1. © Crown copyright Met Office Some thoughts on s12 stratocumulus feedback Adrian Lock EUCLIPSE WP3 meeting, Toulouse, April 2012

2. © Crown copyright Met Office

4. Subsidence pdfs: HadGEM2 vs CGILS s11 s12 Mean w unchanged

5. © Crown copyright Met Office Why do the LES robustly show a significant positive feedback at s12 when subsidence unchanged?

6. © Crown copyright Met Office Why the positive feedback at s12 with subsidence kept constant? Robust changes with +2K: 1)Cloud is warmer greater upward LW flux at cloud top more cloud-top cooling more entrainment thinner cloud? Actually find downward LW flux increases too and balances almost exactly cloud-top cooling unchanged

7. © Crown copyright Met Office Why the positive feedback at s12 with subsidence kept constant? Robust changes with +2K: 1)Cloud is warmer greater upward LW flux at cloud top more cloud-top cooling? Actually find downward LW flux increases too and balances almost exactly cloud-top cooling unchanged 2)Surface LH flux increases (assuming unchanged RH, U, T 1 -T surf ): LHF = c h (q 1 -q sat (T surf )) ~ c h (RH 1 q sat (T 1 )-q sat (T surf )) 3)Δq more negative (larger RH in the PBL dominates larger dq sat /dT in warm free atmosphere): (2)+(3) if entrainment rate unchanged, expect stronger LH fluxes throughout PBL: LH flux

8. © Crown copyright Met Office Impact of enhanced LH fluxes Buoyancy flux = α wθ l + β wq T wq T term is most important in the cloud layer (where β is much larger) Larger wq T larger buoyancy flux more turbulent PBL more entrainment of dry air thinner cloud positive cloud feedback Do the steps of this argument hold up?

9. © Crown copyright Met Office LH flux – entrainment feedback? Larger wq T larger buoyancy flux more turbulent PBL more entrainment of dry air thinner cloud positive cloud feedback … more entrainment of dry air… should also deeper PBL (given identical subsidence) but cloud-top drops less entrainment!

10. © Crown copyright Met Office LH flux – entrainment feedback? MetO LEM fluxes after 3 days LH flux still larger throughout PBL, despite lower inversion (larger Δq must dominate reduced w e ) Buoyancy flux weaker (consistent with reduced w e ) But why reduced w e, to less than in the control?

11. © Crown copyright Met Office Initial adjustment Entrainment is larger initially (<first 6 hours), driven by larger buoyancy flux, as expected cloud thins (+ve feedback), as expected BUT cloud becomes optically thinner in LW too: reduced LW cloud-top cooling reduced buoyancy flux reduced turbulence reduced entrainment cloud-top falls back (relative to control)

12. © Crown copyright Met Office Balanced state So, despite enhanced LH fluxes at all levels, SH flux is reduced because of reduced LW cooling implying reduced buoyancy flux Total Turb Rad

13. © Crown copyright Met Office What if? …the initial s12 cloud layer were deeper? Same initial response (larger w e higher cloud-top and thinner cloud layer)? Noting that cloud depth has a strong control on the integrated buoyancy flux, thinner cloud layer still implies w e reduces but not now to less than control? So equilibrium state would still be a thinner cloud layer (ie positive feedback) but with a higher cloud-top? Chris Jones has tested this in a MLM…

14. © Crown copyright Met Office Initial conditions dont matter!

15. © Crown copyright Met Office What if? What if the circulation changes? Eg, the shape of the w pdf changes? HadGEM2 shows no change in the mean but less ascent and more weak subsidence If w>0 associated with small cloud fraction then this might suggest a negative cloud feedback (but this doesnt happen in HadGEM2) Something to try in time-varying CGILS… Eg, the wind speed changes? (See Webb and Lock, soon!) s12