Case Studies of Midwestern Thundersnow Events

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Case Studies of Midwestern Thundersnow Events
Christopher E. Halcomb
To satisfy the degree requirements for Master of
Science in Atmospheric Science
University of Missouri-Columbia November 13, 2001
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Introduction

• Thundersnow (TS) characteristics
• Typically occurs in meso-? bands
• Form of elevated convection
• Moist, stable boundary layer
  (Curran Pearson 1971)
• Low CAPE values (lt 200 J kg-1)
• Duration of TS observations are often short
• However, sustained TS ? heavy snow totals
• Thanksgiving Week 2000 Buffalo, NY
• 19-20 January 1995 Columbia, MO
• 25 February 1979 Cape Girardeau, MO

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Introduction (cont)

• In theory, byproduct of
• Upright Instability - Positive MUCAPE
  (Moore et al. 1998)
• Gravitational instability
• ? decreases with height
• Convective instability (CI)
• ?e decreases with height
• Conditional symmetric instability (CSI)
• In moist region (RH ? 80)
• ?e surfaces more upright than Mg surfaces (PSI)
• ?es surfaces more upright than Mg surfaces (CSI)
• Negative equivalent potential vorticity (EPV) in
  an otherwise stable environment

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Purpose, Objectives, and Statements of Thesis

• Purpose
• To determine when and where TS occurs most often
• To examine thermodynamic and dynamic
  characteristics of individual TS events
• Main objective
• To improve the accuracy of TS forecasting
• Statements of Thesis
• TS normally occurs in two locations
• NW of surface cyclone
• Due to CI in cyclonically-sheared environment
• NE of surface cyclone
• Due to CSI in anticyclonically-sheared
  environment

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Methodology for Climatology

• Based on CD-ROM of surface data (SA) from
  1961-1990
• Reports of TS extracted with C program
• Characterized by
• Location
• State
• Actual count
• Normalized according to the land area of the
  average state
• Region
• Relative to cyclone center (Digital Atmosphere)
• Distance
• Direction

• Time of occurrence
• Month
• Time of day
• Intensity, TMPF, DWPF, SLP, winds
• Synoptic setting
• Digital Atmosphere
• Assoc. w/ cyclone
• Lake-effect
• Upslope
• Orographic
• Offshore cyclone
• Undetermined cyclone location
• Misc.

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What is an event?

• A single observation of TS
• If more that one for a given time frame
• No more than 6 hours between multiple reports at
  a given station
• No more than 6 hours between reports at multiple,
  nearby stations
• Distance between stations observing TS not more
  than 1100 km
• Halfway point of meso-? scale
• To be associated as Type 1 event
• Within cloud shield/cyclone flow around cyclone
• Not an isolated precipitation report
• Clearly identifiable cyclone center

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Methodology for Case Studies

• 4 Cases examined
• 5 December 1999 Wichita, KS
• 9 December 1999 Lubbock, TX
• 11 March 2000 St. Louis, MO
• 19 April 2000 Ellsworth AFB, SD
• Analyses performed using GEMPAK (PCGRIDDS used
  for 5 December event)
• Surface analyses (METAR)
• Mandatory level analyses (RUC initial fields)
• Dynamics Thermodynamics (RUC initial fields)

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Methodology for Case Studies(cont)

• Thermodynamics (stability)
• Cross section overlays of ?e, Mg, and RH (Moore
  Lambert 1993)
• Alternative method also compared (Schultz
  Schumacher 1999)
• 3-D EPV (McCann 1995)
• Mg not part of equation
• Any cross section line can be used

• Dynamics (forcing)
• Q vectors
• S-component to assess synoptic-scale forcing
• N-component to assess frontal-scale forcing
• Storm-relative moisture transport
• Isentropic maps
• Tropopause/PV maps
• Petterssen surface frontogenesis - ? cross
  sections

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Results of Climatology

• 375 TS events discovered for the period of
  1961-1990
• Preferences
• Regional
• Basin and Range
• Central Plains
• Mid-Atlantic Region to New England
• Temporal
• March and April peak
• Jet stream interaction
• Afternoon and evening hours
• No reason identified

• Classification
• Most TS associated with cyclone
• NW/NE quadrants
• 360 km from center
• Some Type 1 events may be lake-enhanced
• Great Lake and Great Salt Lake both contribute to
  Type 2

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5 December 1999

• TS observed at KIAB, KICT at 0600 UTC
• Cloud-to-ground lightning strikes
• Bands of heavy precip indicated on radar imagery
• Synoptic setting
• West of surface cyclone (1009 hPa)
• Slight east-west tilt with height
• Cyclonic flow regime
• Under strong divergence axis
• Occluded characteristics aloft
• Tropopause/PV anomaly to the south

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300-hPa Heights and Isotachs
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300-hPa Divergence
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Convergence of Qn Dashed areas Convergence
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Convergence of Qs
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5 December 1999 Conclusions

• Upright instability present
• Inferred from 850-700-hPa LI lt0
• Released lower to middle level QG frontogenesis
• 700 hPa trowal co-located with precip.
• Upper tropospheric influences
• Trop/PV anomaly decreased static stability
• Qs field suggests presence of ducted gravity wave
• Divergence axis at 300 hPa

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9 December 1999

• TSSN TSIP observed at KLBB
• 90-minute duration
• 7 inches of snow/sleet in 6 hours
• Banded precipitation structure on radar
• Synoptic setting
• Northwest of surface cyclone (1009 hPa)
• Cyclonic flow regime and west-to east tilt
• Occluded structure aloft
• Tropopause/PV anomaly to south

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19991209 0900 UTC
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9 December 1999Conclusions

• TS occurred as convective instability was
  released by 750 hPa frontogenesis over LBB in
  trowal airstream
• Additional vertical enhancement by nearby trop/PV
  anomaly and gravity wave
• Little difference in EPV3 plots

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11 March 2000

• TS observed at KSTL and much of metro area
• Unconfirmed reports as far south as KCGI
• Cloud-to-ground lightning strikes observed
• Banded precipitation structure
• Northwest of open wave surface cyclone that was
  occluded aloft
• Nearly zonal flow regime above 500 hPa ahead of
  short wave
• Slight west-east tilt (nearly stacked)
• Tropopause/PV anomaly over N. Arkansas

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11 March 2000Conclusions

• TS occurred in the presence of weak symmetric
  stability due to 700 hPa frontogenesis within the
  trowal
• However, there could be unresolved CSI
• Prominent divergence axis over E. MO
• Trop/PV anomaly and ducted gravity wave further
  enhanced UVV
• EPV3 plots very similar

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19 April 2000

• TS reported at KRCA
• Up to 30 (75 cm) acc. in Black Hills
• Precip/snowfall records established
• NW of intense, occluded sfc cyclone
• System stacked at the lower levels
• Cyclonic shear
• Temperature gradient lessened
• 550 hPa Trop/PV anomaly to southeast

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19 April 2000Conclusions

• TS occurred in region of WSS due to 700 hPa
  frontogenesis in strong easterly trowal
  airstream
• Orographic influences possible
• Prominent divergence axis at 300 hPa
• Trop/PV/gravity wave influences
• Enhanced upward motion
• Decreased static stability
• Differences observed between EPV3 fields, but
  diagnosis at KRCA not affected

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Case Studies Overall Impressions

• Each event in NW sector of cyclone
• Trowal airstream/frontogenesis are most important
  forcing mechanisms
• Q fields show that these mechanisms occur as a
  result of synoptic-scale forcing (Qs gt Qn)
• Effects of trop/PV anomalies
• Enhance UVV decrease static stability
• Create trowal as flow becomes more meridional in
  attempt to conserve PV (Hoskins et al. 1985)
• Gravity waves may also act to increase UVV
• Stability may be less important that forcing and
  vertical velocities in production of TS (Schultz
  2000)

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Evaluation of the Statement of Thesis

• Statements of Thesis
• TS normally occurs in two locations
• NW of surface cyclone
• Due to CI in cyclonically-sheared environment
• NE of surface cyclone
• Due to CSI in anticyclonically-sheared
  environment

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Evaluation of the Statement of Thesis (cont)

• Evaluation
• None of the known TS events that season were NE
  of sfc cyclone ? couldnt test
• 2 of 4 events in NW quadrant exhibited upright
  instability
• 2 exhibited weak symmetric stability
• When a system becomes stacked
• Vertical shear/lapse rates decrease
• Stability increases
• Therefore, fine line between unstable and
  symmetrically stable environments
• NW quadrant hypothesis partially true, although
  oversimplified