cfmip ii sensitivity experiments mark webb (met office hadley centre) johannes quaas (mpi)

© Crown copyright 2006

CFMIP II sensitivity experiments Mark Webb (Met Office Hadley Centre)

Johannes Quaas (MPI)

Tomoo Ogura (NIES)

With thanks to Adrian Lock, Damian Wilson,

Andy Jones, Alejandro Bodas Salcedo

CFMIP/ENSEMBLES Workshop Paris, April 2007


Modelling and Prediction of Climate variability and change

Motivation and approach

We propose running sensitivity experiments to investigate the impact of different modelling assumptions on cloud feedbacks across models

Two types of sensitivity experiments are proposed:

1/ Where certain radiative feedbacks loops are cut

2/ Where elements of model physics are simplified


Modelling and Prediction of Climate variability and change For example:

Fix cloud liquid water contents and radiative properties seen by radiation

Does suppressing any cloud liquid water content feedback make cloud feedback more positive?

Does inter-model spread in cloud feedback reduce?

If so, by how much?

1/ Radiative feedback loop cutting experiments


Modelling and Prediction of Climate variability and change For example:

Put a simple stability based low cloud fraction into several models

Do low level cloud feedbacks become more negative/less positive?

What is the effect on inter-model spread?

2/ Replacing parametrizations with simple alternatives


Modelling and Prediction of Climate variability and change HadGEM2 + PC2 development version (Met Office) PC2 is a Tiedtke-like cloud scheme with prognostic equations for cloud liquid, cloud ice and cloud fraction

ECHAM5 – Tiedtke scheme (Johannes Quaas)

MIROC3.2 - statistical/PDF scheme (Tomoo Ogura)

So far we have results for fixed liquid cloud properties for PC2 and ECHAM5

Control runs are 10 year AMIP runs Climate change: control + CMIP 1% patterned SST composite

Liquid cloud droplet effective radius seen by radiation: 7 microns In cloud liquid water content seen by radiation: 0.2 g/kg

Pilot study (three models)



Impact of fixed liquid cloud radiative properties

dNCRF dSWCRF dLWCRF

Wm-2 Wm-2 Wm-2

ECHAM5 ref 0.00 -0.01 0.01ECHAM5 fix 1.32 1.61 -0.29ECHAM5 fix - ref 1.33 1.63 -0.30PC2 ref 0.35 0.46 -0.12PC2 fix 0.46 0.64 -0.18PC2 fix - ref 0.12 0.18 -0.07

Global mean net cloud radiative response is increased in both models and this effect comes mainly from the SW - this is consistent with what we expected However the effect is much larger in ECHAM5 than in PC2, making the two models diverge rather than converge



Impact on control simulations

NCRF SWCRF LWCRF

Wm-2 Wm-2 Wm-2

ECHAM5 ref -23.1 -51.8 28.7ECHAM5 fix -39.6 -97.1 57.4ECHAM5 fix - ref -16.5 -45.2 28.8PC2 ref -18.1 -46.0 28.0PC2 fix -29.5 -58.3 28.8PC2 fix - ref -11.4 -12.2 0.8

Fixing the liquid cloud radiative properties has made both of the models too bright with the biggest impact in ECHAM5

We plan to retune the models by applying a scaling factor to the liquid cloud fraction seen by the radiation. We may also consider using a larger effective radius



Use of tendency diagnostics and GPCI transect

We also plan to use cloud condensate tendency diagnostics to understand the feedback mechanisms operating in the reference and sensitivity experiments

The GCSS Pacific Cross Section Intercomparison ( GPCI ) transect samples stratocumulus, trade cumulus and deep convective regimes as well as the transitions between them

Some examples with PC2 follow….



Low cloud response in the PC2 experiments


Modelling and Prediction of Climate variability and change Low cloud fraction decreases along the GPCI when we fix liquid cloud radiative properties and when we warm the climate

What are the possible explanations?

Hypothesis 1

weaker circulation => reduced subsidence => weaker inversion => cloud breakup

Can we rule out this hypothesis in any of the above cases?





Overlaid contour lines show liquid cloud fraction…


Modelling and Prediction of Climate variability and change Hypothesis 2

Reduced convective mass flux (Held and Soden 2006) => less detrainment from shallow convection => less low level stratiform cloud





Overlaid contour lines show liquid cloud fraction


Modelling and Prediction of Climate variability and change Hypothesis 3 (climate response only)

Upper troposphere warms more than lower troposphere as climate models warm (e.g Santer 2005) => warmer (and possibly moister) free troposphere => less LW cooling at BL cloud top => less condensation => less cloud water / cloud fraction

( Note that the effect on cloud fraction could well be the opposite in any model where the cloud fraction is represented as an increasing as function of stability )





Overlaid contour lines show liquid cloud fraction




Hypothesis for reduction ref climate fix climate ref climate fix climate fix - refin low cloud response response response response

145W/15N 145W/15N 130W/25N 130W/25N 130W/25N

weaker subsidence

reduced conv condensation

warming aloft/less LW cooling

Dynamical forcing may be responsible for some but not all changes

Shallow convection may well drive low cloud feedbacks in the trades but doesn’t explain the response closer to the coast

Cloud top cooling may well play a role in driving the reductions in low level clouds, particularly closer to the coast


Modelling and Prediction of Climate variability and change 1/ Replace liquid cloud fraction seen by radiation with a simple stability based relationship

2/ Make the radiation code see warmer temperatures above the BL and see if this reduces cloud top cooling and in turn reduces low level cloud

3/ Simplified mixed phase feedback experiment

4/ Simplified autoconversion formulation experiment

Other suggestions?

Other potential sensitivity tests:


Modelling and Prediction of Climate variability and change The pilot study may demonstrate sensitivity tests to be useful, but the experiments will require retuning

Cloud condensate tendency diagnostics provide extra information that can be used to test or suggest hypotheses on the roles of different physical processes in cloud feedback mechanisms

Feedback patterns in 10 year AMIP + CMIP 1% patterned SST experiments are quite noisy compared to slab responses patterns

Conclusions




Overlaid contour lines show liquid cloud fraction…





145W/15N 145W/15N 130W/25N 130W/25N 130W/25N

weaker subsidence





145W/15N 145W/15N 130W/25N 130W/25N 130W/25N

weaker subsidence

reduced conv condensation

cfmip ii sensitivity experiments mark webb (met office hadley centre) johannes quaas (mpi)

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