, L.F., and A. Seimon, 1988: A Case Study of an Unusually Intense Atmospheric Gravity Wave. Mon. Wea. Rev., 116, 1857–1886.– Fiorino, S.T., and J. Correia Jr, 2002: Analysis of a Mesoscale Gravity Wave Event Using Empirical Orthogonal Functions. Earth

Interactions, 6, 1–19.– Einaudi, F., W. Clark, J. Green, T. VanZandt, and D. Fua, 1987: Gravity Waves and Convection in Colorado during July 1983. J. Atmos.

Sci., 44, 1534–1553.– Raymond, D.J., 1976: Wave-CISK and Convective Mesosystems. J. Atmos. Sci., 33, 2392–2398.– Kaplan, Mike, 2005, “Mesoscale Gravity Waves”, MEA 444. http://www.dri.edu/People/Mike.Kaplan/mea444/lectures/lect03.ppt

• Images– 4-quad Jet http://www4.ncsu.edu/~nwsfo/storage/training/jets/straightjet.html – CISK http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hurr/grow/cisk.rxml