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Evening Discussion: Toward a better understanding of PBL cloud feedbacks on climate sensitivity

Evening Discussion: Toward a better understanding of PBL cloud feedbacks on climate sensitivity. Some introductory material Chris Bretherton University of Washington. Low-latitude PBL clouds interact with climate change in model-dependent ways. CCSM3. Ping Zhu.

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Evening Discussion: Toward a better understanding of PBL cloud feedbacks on climate sensitivity

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  1. Evening Discussion:Toward a better understanding of PBL cloud feedbacks on climate sensitivity Some introductory material Chris Bretherton University of Washington

  2. Low-latitude PBL clouds interact with climate change in model-dependent ways CCSM3 Ping Zhu

  3. Regime-sorting of DCRF – A CPT diagnostic activity GFDL NCAR PBL cld +2K Subsidence regimes important but not dominant in DNCRF. D(PBL cld) depends on GCM and somewhat on scenario. CMIP 2xCO2 Wyant et al. (2005) following Bony et al. (2004)

  4. Low cloud DNCRF mostly from Dcldfrc, also from DLWP Liquid + Ice Path Cloud Forcing Cloud Fraction

  5. 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)

  6. (-) 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.

  7. Alternative LTS measures more climate-stable? 700 • Estimated Inversion Strength (Rob Wood) EIS = LTS- (z700-LCL)*(dq/dz)moistad(700 mb) LCL 1000 LTS = q700 – q1000 EIS = q+LCL,ma – q1000 (Klein and Hartmann 1993) • Collapses midlatitude vs. low-lat. Sc regimes better. • EIS less sensitive than LTS to steeper moist adiabat Þpredicts less low cloud feedback on climate sensitivity.

  8. 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 q profile - empirically-constrained RH profile - 2-layer energy balance. q q Dq (e. g. from const RH) w Dqe* from DSST(ITCZ) U DSST

  9. 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, q 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.

  10. 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?

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