Cell Initiation, Interaction, and Morphology in the 1996 Illinois Tornado Outbreak

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Cell Initiation, Interaction, and Morphology in
the 1996 Illinois Tornado Outbreak
Brian F. Jewett and Robert B. Wilhelmson U.I.
Atmospheric Sciences NCSA
Bruce D. Lee University of Northern Colorado
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April 19, 1996 Damage

• Most Illinois tornadoes in a single day (36)
• Average IL tornadoes in a year 27
• Tornadoes hit Jacksonville, Bloomington,
  Decatur, Champaign-Urbana, Ogden
• 1 fatality in IL (I-74 near Ogden)

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April 19, 1996 Motivation

• Many tornadoes - but
• Few(er) tornadic storms
• Large number of initial storms fewer mature
  storms later

• Most tornadoes short-lived
• Some tornadoes up to low F-3 intensity (C-U
  Ogden)

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April 19, 1996 Motivation

• Frequent instances of cell splitting and merging
• Result was fewer, but tornadic, storms moving
  across IL
• Tornadogenesis found to often accompany later
  mergers

Cell evolution in western Illinois
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Methodology MM5

• Cyclone evolution MCS simulated using MM5
• Up to 5 domains (1 km grid)
• Start simulation at 00z run 27h

Inner 4 model domains

• Mesoscale evolution reasonable
• MM5 results also exhibit cell splitting and
  merging
• Split/merge details strongly dependent on
  resolution

1-km rainwater at 2236, looking N
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Convective Initiation

• Model storms formed correct time (aftn)
  location (e. Missouri)
• Storms formed on, moved off surface
  dryline
• Convection appeared to remain near leading
  edge of 600 mb qe gradient
• CFA?

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Convective Initiation

• Cross section at mature time shows apparent
  cold front aloft
• Earlier fields less clear qe gradient due
  almost entirely to dry air
• Convection fires in model as leading qe
  gradient moves over surface drytrough
• Coincidence/causality?

qe (red contours) rainwater (green outline,
shading)
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Idealized Studies

• Idealized 3-D simulations were made with WRF to
  investigate cell interaction without
• pre-existing boundaries
• changes in instability/shear across the domain
• changes in instability/shear over time

We explored the development mature structure of
a pair of storm cells, varying only their
original position. Questions what types of
interaction are favorable? How sensitive is storm
evolution to cell orientation?
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Methodology WRF

• Soundings extracted from MM5 simulation at one
  of three locations on dryline.
• Location / time chosen where convection
  formed in the MM5 simulation.
• WRF runs
• Sounding A
• 1 km grid spacing
• 2.5 hour simulations
• horizontally uniform

hodograph from location A
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Methodology WRF

• 230 WRF runs were made, two control
  (single-cell)
• Each corresponded to a particular
  arrangement of a pair of initial storm cells
• In figure at left
• Each square 1 simulation
• 1st storm in the middle
• 2nd at one of blue squares
• Center cell stronger

Matrix of WRF simulations
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Rotational properties
Data was saved each minute, for 2.5 hours.
Statistics gathered

• Max 1 km, overall updraft
• Peak surface wind speed

• Peak surface vorticity
• Max rotation duration

Magnitude
Duration
Most cases of interaction were destructive
cells stronger w/o neighbor
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Rotational properties
Vorticity properties (a single number) for each
run were plotted in the same coordinates as the
initial run matrix, showing the relationship
between surface-based rotation initial cell
layout.
Magnitude
Duration
Run matrix
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Rotational Magnitude
Of interest was the peak surface vorticity when
the secondary cell formed to the southwest of the
stronger, center storm. The opposite was also
true - weaker rotation resulted when the 2nd cell
formed to the northeast. A nearly N-S
orientation led to weaker rotation.
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Rotational Duration
Most surprising here was the very sharp gradient
at the bottom of the figure. Small changes in
the initial cell placement led to large
differences in duration, between storms with
short- vs. long-lived rotation. Duration here
was defined as the largest time period over which
a particular surface vorticity maximum exceeded
0.015 s-1.
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Rotation Sensitivity
Two cases are now shown, in which the secondary
cell was southwest (left) or south-southwest
(right) of the center storm. The two cases
differ only in the 5-km difference in east-west
placement of the second cell.
30 min
2-km rainwater (shading) and surface rotation
(contoured)
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Rotation Sensitivity
By 2.5 hours, the two simulations had diverged
significantly. The SW case had strong surface
rotation - in the hook. The SSW storm at times
had stronger - but less steady - surface rotation.
2.5 hours
SW
SSW
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Rotation Sensitivity
Surface time series of peak vorticity from the
two cases shows the difference in steadiness.
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Summary Conclusions

• The April, 1996 Midwest tornado outbreak has been
  modeled to study convective initiation and
  development.
• Storm initiation occurred when dry air aloft,
  which represented a modest qe gradient, became
  collocated with (above) the surface drytrough in
  the model fields.
• Cell splitting and merging occurred in the MM5
  model simulations, and was highly sensitive to
  resolution.
• Splitting and merging is being studied with
  idealized WRF simulations of a pair of storm
  cells. The evolution and mature rotational
  characteristics were highly sensitive to small
  changes in initial cell orientation.

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Ongoing Future Work

• Analysis of the MM5 simulation will explore the
  role of the CFA in convective initiation and
  propagation.
• WRF simulations will take advantage of more
  sophisticated model physics. In addition, future
  model work will assess cell interaction within
  different shear and buoyancy profiles.
• Delayed cell initiation, not yet explored in our
  work, will allow interaction of mature storms
  with new, non-rotating ones - another mode of
  interaction seen in the outbreak.
• The computational demands of this work are being
  used as a testbed for NCSA Portal development.

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Thank you.