Low-Latitude Cloud Feedbacks CPT Chris Bretherton University of Washington US CLIVAR activity sponsored by NSF and NOAA at ~$1M/yr, along with 2 ocean ‘climate process teams.’ 1 Oct Sept A pilot project – can a multi-institution, multi-model effort integrating observationalists, diagnosticians, process-scale and large-scale modelers improve climate models faster? NCAR, GFDL, GMAO(NASA) models.

Low-latitude cloud feedback CPT Goal: Reduce uncertainty in low-latitude cloud feedbacks on climate sensitivity. In-depth diagnosis of cloud feedbacks in models. Implement ‘best-practices’ parameterizations honed via single-column methods. Start with boundary-layer clouds, move to deep convective systems.

The time is ripe… Cloud feedbacks on climate sensitivity are a long- standing problem. ~1 yr ago, CAM2 climate sensitivity was 1.5 K, AM2.10 was ~5 K for 2xCO 2 Subtropical PBL cloud feedbacks responsible for much of this difference. Now, CAM3 climate sensitivity is ~2.9 K AM2.12 is ~2.6 K Both models have evolved, but this demands clear explanation...can we prove newer is better? Over short run, diagnostic findings may help IPCC AR4. Over longer run, IPCC model finalization allows more thoughtful parameterization development.

CPT organization Core group (C. Bretherton, M. Khairoutdinov, C. Lappen, B. Mapes, J. Norris, R. Pincus, B. Stevens, K. Xu, M. Zhang): Parameterization, diagnosis, observational hooks. Advisory group (B. Albrecht, A. Betts, C. Fairall, T. del Genio, S. Ghan, G. McFarquhar, R. Mechoso, H. Pan, D. Randall, D. Raymond, J. Teixeira, R. Weller) NCAR–Kiehl, Rasch, Collins; Liaison: hiring underway GFDL-Klein (thru 3/04), Held, Donner; Liaison: Zhao GMAO-Bacmeister, Suarez. Not an exclusive effort - active coordination with CCSM AMWG (one reason for this talk!), GCSS, European, Canadian efforts, CFMIP.

Strategy Hypothesis-driven diagnoses of biases, cloud feedbacks Single-column methods and intercomparison. PI-specific efforts -New PBL, ShCu, microphysical parameterizations -Superparameterization cloud response to climate change (e.g. specified dSST) -Use of long-term cloud datasets and new satellite obs. -CRM/LES simulations of cloud feedbacks to specified greenhouse-type large-scale forcing changes. -Novel model diagnostics.

CPT Meeting Nov, 2003 at NCAR 1. Focus teams for process diagnosis and new physics: -Cloud topped boundary layers (Bretherton) -Deep tropical convective systems (Mapes) -Representing subgrid heterogeneity in microphysics and radiation (Pincus) 2. Multimodel simulation archive 3. Column output locations 4. Group projects -Rad.-conv. equilibrium intercomparison -Intercomp. of CTBL response to different climate forcing components (e.g. changed free-tropospheric subsidence and humidity, LTS)

Cloud response mechanisms to climate change Deep convective clouds - Freezing level increase - Anvil top temperature feedbacks (Hartmann) - LWCF vs. SWCF – is cancellation universal? PBL clouds: - Stronger trade inversion due to stabler moist adiabat - More emissive free troposphere. - Subsidence changes.

Simulation archive Monthly climatology history files from recent versions of models suitable for climate sensitivity analysis. GFDL NCAR GMAO AM2.12 (59e) CAM3(rio33) NSIPP ctrl y y y dSST +2/[-2] y y y dSST_CMIP y y y SOM (1x,2x CO2) y y Single column model y y

Some sample diagnostics so far… Climatologies Bony-grams (binning vs. 500 mb 

Control runs Annual mean  (500 hPa) ERA15 NCAR GFDL Annual mean Control run  500 mb) (mb d -1 ) GMAO

Annual mean Control run -SWCF (W m -2 ) ERBE NCAR GFDL GMAO

NCAR SST + 2 Bonygram

SST+2 Bonygram comparison NCAR GFDL GMAO

NCAR SST+2 vs. 2xCO2 SOM NCAR SST+2 vs. SOM 2xCO 2

Summary CPT is spinning up. Regular reports at AMWG meetings Many opportunities for collaboration with other AMWG members.