cpt'UCLA

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cpt.UCLA

• Our work is motivated by two observations
• our understanding of cloud feedbacks is zonally
  symmetric.
• all pbl parameterizations strive to well
  represent the mixed layer limit (at the right
  time).

Bjorn Stevens, Yunyan Zhang, Brian Medeiros
http//www.atmos.ucla.edu/brianpm/ucla_cpt.html
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how earth-like are the aqua-planets?

• Motivation
• Methodology - the experiments
• Some results
• GFDL AM2
• NCAR CAM3
• A few extra CAM3 figures
• Discussion topics the future

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aqua planet - what why

• simplified framework to help isolate feedbacks
• full 3D GCM with dynamics and physics
• prescribed SST
• no land or sea-ice
• perpetual equinox solar conditions (no seasons)
• gives a zonally symmetric configuration to
  compare with simple models
• useful for model intercomparison
• Is aqua planet sensitivity similar to full GCM?
• Do different GCMs have similar aqua planets?

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APE
APE SSTs

• start with full model (dynamics physics)
• remove zonal asymmetry (land sea-ice)
• prescribe SST (APE analytic expressions aqua,
  flat, qobs)
• Warm SST by 2 K
• Where to have SST 0?
• Preserve ?ySST (or do as good as we can)

AQUA SST ? sin2(a1?) FLAT SST ? sin4(a2?)
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SST 2

• warming equatorward of 60 latitude
• maximum SST 29 C (at equator)
• match gradient at a single latitude
• minimize RMS difference
• result is a modifed cess perturbation in the
  tropics and midlatitudes
• high latitude SST remains at 0 C

QOBS SST 0.5( AQUA FLAT )
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aqua planet 2K

• NCAR CAM3 GFDL AM2 were run in the three APE
  configurations for 3.5 years each
• 2K runs were performed for all three APE SST
  profiles
• An additional Cess experiment was included with
  the full GCM (land sea-ice)

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CAM3 global sensitivity

• aqua planets slightly less sensitive than cess
• Cess has largest sensitivity, and smallest
  albedo changes
• climate sensitivity parameter ratio of change in
  surface temperature to change in direct radiative
  forcing (dominated by change in OLR).
• cloud feedback parameter ratio of change in
  total cloud forcing to direct radiative forcing.
• CAM cloud feedback is negative in all
  configurations

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AM2 global sensitivity

• Cess has largest sensitivity, and smallest
  albedo changes (again)
• AM2 cloud feedback is positive in Cess, but
  negative in aqua planets (only globally)
• albedo change due almost entirely to cloud albedo
  for aqua planets, but not Cess

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CAM3 tropical (30-30) sensitivity
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AM2 tropical (30-30) sensitivity
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compare global cloud response

• Very similar TOA radiative changes
• AM2 global cloud feedback changes sign in aqua
  planet
• qobs consistently most similar to full GCM

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compare tropical cloud response

• TOA radiative forcing larger in CAM
• CAM radiative forcing similar to global forcing
• AM2 forcing about half global value
• Cloud feedback opposite sign between models
• intra-GCM tropical cloud feedback consistent in
  sign and magnitude

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CAM3 clouds

• Aqua planets have larger changes in cloud
  fraction
• total low cloud fraction change is consistent
  among configurations
• qobs is most CAM-like

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AM2 clouds

• Tropics (30S-30N) show decrease in total cloud
• only cess shows increase in high clouds.
• Tropical total and low cloud changes have same
  sign among runs.

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qobs cloud fractions
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aqua planet convective precipitation

• very similar double ITCZ in QOBS
• change in 2 case is quite similar
• other configurations are also very alike, with
  exception of aqua (single ITCZ in AM2)

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CAM vertical motion

• seasonal effects make cess look weakest
• wider SST maximum makes wider spaced ITCZs and
  weaker circulation
• double ITCZ in qobs and flat (and equatorial
  subsidence)

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CAM liquid water

• more intense circulation in aqua and qobs
  (area of convection/susidence)
• warmer climate produces more cirrus in convective
  regions and more stratus in subsidence
• warmer climate has less mid-level cloud

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summary

• Aqua planet configuration does seem to have
  similar climate sensitivity as full GCM
• qualitatively captures response in omega space
• discriminates between AM and CAM cloud feedbacks
• reasonable predictor of low cloud response
• Caveats (how big, how long, how right?) more
  work...
• A laboratory for investigating climate feedbacks?

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aqua planet

• Double ITCZ in qobs and flat, less pronounced in
  aqua and CAM
• subtropical local maxima in aqua and flat?
• difference dominated by poleward shift of ITCZs
  in flat

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aqua planet

• qobs most similar in extratropics to CAM
• substantial poleward shift of midlatitude storms
  (stronger in aqua planet)
• aqua planets have more large-scale precipitation
  than CAM.

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aqua planet