Finescale structure of a cold front

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Fine-scale structure of a cold front

• A cloud radar view of the May 24 Shamrock cold
  front
• Bart Geerts, Rick Damiani, Sam Haimov, Dave Leon,
  and Tim Trudel
• University of Wyoming

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Synoptic situation at 18 UTC on 24 May 2002,
based on the ETA initialization. Equivalent
potential temperature (color field) and winds
(blue, a full barb equals 10 kts) at 900 mb, sea
level pressure (yellow contours), and 300 mb
geopotential height (red contours).
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21 Z
18 Z
  GOES 8 visible satellite image, operational
surface observations, and subjective frontal
analysis Black line shows location of dropsonde
transect
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Dropsonde transect (2022-2057 UTC)
q,q
qe, RH
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5/24, 2107 UTC, view towards SE
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1942 UTC
AMA
1952
UWKA
N
1943
cold front
cold front
dryline
WCR up-looking, flight level 165 m
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164
344
flight level 165 m
W
breaking K-H waves
stratus clouds
Cold frontal propagation speed 7 m/s - this
corresponds with
11 aspect ratio
2002_05_24_2029 UTC
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Dual-Doppler synthesis

• Radial data corrected for aircraft motion
• Actual angles used to estimate (u,w) at various
  ranges below the aircraft.
• (u,w) redistributed on Cartesian grid (30m x 30m)
  (not all vectors are shown)
• Some data points are eliminated based on
  noisepower ratio, not on local velocity
  variance vectors are not filtered or smoothened
• Vertical velocities are adjusted for insect
  motion (Geerts and Miao 2004) with a maximum
  adjustment of /- 1.5 m/s
• Streamlines and vorticities are based on filtered
  (u,w)

single up
dual down
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q
q
2300 m
stable layer?
2002_05_24_1958 UTC
11 aspect ratio front-relative flow
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Triple point transect
2012
Flight level 2300 m AGL
King Air
cold front _at_2009
2007
NNW 344
frontal motion
SSE 164
front dryline
moist air
cold air
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Fig 2
2002_05_24_2009 UTC
11 aspect ratio front-relative flow
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vertical velocity
Fig 3
horizontal vorticity
2002_05_24_2009 UTC
11 aspect ratio
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q and q at flight level (1000 m)
reflectivity and streamlines
vertical velocity
horizontal vorticity
11 aspect ratio front-relative flow
2002_05_24_2024 UTC
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Fig 5
reflectivity and streamlines
2002_05_24_2024 UTC
2002_05_24_2054 UTC
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reflectivity and streamlines
horizontal vorticity (colors)
vertical velocity (contours)
2002_05_24_2024 UTC
2002_05_24_2054 UTC
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summary

• Leading edge of cold front appears as a density
  current with nose, head, rear-to-front inflow
  current and front-to-rear acceleration over the
  front
• Vertical transects of GC head highly variable in
  slope/depth
• baroclinically-generated vorticity quickly
  breaks up (large-amplitude K-H billows evolving
  into trailing gravity waves)