Cpt.UCLA Our work is motivated by two observations ‣ our understanding of cloud feedbacks is zonally symmetric. ‣ all pbl parameterizations strive to well.
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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). 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?
APE 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 ∂ y SST (or do as good as we can) 1 “AQUA” SST ∝ sin 2 (α 1 ϕ ) 2 “FLAT” SST ∝ sin 4 (α 2 ϕ ) 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”
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.
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
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”
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.