Low-Latitude Cloud Feedbacks CPT Chris Bretherton University of Washington, Seattle, USA Goal: Better simulation and understanding of lowlatitude [boundary layer] cloud feedbacks in present and perturbed climates within NCAR, GFDL, GMAO AGCMs and in the superparameterized CAM. 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 Regime-binned cloud climatology Wyant et al. 2006 (Clim Dyn, GRL) ISCCP simulator results from superparameterization
more realistic than GCMs, though not perfect. CAM3-SP SST+2 climate sensitivity Based on 3.5 yr ctrl, SST+2 runs Strong negative shortwave cloud feedbacks in tropics, extratropics, esp. from subsidence regimes. Mean BL cloud thickness and fraction both increase CAM3-SP = 0.41 K/(W m-2) vs. CAM3 = 0.54 K/(W m-2) Global CRM, DARE results similar. Wyant et al. 2006 (GRL)
Single-column analysis SE Pac Sc (85W 20S), October, every timestep CAM3 GFDL Mapes 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 RH Fixed (moist adiabat) T(z) Warm SST Cold SST
Forcings (maintain steady state w/o CTBL) Free trop RH=15% Tw=29oC TOGA-COARE Ts = T w - 4 Tw=27oC T Forcing radiative realistic
shape q Forcing Interesting feedback Anomalous radiative cooling due to underlying cold boundary layer affects free trop. temperature profile. T-T0 (SCAM3) PBL top SST=23oC,25oC
SCM intercomparison M. Zhang 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. SCAM3 equilibrium
Control clouds Time Series of clouds at 900 mb LS Time Series of q tendency at 900 mb phy ZM TOT
liq DPC c-e q HK SHC Days Days dqdt
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 Aquaplanet climate sensitivity
Meideiros/Stevens Aquaplanet simulations are simpler but show remarkably similar low-lat cloud feedbacks to full +2K Cess runs. CAPT forecast mode analysis (Hannay/Klein) CAM3, CAM3-UW and AM2 so far. JJA 1998, GCSS NE Pacific cross-section. - EUROCS project JJA 1998 - GCSS intercomparison
JJA 1998/2003 - Observations ISCCP data SSM/I product TOVS atmosphere GPCP precipitation AIRS data - Reanalyses NCEP/ERA40 Mean errors from CAM3 T42 daily forecasts
(Hannay) Systematic biases set up fast (1 day in ITCZ, 5 days in subtropics). Can investigate cloud errors from satellite observations. Clouds CPT Research Foci 1. Direct comparisons of single-column versions of the three AGCMs, LES, bulk models for idealized CTBLs, with focus on understanding climate sensitivity.
2. Improved parameterization of shallow convective cloud cover and microphysics. 3. Incorporation of an LES into a superparameterization (MMF) framework for better CTBL simulations. 4. CAPT forecast-mode and climate-mode column analysis of low-latitude CTBLs in the three GCMs. 5. Zonally symmetric aquaplanet low cloud sensitivity.
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