Thundersnow Proximity Soundings

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Thundersnow Proximity Soundings

• Patrick S. Market
• Atmospheric Science Program
• University of Missouri-Columbia

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Introduction

• Horizontal composite fields useful, but
• Reveal otherwise typical snowstorms
• Many of the standard approaches to snow
  forecasting would apply
• Need to distinguish what sets these convective
  snow storms apart from
• Aspects of the sounding

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Data and Approach

• 12 proximity soundings were found
• All associated with an extratropical cyclone
• Assume
• some feature (instability) makes these events
  unique, and
• convection resulting from instability is the
  source of charge separation for lightning
  production

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Data and Approach

• Four events occurred at exact time and location
  of a sounding
• Four events occurred at exact time of a sounding
  but were not at the exact location the sounding
  occurred
• the location of thundersnow was within 90
  nautical miles (167 km) of the sounding station
• Four events occurred within 90 nmi of sounding
  station and the TS report occurred 0-3 h after
  balloon release

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What we found Part I

• Thundersnow northeast of a cyclone center
• NEC
• N4
• Potentially neutral, but
• profiles rich in
• moisture
• forcing for ascent

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What we found Part II

• Thundersnow northwest of a cyclone center
• NWC
• N8
• Elevated layer of potential instability
• Not deep, but exists in a region of
• deep moisture
• forcing for ascent

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Previous work

• Curran and Pearson (1971)
• Found only 76 obs of TS (period - 2/68 TO 4/71)
• Obs eliminated if they did not occur
• Three hours before or after
• 0000 or 1200 UTC, and
• within 90 nmi of a rawinsonde station
• Only 13 observations remained
• All of these occurred east of the Rocky Mountains

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Previous work (CP71 reanalysis)
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Our results Mean profiles

• Mean profiles used raw data from 12 balloon
  flights
• not fitted to a grid first
• Proximity soundings show
• mean temperature
• mean dew point
• mean wind speeds, and
• median wind direction

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Our results Mean profiles

• NWC mean proximity sounding
• N8
• NO CAPE present NO PI present
• However, moist-neutral LR from 700 mb to 550 mb
• Also, Winds
• veer from surface to 600 mb (WAA)
• back from 600 mb to 400 mb (CAA)
• veer from 400 mb to 200 mb (WAA)

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Our results Mean profiles
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Our results Mean profiles

• NEC mean proximity sounding
• N4
• NO CAPE present NO PI present
• Moist-neutral LR from 500 to 400 mb
• Shallower than NWC
• Higher that NWC
• Deep WAA profile

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Our results Mean profiles
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Additional Analyses
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Our results Derived profiles

• Mean derived (e.g., F ) profiles generated
• Raob data objectively analyzed to a grid
• horizontal grid spacing of 150 km
• 50 mb vertical spacing.
• Only TSSN at 0000, 0300, 1200, or 1500.
• Location of the TSSN w.r.t. location of the
  sounding not a limiting factor as before
• Samples now larger
• NEC N9
• NWC N13

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Our results Derived profiles

• NWC cases
• Potentially stable
• Negative EPV not a recurrent theme
• Weak values do manifest at 600 mb

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Our results Derived profiles
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Our results Derived profiles
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Our results Derived profiles
log
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Our results Derived profiles

• NEC cases
• Potentially stable
• Negative EPV not a recurrent theme
• Weak values do manifest
• Deeper layer than NWC cases
• 700-500 mb

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Our results Derived profiles
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Our results Derived profiles
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Our results Derived profiles
log
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Our results Typical profiles

• Also based upon objectively analyzed data fitted
  to a grid, but
• Blind averages were not used here, as before
• Cases examined individually to assess the
  existence of PI, and if present, where
• While not a quantitative assessment of profiles,
  typical profiles reflect patterns seen most often.

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Our results Typical profiles

• NWC cases
• 10 of 13 profiles revealed a PI layer somewhere
• Two most common locations for PI
• near the surface, usually 950 mb to 900 mb
• usually late-fall or early spring
• in the range of 700 mb to 600 mb
• occurs throughout the year

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Our results Typical profiles

• NWC cases
• Profiles speaks of a source of heat and moisture
  aloft
• In a well-developed cyclone, warm moist air
  northwest of the surface low is most likely the
  result of the warm conveyer belt, especially the
  trowal

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Our results Typical profile (NWC)
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Our results Typical profiles

• NEC cases
• 5 of 9 profiles revealed at least one PI layer
• near the surface, from 1000 mb to 950 mb
• Second location of nearly neutral qe change was
  found higher in the atmosphere
• 650 mb to 600 mb
• profile here is moist neutral

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Our results Typical profile (NEC)
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Conclusions

• Not statistically significant, yet
• Profiles would seem to support the hypothesis
• potential instability related convection
  dominating in NWC thundersnow cases
• potential symmetric instability related
  convection dominating in NEC cases