Part I' Finescale structure of a cold front, as detected with a Wband radar

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Part I. Fine-scale structure of a cold front,as
detected with a W-band radar

• 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

Toulouse IHOP workshop, June 2004
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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
dryline
WCR up-looking, flight level 165 m
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164
344
flight level 165 m
W
stratus clouds
breaking K-H waves
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, JTech) 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
dryline 1.5 km to the right
2002_05_24_1958 UTC
11 aspect ratio front-relative flow
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Triple point transect
King Air
2012
cold front _at_2009
2007
cold front
Flight level 2300 m AGL
dryline
NNW 344
frontal motion
SSE 164
front dryline
moist air
cold air
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2002_05_24_2009 UTC
11 aspect ratio front-relative flow
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vertical velocity
horizontal vorticity
2002_05_24_2009 UTC
11 aspect ratio
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q and q at flight level (1000 m)
q
reflectivity and streamlines
Flight-level updraft max 9.8 m/s, 500 m average
4.6 m/s
vertical velocity
horizontal vorticity
11 aspect ratio front-relative flow
2002_05_24_2024 UTC
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2002_05_24_2024 UTC
2002_05_24_2054 UTC
reflectivity and streamlines
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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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May 24 findings

• 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, as observed in lab experiments (e.g.
  Simpson) and as numerically simulated (e.g. Xue
  et al 1997)
• Vertical transects of GC head highly variable in
  slope/depth, but consistently associated with a
  strong updraft 500 m wide. This frontal updraft
  did not light the fire because something else
  (dryline? frontogenetic circulation? ) already
  had.
• Large amounts of vorticity is baroclinically
  generated vorticity and quickly breaks up,
  resulting in large-amplitude K-H billows that
  evolve into lee-type gravity waves.

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Part II vertical structure of the fair-weather
continental convective boundary layer vertical
velocity and insect flight behavior
characteristics of echo plumes (thermals?)

• Bart Geerts and Qun Miao
• Department of Atmospheric Science
• University of Wyoming
• Acknowledgement Peggy LeMone, NCAR

Toulouse IHOP workshop, June 2004
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mid-level flight, radar looking up down
29 May 02
aspect ratio 21
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Low-level flight, radar looking up
aspect ratio 41
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Thermodaynamic BL depth corresponds well with the
WCR-inferred one
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Note the prevalence of downdrafts
CBL top
aspect ratio 21
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Development of the CBL (14 June BLE flight)
aspect ratio 11
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Echo plumes tend to be associated with updrafts
r 0.39
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WCR velocity spectrum corresponds with the gust
probe spectrum, but high frequencies (lt100 m) are
dominated by noise

• - wa (gust probe) accurately describes
  atmospheric turbulence
• the average wa may be off (integration error)
• -gt leg-mean wa values are set to zero

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WCR vertical velocities show a clear downward
bias at all levels
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This downward bias is stronger in strong
updrafts
radar
average of first gate above and first gate below
the aircraft
gust probe
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This updraft-dependent bias does not exist in
the ice-filled CBL over warm water
IHOP
Lake Michigan cold-air outbreak
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Analysis method average all WCR velocities in
each wa bin over 1 minute (1500 profiles)
IHOP
Lake Michigan cloud streets
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bias vs vertical air motion on 3 days
Note - average bias 0.45 m/s - bias
increases with updraft strength, almost linearly
- the slope is almost 50 ! - wa
distribution is positively skewed (LeMone 1990)
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This bias appears to be largely independent of

• Height in the CBL
• Echo strength
• Time of day

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Radar should give air vertical motion, thus we
correct for insect opposition
Average regression, for three days
Or
Thus the corrected radar vertical velocity is
uncorrected
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Insect opposition to updrafts

• The cloud radar detects mainly micro-insects in
  the continental CBL
• Micro-insects are weak flyers (aerial plankton,
  Russell and Wilson 1997)
• Micro-insect dispersal, necessary to find new
  feeding and breeding grounds, occurs during the
  daytime
• Dispersal is more effective by opposing updrafts
  (longer suspension time)
• The traditional entomological theory is that
  micro-insects slow or stop their wingbeat due to
  low temps in the upper CBL, since insects are
  poikilotherms
• This theory does not fly since none of the temp
  thresholds was reached
• A simple numerical model shows that the updraft
  opposition theory produces high insect
  concentrations (strong echoes) in sustained
  updrafts. The temperature control theory produces
  weaker echoes, and they occur to the side of the
  updrafts.
• Radar fine-lines and other echo plumes in the CBL
  thus are an indication of the sustained nature of
  sfc convergence and updrafts

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Adjusted radar vertical velocities
original
corrected
Echo plumes correspond with updrafts The vertical
velocity is skewed
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Stronger plumes correspond with stronger updrafts
(more buoyant thermals ??)
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Conditional sampling echo plumes have a mean
reflectivity gt1dBZ above the background mean
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Height (m)
TIME (UTC)
Reflectivity (dBZ)
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Plume width and spacing
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May 29 western track plume statistics
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Plume thermodynamic properties variation with
height in the CBL
q
w
q
qv
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Plume characteristics (May 29, June 6, June 17)

• Plume width is highly variable, about 500-700 m
• Plume spacing is 2-3 times larger
• About 20-25 of the CBL is considered plume
• In these plumes,
• q is positive and maximum in the upper CBL
  (.5-.7 g/kg)
• Updrafts occur (wgt0), maximum at mid- to
  upper-levels (about 1 m/s)
• qv is positive except in the upper CBL, where it
  is usually negative. qv 0.2K at low-mid levels
  in the CBL.

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Stronger plumes smaller fraction of CBL,
stronger updraft, higher buoyancy
w 0.7 m/s
q 0.25 g/kg
20 of CBL
1
qv 0.3 K
q 0.2 K
May 29, 2002 10 flight legs
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Plume strength and thermodynamic properties, two
other days water vaporpotential
temperaturebuoyancy,vertical velocity
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Conclusions

• The CBL reflectivity field is dominated by
  well-defined plumes, most of which penetrate to
  the CBL top. These plumes tend to be associated
  with updrafts.
• The scatterers in the CBL (small insects) tend to
  oppose updrafts. This opposition increases with
  updraft strength. This explains the existence of
  echo plumes and radar fine-lines in an
  otherwise well-mixed layer.
• Aircraft and radar data confirm that updrafts are
  generally stronger but more confined than
  downdrafts.
• Echo plumes have the characteristics of thermals
• Positive water vapor anomaly, increasing with
  height
• Positive buoyancy except at upper levels
• Updrafts peaking aloft

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Echoes are passive tracers? simple dispersal
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Dead particle scenarioEchoes fall at a constant
speed
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Temperature control scenario insects fall like
dead particles above 0.7 zi
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Updraft opposition scenario
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Correlation between vertical velocity and insect
concentration
dead particles
passive tracers
temperature threshold
updraft opposition