GCSS BLCWG update

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GCSS BLCWG update

• Chris Bretherton, BLCWG Chair
• Thanks to
• Andy Ackerman (LES case leader)
• Margreet vanZanten/Bjorn Stevens
• SCM and LES case participants
• Other BLCWG attendees

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RF01 case LES (Stevens et al. 2005 MWR)

• 10 groups submitted 16 LES runs.
• Observed cloud deepens and thickens very slightly
  in 8 hrs with well-constrained we 0.35 cm s-1.
  Almost all LES models predict we to within 30
  (good) but most considerably underestimate LWP.

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• A subset (4 of 16 LESs) predicted LWP better.
  These models used subgrid schemes which strongly
  inhibited SGS vertical mixing within the
  inversion layer. Their we and turbulent velocity
  variance profiles were also most consistent with
  observations.

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RF01 SCM LWP and entrainment evolution

• Within an hour, 10 SCMs diverge toward a wide
  range of LWP bracketing obs.
• After this time, in most SCMs, LWP quasisteady
  with we within 50 of observed.
• Most SCMs similar at high vs. normal resolution,
  correctly predict no drizzle.

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DYCOMS RF02 nocturnal drizzling stratocumulus
Nd 45-70 cm-3
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Participating SCMs
Name SCM Turbulence Cld. Frac. Microphys.
Austin CCCMa4?
Chlond ECHAM4-5 moist TKE we pdf Sundquist
Kitagawa JMA 1st-order K RH-pdf Sundquist
Lappen CAM3 CAM3UW Nonlocal, sfc-based. K-profile, explicit-we RH/stab RH Autoconv./coll., N 65 cm-3
Larson 2GPDF-HOC From pdf Khair.-Kogan w. joint pdf
Lock UKMO Nonlocal, explicit-we RH-pdf Autoconv./coll., N 100 cm-3
Menon GISS SCM Dry adjustment RH/stab Autoconv./coll. (del Genio)
Roode RACMO EC CY23R4 K-profile, explicit-we Tiedtke Sundquist, PµLWC
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Surface drizzle vs. LWP

• Diverse sensitivities.
• Microphysical parameterizations or droplet size
  assumptions?

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Cloud-base drizzle vs. LWP
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Precip vs. no-precip sensitivity studies

• In drizzly models (except JMA), LWP increased
  substantially by drizzle suppression.

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Discussion

• Small revisions to case (Nd, u, fixed fluxes)
  revised submissions due Aug. 1 (must include real
  drizzle).
• WG preferred not to initiate new case or meeting
  yet better to digest results we have and try to
  improve model performance on these two cases.
  Provides an opportunity for simplified
  microphysics-only intercomparison in a specified
  flow field.
• Lots of interest in a RICO case when obs. are
  mature, and in some global GCM sensitivity
  studies (perhaps in intercomparison mode, perhaps
  using Pacific x-sect) to drizzle and
  sedimentation in boundary layer cloud.