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).
http://www.atmos.ucla.edu/~brianpm/ucla_cpt.html
Bjorn Stevens, Yunyan Zhang, Brian Medeiros
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
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?
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 sin∝ 2(α11ϕ)“FLAT” SST sin∝ 4(α22ϕ)
APE SSTs
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” )
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)
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
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”
CAM3 tropical (30-30) sensitivity
AM2 tropical (30-30) sensitivity
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
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
CAM3 clouds
‣ Aqua planets have larger changes in cloud fraction
‣ total & low cloud fraction change is consistent among configurations
‣ “qobs” is most CAM-like
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.
“qobs” cloud fractions
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)
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)
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
‣ 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?
summary
