The Lifecyle of a Springtime Arctic MixedPhase

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The Lifecyle of a Springtime Arctic Mixed-Phase
Cloudy Boundary Layer observed during SHEBA
Paquita Zuidema
University of Colorado/ NOAA Environmental
Technology Laboratory, Boulder, CO
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Surface Heat Budget of the Arctic
SHEBA
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Early May 76N, 165 W
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WHY ?

• GCMs indicate Arctic highly responsive to
  increasing greenhouse gases (e.g. IPCC)
• Clouds strongly influence the arctic surface and
• atmosphere, primarily through radiative
  interactions
• Factors controlling arctic cloudiness not well
  known
• Springtime conditions of particular interest

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What Ive done

• Picked a multi-day time period containing
  aircraft data as well as the ship-board data
• Used the aircraft data to help strengthen the
  shipboard assessment of the cloud properties, so
  that a 9-day cloud characterization could be done
  w/ confidence
• Used the cloud characterization to assess the
  clouds radiative impact and elucidate the cloud
  lifecycle

Why so challenging ?? both ice and liquid phases
are present (cloud T -15C)
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Surface-based Instrumentation May 1-8 time series
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dBZ
-5
-45
-20
6
35 GHz cloud radar ice cloud properties
km
4
2
depolarization lidar-determined liquid cloud base
Microwave radiometer-derived liquid water paths
100
g/m2
2
3
4
5
6
7
8
1
day
day
4X daily soundings. Near-surface T -20 C,
inversion T -10 C
4
1
8
lidar cloud base
z
-30C
-10C
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Details of the cloud characterization can be
found athttp//www.etl.noaa.gov/pzuidema
publication now in press with J. Atmos. Sci.
Cloud radar reflectivity
-50
0
dBZ
Brad Baker Paul Lawson - aircraft data and
analysis knowledge Yong Han - new and improved
liquid water paths Janet Intrieri -
depolarization lidar data Jeff Key - Streamer
radiative transfer code Sergey Matrosov - cloud
radar retrieval of ice cloud properties Robert
Stone - sunphotometer-derived aerosol optical
depths Matthew Shupe Taneil Uttal -
well-organized, web-accessible datasets
-50
2
Height (km)
1
Temperature inversion
Aircraft path
Lidar cloud base
2400
UTC
2200
2300
time
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Main results from cloud characterization

• Liquid cloud phase adiabatically-distributed
• Radar ice microphysical retrievals compare well
  (enough) to aircraft-derived values
• Liquid optical depth usually far exceeds ice
  optical depth (mean values of 10 and 0.2
  respectively)

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impact of the ice

• 1) upper ice cloud sedimentation associated with
  near-complete or complete LWP dissipation (May 4
  6)
• 2) local IWC variability associated with smaller
  LWP changes, time scale few hours

At T-20C, air saturated wrt water is 20
supersaturated wrt ice
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Ice water content/LWP time series
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Mechanism for local ice production

• Liquid droplets of diameter gt 20 micron freeze
  preferentially, grow, fall out
• New ice particles not produced again until
  collision-coalescence builds up population of
  larger drops
• Only small population of large drops required
• Hobbs and Rangno, 1985 Rangno and Hobbs, 2001
  Korolev et al. 2003 Morrison et al. 2004
• Availability of contact nuclei also important
• Little previous documentation within cloud radar
  data

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Local ice production more evident when boundary
layer is deeper and LWPs are higher
May 3 counter-example variable aerosol
entrainment ?!?!
Quick replenishment of liquid longer-time-scale
variability in cloud optical depth related to
boundary layer depth changes
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May 1-3 Mean Sea Level Pressure
Weak low N/NW of ship followed by weak/broad
high moving from SW to NE
Data courtesy of NOAA Climate Diagnostics Center
May 4-9 Mean Sea Level Pressure
Boundary-layer depth synchronizes w/
large-scale subsidence
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Why is this cloud so long-lived ????

• Measured ice nuclei concentrations are high
  (mean 18/L, with
• Maxima of 73/L on May 4 and 1654/L (!) on May 7
  (Rogers et al. 2001)
• This contradicts modeling studies that find
  quick depletion w/ IN
• conc of 4/L (e.g. Harrington et al. 1999)

We find

• Quick replenishment of liquid, suggesting strong
  water vapor fluxes,
• either local or advected
• When liquid is present,
• Cloud-top radiative cooling rates can exceed 65
  K/day
• gt Strong enough cooling to maintain cloud for
  any IN value (Pinto 1998)
• gt Promotes turbulent mixing down to surface,
  facilitating surface fluxes

How did this cloud finally dissipate ????
Strong variability in large-scale subsidence
rates part of answer
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What might a future climate change scenario look
like at this location ?
Recent observations indicate increasing
springtime Arctic Cloudiness and possibly in
cloud optical depth (Stone et al., 2002, Wang
Key, 2003, Dutton et al., 2003)
At this location (76N, 165W) an increase in
springtime cloud optical depth may not
significantly alter the surface radiation
budget, because most cloudy columns are already
optically opaque.
Changes in large-scale dynamics (e.g., more
synoptic activity bringing in more upper-level
ice clouds, or changes in the mean subsidence
rate) may be more influential
Future impact of clouds upon the surface energy
budget best understood if both the underlying
mixed-phase cloud processes, and their dependence
upon the large-scale dynamics, are known