© Crown copyright Met Office Some thoughts on s12 stratocumulus feedback Adrian Lock EUCLIPSE WP3...

© Crown copyright Met Office

Some thoughts on s12 stratocumulus feedback

Adrian LockEUCLIPSE WP3 meeting, Toulouse, April 2012


Subsidence pdfs: HadGEM2 vs CGILS

s11

s12

Mean w unchanged


Why do the LES robustly show a significant positive feedback at s12 when subsidence unchanged?


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


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, T1-Tsurf):

LHF = ch(q1-qsat(Tsurf)) ~ ch(RH1qsat(T1)-qsat(Tsurf))

3) Δq more negative (larger RH in the PBL dominates larger dqsat/dT in warm free atmosphere):

(2)+(3) → if entrainment rate unchanged, expect stronger LH fluxes throughout PBL:

0

dT

dqRH

dT

dqRH

dT

qd PBLsatPBL

FAsatFA

LH flux


Impact of enhanced LH fluxes

• Buoyancy flux = α wθl + β wqT

• wqT term is most important in the cloud layer (where β is much larger)

• Larger wqT → larger buoyancy flux → more turbulent PBL → more entrainment of dry air → thinner cloud → positive cloud feedback

• Do the steps of this argument hold up?


LH flux – entrainment feedback?

• Larger wqT → 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!


LH flux – entrainment feedback?MetO LEM fluxes after 3 days

• LH flux still larger throughout PBL, despite lower inversion (larger Δq must dominate reduced we)

• Buoyancy flux weaker (consistent with reduced we)

• But why reduced we, to less than in the control?


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)


Balanced state

• So, despite enhanced LH fluxes at all levels, SH flux is reduced because of reduced LW cooling implying reduced buoyancy flux

Total

TurbRad


What if?

• …the initial s12 cloud layer were deeper?

• Same initial response (larger we → 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 we 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…


Initial conditions don’t matter!


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 doesn’t happen in HadGEM2)

• Something to try in time-varying CGILS…

• Eg, the wind speed changes? (See Webb and Lock, soon!)

s12

© Crown copyright Met Office Some thoughts on s12 stratocumulus feedback Adrian Lock EUCLIPSE WP3...

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