© crown copyright met office towards understanding the mechanisms responsible for different...

© Crown copyright Met Office

Towards understanding the mechanisms responsible for different cloud-climate responses in GCMs.Mark Webb, Adrian Lock (Met Office), Tomoo Ogura (NIES)

4th PAN-GCSS meeting: “Advances in modelling and observing clouds and convection.” Toulouse , June 2008


Acknowledgements

Sandrine Bony, Chris Bretherton, Gill Martin,

William Ingram, Bjorn Stevens, Joao Teixeira,

George Tselioudis, Mark Ringer, Keith Williams,

Rob Wood


CFMIP-GCSS activities for better understanding of cloud-climate feedback processes

• Process studies based on the analysis of high-frequency climate model output e.g. GPCI, ARM sites

• Sensitivity tests (in GCMs and SCMs)

• Idealised low cloud feedback studies with SCM/CRM/LES


Standard deviation of cloud radiative responses to CO2 doubling (W/m-2)


SW cloud radiative response Wm

-2

TWP GPCI SE Pacific

Wm

-2

Triangles show ensemble standard deviations of local valuesSquares show correlation of local and global values across ensemble

1


LW cloud radiative response Wm

-2

TWP GPCI SE Pacific

Wm

-2Wm

-2

Triangles show ensemble standard deviations of local valuesSquares show correlation of local and global values across ensemble

1


HadGEM3 development version containing: PC2 cloud scheme with prognostic cloud fraction, liquid and ice Lock boundary layer scheme

Standard experiments (STD)

a/ 10 year atmos-only forced with SST/sea ice obs climatology b/ CO2 forcing: fixed SST + 2xCO2 (Hansen method) c/ Climate change: control + CMIP 1% patterned SST composite d/ Climate change: control + 2K uniform SST

CFMIP-2 Pilot experiments

© Crown copyright Met Office Control 2CO2 Patterned SST Uniform SST

GPCI


SETP


Lock scheme uses following BL types:

1. Stable 2. Sc over stable 3. Well mixed 4. Decoupled Sc not over Cu decreasing cloud fraction/water 5. Decoupled Sc over Cu higher cloud top 6. Cumulus capped positive SW response? If so we would expect to see consistent changes in frequency of BL types and the SW CRF response

Could the SW responses be mainly due to transitions between boundary layer types?


Control 2CO2 Patterned SST Uniform SST


SETP


This idea is not supported for 2CO2, but is for the SST forcings: 3. Well mixed Sc 4. Decoupled Sc not over Cu 5. Decoupled Sc over Cu 6. Cumulus capped

SW CRF response Pat +2K +ve +ve -ve Transitions between BL types are larger in the SST forced runs, so these may be dominating smaller changes in the SW caused by cloud changes within the BL regimes

Are the SW responses mainly due totransitions between BL types?


In PC2 the largest source terms for clouds in this region are: - condensation due to LW cooling (mainly at cloud top) - condensate detrained from shallow convection We can repeat the experiments removing these terms:

1/ liquid condensate formed through LW cooling falls out (No CTC) 2/ liquid condensate from convective detrainment falls out (No CD)

We might expect LW cooling to be more important nearer the coast, and convective detrainment to be more important in the centre of the section

How can we trace these responses back to modelling assumptions?


In HadGEM3/PC2, SETP SW responses to SST forcing are largely consistent with transitions between BL types Responses near the coast coincide with transitions between non-convective BL types, and require condensation from LW cooling but not convective detrainment.

Responses further west coincide mainly with transitions between decoupled Sc over Cu and trade Cu, in opposite directions for uniform and patterned SST perturbations.

These require convective detrainment, but are weaker without LW cooling

Summary


Caldwell and Bretherton (submitted) argue that reductions in LW cooling due to increases in CO2 or water vapour can lead to reduced cloud top entrainment and a shallower BL

We can change or remove the dependence of entrainment on LW cooling to see if this mechanism explains the reductions in cloud near BL top in our results

Why does shallow convective detrainment reduce? Does reduced LW cooling of the BL force a reduction in latent heat release from shallow convection?

We could test this by discarding the latent heating from shallow convection.

Future work


The South East Tropical Pacific is a convenient area to study shallow cloud feedbacks in GCMs

Sensitivity tests in CFMIP-2 could help us to understand cloud response mechanisms at play in other GCMs

We will be able to do many more such sensitivity tests if we can develop relevant SCM forcing cases

Capturing the important shallow cloud feedbacks across models may well require cases for well mixed, convective and transition boundary layers

Concluding remarks

© crown copyright met office towards understanding the mechanisms responsible for different...

Documents

adrian lock met office

fixed sst

sst forcings

uniform sst cfmip

scmcrmles slide

patterned sst perturbations

lw cooling summary slide

cloud changes