Evening Discussion: Toward a better understanding of PBL cloud feedbacks on climate sensitivity Some introductory material Chris Bretherton University of Washington
Jan 14, 2016
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?