Evening Discussion:Toward a better understanding of PBL cloud

feedbacks on climate sensitivity

Some introductory material

Chris BrethertonUniversity of Washington

CCSM3

Low-latitude PBL clouds interact with climate changein model-dependent ways

Ping Zhu

+2K

CMIP

2xCO2

NCAR GFDL

Regime-sorting of CRF – A CPT diagnostic activity

Wyant et al. (2005) following Bony et al. (2004)

Subsidence regimesimportant but not dominant in NCRF.

PBL cld) depends on GCM and somewhat on scenario.

PBLcld

Low cloud NCRF mostly from cldfrc, also from LWP

Cloud Fraction Liquid + Ice Path Cloud Forcing

Mechanisms of PBL cloud feedback

Is every geographical region different?

Lack of published physical mechanisms:

(-) In warmer climate, adiabatic dLWC/dz larger, so PBL clouds of given thickness are more reflective (Somerville and Remer 1984).

… but not much evidence for such trends with T.

Tselioudis and Rossow (1994)

• (-) In warmer climate, steeper moist adiabat raises lower tropospheric stability, increasing low cloud cover (Miller 1997 based on Klein and Hartmann 1993).

…but how to apply ‘Klein line’ to changed climate (or even its robustness in current climate) is questionable.

• Observed TOA net CRF moderately correlated to LTS.• In CAM3, LTS increases ~2 K in 2xCO2 climate with little

NCRF change.

Alternative LTS measures more climate-stable?

• Estimated Inversion Strength (Rob Wood)

EIS = LTS - (z700-LCL)*(d/dz)moistad(700 mb)

LTS = 700 – 1000 EIS = +LCL,ma –

700

1000

LCL

• Collapses midlatitude vs. low-lat. Sc regimes better. • EIS less sensitive than LTS to steeper moist adiabat

predicts less low cloud feedback on climate sensitivity.

(Klein and Hartmann 1993)

Betts-Ridgway (1989) type approaches

• Subject idealized subtropical CTBL to a perturbed moist-adiabatic change in free-tropospheric conditions.

• B&R used - mixing-line PBL with

specified trade-cumulus cloud fraction

- moist-adiabatic free-tropospheric profile

- empirically-constrained RH profile

- 2-layer energy balance. SST

e* from SST(ITCZ)

q (e. g. from const RH)

q

w

U

Issues

• Can explore effects from changing RH, U, SSTITCZ-SSTSc.

This type of study could be done with PBL models of varying complexity (from mixed layer to LES).

• There are setup subtleties:– Radiative subsidence profile dependent on subtle

features in RH, profiles.– In Sc regions, also need some free-tropospheric cold

advection to get reasonable PBL-top subsidence rates.

– So far, we haven’t got a good LES-simulated steady-state Sc regime with enough LWP.

Other approaches?

• The purpose of this discussion is to brainstorm about other possible approaches to systematically studying PBL cloud feedbacks on climate sensitivity in a way that goes beyond just diagnosing global simulations with GCMs that have flawed parameterizations.

• Your ideas and feedback?