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
SHEBA
Surface Heat Budget
of the Arctic
Early May
~ 76N, 165 W
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
What I’ve 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
cloud’s radiative impact and elucidate the cloud
lifecycle
Why so challenging ?? both ice and liquid
phases are present (cloud T ~ -15C)
Surface-based Instrumentation: May 1-8 time series
8
-45
-20
-5
dBZ
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/m^2
1
1
2
3
4 day
6
7
day 5
4X daily soundings. Near-surface T ~ -20 C, inversion T ~-10 C
lidar cloud base
4
8
8
z
Details of the cloud characterization can be found at
http://www.etl.noaa.gov/~pzuidema
publication now in press with J. Atmos. Sci.
Cloud radar reflectivity
0
dBZ
Brad Baker & Paul Lawson - aircraft
-50data 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
2
Height (km)
-50
1
Temperature inversion
Aircraft path
Lidar cloud base
22:00
UTC
23:00
time
24:00
Main results from cloud
characterization
• Liquid cloud phase adiabaticallydistributed
• 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)
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
Ice water content/LWP time series
Mechanism for local ice production:
• Liquid droplets of diameter > ~ 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
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
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
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
=> Strong enough cooling to maintain cloud for any IN value (Pinto 1998)
=> 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
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