Low-Latitude Cloud Feedbacks CPT

Chris Bretherton

University of Washington

• US CLIVAR activity sponsored by NSF and NOAAat ~$1M/yr, along with 2 ocean ‘climate process teams.’

• 1 Oct. 2003-30 Sept. 2006.• 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

2xCO2

Subtropical PBL cloud feedbacks responsible for much of this difference.Now, CAM3 climate sensitivity is ~2.9 K

AM2.12 is ~2.6 KBoth 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 20-21 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 runsAnnual mean(500 hPa)

ERA15

NCAR

GFDL

Annual meanControl run

500 mb)(mb d-1)

GMAO

Annual meanControl run

-SWCF(W m-2)

ERBE

NCAR

GFDL

GMAO

NCARSST + 2

Bonygram

SST+2Bonygram

comparison

NCAR

GFDL

GMAO

NCARSST+2

vs. 2xCO2 SOM NCAR

SST+2vs.

SOM 2xCO2

Summary

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

AMWG members.