Some thoughts on s12
stratocumulus feedback
Adrian Lock
EUCLIPSE WP3 meeting, Toulouse, April 2012
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Subsidence pdfs:
HadGEM2
vs
s11
Mean w
unchanged
s12
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CGILS
Why do the LES robustly show a
significant positive feedback at s12 when
subsidence unchanged?
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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?
•
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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):
FA
PBL
dq
FA dqsat
PBL dqsat
 RH
 RH
0
dT
dT
dT
(2)+(3) → if entrainment rate unchanged, expect stronger LH
fluxes throughout PBL:
LH flux
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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?
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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!
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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?
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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)
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Balanced state
• So, despite enhanced LH fluxes at all levels,
SH flux is reduced because of reduced LW
cooling implying reduced buoyancy flux
Total
Rad
Turb
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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…
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Initial conditions don’t matter!
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What if?
s12
• 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!)
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