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Low-Latitude Cloud Feedbacks CPT

Study on boundary layer cloud feedbacks in present and perturbed climates, aiming for better simulation and understanding. Collaboration with AGCMs and superparameterized CAM. |

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Low-Latitude Cloud Feedbacks CPT

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  1. Low-Latitude Cloud Feedbacks CPT Goal: Better simulation andunderstanding of low-latitude [boundary layer] cloud feedbacks in present and perturbed climates within NCAR, GFDL, GMAO AGCMs and in the superparameterized CAM. Chris Bretherton University of Washington, Seattle, USA

  2. CPT history • October 2003 - Sept. 2006, NSF/NOAA funded • CPT overviews: Spring 04, 06 US CLlVAR Variations • 8 funded PIs (Bretherton, Khairoutdinov, Lappen, Mapes, Pincus, B. Stevens, Xu, M. Zhang) + NCAR (Kiehl), GFDL (Held), some GMAO participation. Liaisons at NCAR (Cecile Hannay), GFDL (Ming Zhao). • 7 submitted publications. • Collaborations with CAPT, GCSS. • Proposal submitted for 2 year continuation, restricted to boundary layer cloud feedbacks. • Annual meetings + telecons. • www.atmos.washington.edu/~breth/CPT-clouds.html

  3. Regime-binned cloud climatology Wyant et al. 2006 (Clim Dyn, GRL) ISCCP simulator results from superparameterization more realistic than GCMs, though not perfect.

  4. CAM3-SP SST+2 climate sensitivity • Based on 3.5 yr ctrl, SST+2runs • Strong negative shortwave cloud feedbacks in tropics, extratropics, esp. from subsidence regimes. • Mean BL cloud thickness and fraction both increase • CAM3-SP l = 0.41 K/(W m-2) • vs. CAM3 l = 0.54 K/(W m-2) • Global CRM, DARE results similar. Wyant et al. 2006 (GRL)

  5. Single-column analysis SE Pac Sc (85W 20S), October, every timestep CAM3 GFDL Mapes

  6. RH Fixed (moist adiabat) T(z) Warm SST Cold SST SCM intercomparison (a la Betts-Ridgway, Larsen et al) • Single column in subtropical subsidence regime • Radiative/advective forcings maintain moist-adiabatic, fixed RH profile in free trop. • Plausible framework for analyzing • intermodel CTBL structure differences (previous slide) • cloud response to climate warming. • Led by Minghua Zhang

  7. Forcings (maintain steady state w/o CTBL) Free trop RH=15% Tw=29oC TOGA-COARE Ts = Tw- 4 Tw=27oC T Forcing q Forcing radiative realistic shape

  8. T-T0 (SCAM3) SST=23oC,25oC Interesting feedback • Anomalous radiative cooling due to underlying cold boundary layer affects free trop. temperature profile. PBL top

  9. SCM intercomparison • Cloud profiles in the single-column versions of our 3 GCMs exhibit very similar biases to those seen in our Bony analysis of the full models. • SCM +2K cloud feedbacks (not shown) also analogous to full GCMs. M. Zhang

  10. SCAM3 equilibrium Control clouds Time Series of clouds at 900 mb Time Series of q tendency at 900 mb LS phy ZM TOT liq DPC c-e HK dqdt q SHC Days Days

  11. The Cycle: • ZM and Surface Turbulence – Quasi-equilibrium • Evaporative cooling aloft activates PBL scheme • PBL scheme kills the ZM scheme • PBL scheme activates the Hack scheme • The Hack scheme stabilizes itself • The Hack scheme dries the air aloft • Surface evaporation and the dry air aloft re-activates the ZM scheme • Clouds are formed from the ZM water source

  12. Aquaplanet climate sensitivity Meideiros/Stevens Aquaplanet simulations are simpler but show remarkably similar low-lat cloud feedbacks to full +2K Cess runs.

  13. EUROCS project • JJA 1998 • GCSS intercomparison • JJA 1998/2003 • Observations • ISCCP data • SSM/I product • TOVS atmosphere • GPCP precipitation • AIRS data • Reanalyses • NCEP/ERA40 CAPT forecast mode analysis (Hannay/Klein) • CAM3, CAM3-UW and AM2 so far. • JJA 1998, GCSS NE Pacific cross-section.

  14. Mean errors from CAM3 T42 daily forecasts • Systematic biases set up fast (1 day in ITCZ, 5 days in subtropics). • Can investigate cloud errors from satellite observations. (Hannay)

  15. Clouds CPT Research Foci • Direct comparisons of single-column versions of the three AGCMs, LES, bulk models for idealized CTBLs, with focus on understanding climate sensitivity. • Improved parameterization of shallow convective cloud cover and microphysics. • Incorporation of an LES into a superparameterization (MMF) framework for better CTBL simulations. • CAPT forecast-mode and climate-mode column analysis of low-latitude CTBLs in the three GCMs. • Zonally symmetric aquaplanet low cloud sensitivity.

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