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

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Presentation transcript:

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

© Crown copyright Met Office

Subsidence pdfs: HadGEM2 vs CGILS s11 s12 Mean w unchanged

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

© 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

© 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

© 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?

© 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!

© 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?

© 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)

© 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

© 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…

© Crown copyright Met Office Initial conditions dont matter!

© 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