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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?
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
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.