The EPIC 2001 Stratocumulus Cruise

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The EPIC 2001 Stratocumulus Cruise
Chris Bretherton University of Washington
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Sc region posters

• 8. Comstock et al. Drizzle in the SE Pacific
  stratocumulus region
• 9. Yuter et al. Mesoscale Variability of
  Stratocumulus Clouds and Drizzle
• 10. Wood et al. A continental gravity wave
  influence on remote marine SE Pacific cloud
• 12. Caldwell et al. Mixed Layer Budget Analysis
  of Stratocumulus Dynamics in EPIC
• 13. Xu et al. Intraseasonal Oscillation of the
  Stratocumulus Cloud Deck over the Southeast
  Pacific
• 14. Zuidema and Frisch On the vertical profile of
  stratus liquid water flux using a millimeter
  cloud radar
• 15. Ayers et al. Comparison of In Situ, Surface,
  and Satellite Cloud Measurements over the Eastern
  Pacific
• 16. Fairall et al. Diurnal Variations of Solar
  Optical and Microphysical properties of
  Stratocumulus Clouds from EPIC2001 - Leg II
• 19. Wang et al. A Regional Atmospheric Model
  Study of Boundary Layer Clouds over the Eastern
  Pacific off South America and Their Large-Scale
  Forcing
• 20. Intrieri et al. Lidar Observations of the
  Marine Boundary Layer Wind Structure and Stratus
  Clouds during the EPIC 2001 Cruise
• 22. Minnis et al. Diurnal Variations in Cloud
  Properties over the Eastern Pacific From GOES-8
  During EPIC 2001

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Sc Cruise Main Goals and Results(Bretherton et
al. 2004, BAMS)

• Document cloud and boundary layer structure in
  the SE Pacific
• Surprisingly well-mixed CTBL compared to NE
  Pacific, NE Atlantic, with pronounced diurnal
  cycle.
• Assess the importance of drizzle processes to
  cloud thickness and extent
• In clean regimes, evaporating drizzle
  significantly affects turbulence, promotes
  mesoscale variability.
• Compare results with other Sc regimes and with
  large-scale models
• CTBL deeper than GCMs predict at 20S 85 W.
• Maintain WHOI IMET buoy at 20S 85W
• Buoy providing rich validation, long-term
  context

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EPIC Sc cruise 9-24 Oct. 2001
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(No Transcript)
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(Kim Comstock and Peter Caldwell, UW)
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15-day MM5 simulation shows upsidence wave
forced by heating over Andes with late
afternoon convergence at coast, midnight
ascent at buoy. See P10 Wood et al. for
ECMWF, Quikscat analysis of wave.
18LT
06LT
Garreaud and Munoz (2004)
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Portrait of a Drizzly Period(P9 Yuter et al.
examines drizzle-cellularity correlations)
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Heavy Drizzle Events Increase Air-Sea Temperature
Difference
P9 Yuter et al.
Top Time series of actual drizzle area fraction
from the C-band radar. Bottom Time series of
air-sea temperature difference, used as a drizzle
indicator. Lines represent drizzle thresholds
corresponding drizzle events are shaded.
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Remotely-sensed cloud microphysics from
EPIC2001-Sc (see P8 for drizzle, P17 for LWP,
droplet conc. retrieval)
Bretherton et al. (2004)
Significant drizzle in clean periods, but mainly
evaporates
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Derived formula for cloud base drizzle rate
useful for GCMs
(See P8 Comstock et al. for more)
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6-day buoy period soundings vs. global models

• Forecast models and esp. GCMs underestimate ABL
  depth.
• Cloud is too shallow, thin (but surface drizzle
  still too high).

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Mixed layer model of SE Pac Sc (P12 Caldwell et
al.)

• Forcings (SST, subsidence, winds, hor. adv.)
  EPIC buoy period obs. ECMWF
• Uses drizzle, entrainment closures that match
  EPIC data.

MLM
MLM cloud over-deepens, esp. without drizzle
MLM (no drizzle)
EPIC
Cloud top
Cloud base

• Drizzle inhibits entrainment, lowers cloud top,
  thins cloud

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P14 DOPPLER LIDAR OBSERVATIONS of STRATUS CLOUDS
Doppler lidar detects clear air motions below the
stratus deck to reveal distinct up/downdraft
structures extending from the surface into cloud
base. Below is an example obtained during a
drizzle event on 17 October 2001.
Vertical Velocity Distributions with Height
Drizzle
Lidar Vertical Velocity
During the drizzle period, the ws at lowest
ranges peak at 0 m/s while the ranges closest to
the cloud shift toward negative or downdraft
values.
Non-Drizzle
Lidar SNR
During the non-drizzle period, the ws in the
lowest ranges peak in the positive values or
updraft while the range closest to the cloud is
centered at 0 m/s.

MMCR dBZ
-2 -1 0
1 2
(m/s)
Histogram of vertical velocities. Solid lines
denote data at the lowest range (150 m), bin
size 20 cm/s.
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P14 Frisch et al.

• Estimate droplet sedimentation rate in
  non-drizzling EPIC Sc from mm-radar. This is
  associated with slight latent heating in the
  cloud and evaporative cooling below cloud.

Oct 23
Oct 24
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Satellite-Derived Cloud Properties over the
Eastern Pacific (P15 Ayers et al. compares with
in-situ EPIC Sc obs. P22)
1945 UTC Oct. 16 2001
Cloud Phase
Optical Depth
Cloud Temp.
Droplet Radius
LWP
Cloud Height
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Intraseasonal Oscillation of the Southeast
Pacific Stratus Cloud Deck Haiming Xu,
Shang-Ping Xie, and Yuqing Wang (IPRC,
University of Hawaii)
a
TMI CLW
b
TMI CLW Power Spectrum
99 significant level 95 significant level
TMI CLW
c
TMI column cloud liquid water (panel a 10-2mm),
displays a pronounced intraseasonal oscillation
(ISO) over the Southeast Pacific stratus cloud
deck, with periods of 40-50 days (b). This ISO is
confirmed by in-situ measurements of surface
downward shortwave radiation (dashed in c Wm-2)
at the EPIC status buoy (20.15ºS, 85.15ºW). The
correlation coefficient between the high-passed
time series of TMI CLW and buoy radiation is
0.61.
Buoy SW Radiation (20ºS, 85ºW)
Corr 0.61
Aliasing with 45 day TRMM orbital period?
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P19 Wang et al. Radiative feedbacks between SE
Pacific boundary layer clouds, ITCZ convection
and E Pacific Hadley circulation.
IPRC-RegCM driven by the NCEP/NCAR reanalysis and
Reynolds weekly SST for Aug-Oct 1999.
Cloud-induced net diabatic heating rate
(contours) and meridional circulation (arrows)
Radiative cooling from boundary layer clouds
enhances SE Pacific subsidence, Hadley
circulation and cross-equatorial flow, increasing
ITCZ precipitation 10-15 as suggested by Nigam
(1997).
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Conclusions

• EPIC 2001 Sc cruise fulfilled its scientific
  goals and filled a major knowledge gap about the
  SE Pacific Sc region. A representative, but
  cloudy, period was sampled.
• Some model-observation comparison done, but we
  need to demonstrate and promote use of the 6-day
  buoy period as a standard SCM validation dataset
  (GCSS?) and further study Sc feedbacks with
  large-scale coupled circulation.
• Drizzle-cellularity-aerosol-mean LWP
  relationships seen in the data bear more study,
  using LES, longer-term buoy obs., satellite
  retrievals, and other field data.
• VOCALS (the VAMOS Ocean Cloud Atmosphere Land
  Study) is promoting continued work in this region.