reducing uncertainty in the prediction of global warming some pesky cloud obstacles
DESCRIPTION
Reducing uncertainty in the prediction of global warming some pesky cloud obstacles. Brian Mapes doubting reductionist University of Miami. Sources. Radiation: Robin Hogan, ECMWF Ann. Seminar Sep 2008 available on web: presentation and writeup - PowerPoint PPT PresentationTRANSCRIPT
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Reducing uncertainty in the Reducing uncertainty in the prediction of global warming prediction of global warming
some pesky cloud obstaclessome pesky cloud obstacles
Brian MapesBrian Mapesdoubting reductionistdoubting reductionistUniversity of MiamiUniversity of Miami
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Sources• Radiation:
– Robin Hogan, ECMWF Ann. Seminar Sep 2008 – available on web: presentation and writeup
– Consults with live-in radiation guru P. Zuidema• Cloud feedbacks:
– Largely from reading list• J. Clim. reviews by Stephens 2005 and Bony 2006
– Email correspondences and conversations• Bruce Weilicki, NASA Langley (ERB matters)• Larry DiGiralmo, Illinois (some scale issues) • Brian Soden, Miami
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Outline• Preamble: clouds as a climate feedback• A step backward: stating the problem flatly• Integrals: triumphs of atm. RT physics• Now about cloudiness (x,y,z,t)...
– Statistical descriptions from observations– Formulating GCMs to be relatable to above– Tuning
» compensating errors (better than some other kind!) » any help for sensitivity?
• Prospects for understanding cloud changes– in models: so what? – analogues in observations? – via conceptualizations
... a lot is being learned even as uncertainty fails to shrink
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Univariate conceptual model
from Stephens review - critique
• The system The system comprises a whole lot of things• Global mean Ts well defined but how meaningful?
• What is the phys/phil status of a math. average?» e.g. can be acausal (instant across space, nonlocal in time, etc.)
• Relevant for interpreting what parts of Q are “F(Ts)”
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Feedbacks and sensitivity
• ‘base’ negative feedback: ~ -3.2 W m-2 per K– Largely Planck feedback
• -3.8 = d/dT(T4) at global Teff = 255K
• Sensitivity 1/(feedbacks -3.2)
• 0 unstable climate (infinite sensitivity)
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Bony et al. 2006
3.2
Runaway warming!!
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Why are ~all GCM cloud feedbacks positive?
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courtesy of I Heldwho credits B. Soden
Net Net cloud feedbackfrom 1%/ yr CMIP3/AR4simulations
SW
and
LW
clo
ud
feed
back
brighter
in
warm
er world
dark
er
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changes in cloudiness
• IPCC ch 10• The mid-level mid-
latitude decreases are very consistent, amounting to as much as one-fifth of the average cloud fraction simulated for 1980 to 1999.
• Much of the low and middle latitudes experience a decrease in cloud cover, simulated with some consistency. There are a few low-latitude regions of increase, as well as substantial increases at high latitudes.
multimodelnet cloud feedback
Soden Held...2008
wherecloud causesless emission,
darker albedo, or both
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Outline
• Preamble: clouds as a climate feedback• A step backward: stating the problem flatly• Triumphs of atm. RT column physics• Now about cloudiness (x,y,z,t)...
– Statistical descriptions from observations– Formulating GCMs to be constrainable from above– Tuning = right mean answer for nonright reasons
• Prospects for understanding cloud changes– in ensembles of runs of ensembles of GCMs– via conceptualizations
• a lot is being learned as uncertainty fails to shrink
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Radiation budget: a vast integral
• global warming =
• [TOArad] =
∫∫dd ∫d ∫dz ∫∫dR
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knowns
• complete integral ===~ 0 over long integration times– and presumably in preindustrial Holocene
• must be maintained by overall negative feedback– Planck still king
• Cleanly separable into compensating LW and SW halves, each 235 Wm-2 in global mean– equal and opposite– depend on planetary albedo and Temis
• quiz: which was/is easier to guess/ measure?
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Blankly: how do we compute an integral?
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Steps in integrating
micro meso macro
Maxwelleqs.
particleensemble
angleintegral
wavelengthintegral
small-scalestructure
geospace(lat, lon)
seasons,ENSO...
z up to TOA(overlap)
atmospheric radiation physics GCM grid sumssubgrid schemes
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The Scale Problem“macro-” and “micro-” (physics, economics, etc.)
– both intellectually on firm ground, if hard to reconcile
• Micro: – Basic units obey local laws of interaction
• physics: “air parcel” jostlings, thermo– humanities: human nature, drives, responses to stimuli
• Macro: – Whole system constrained by integral laws of conservation
• physics: conservation of mass, energy, momentum– humanity: demographics (fertility, nutrition, etc.).
• Meso: in between: vast, important, but mushy– Only statistics... are they laws, or just descriptions?
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Yechh – take me back to physics
• Preamble: clouds as a climate feedback• A step backward: stating the problem flatly• Triumphs of atm. RT column physics• Now about cloudiness (x,y,z,t)...
– Statistical descriptions from observations– Formulating GCMs to be constrainable from above– Tuning = right mean answer for nonright reasons
• Prospects for understanding cloud changes– in ensembles of runs of ensembles of GCMs– via conceptualizations
• a lot is being learned as uncertainty fails to shrink
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Elementary and rigorous
• Maxwell’s equations» (from Robin Hogan, Reading U, ECMWF seminar 2008)
• Now just integrate all energy over all matter!
http://www.met.rdg.ac.uk/clouds/maxwell/
total “CRF”
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From particles to continuumMaxwell E,B for .
ensemble
Robin Hogan ECMWF Seminar 2008Key bulk variable: Extinction (units: m-1)
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From particles to continuum
• Bulk variable: Extinction (units: m-1)– Shortwave: ~all scattering, ~no absorption
• proportional to cross section (condensate volume(condensate volume/r/ree))– re is “effective radius” (3rd moment/2nd moment of DSD)
– Longwave: mostly absorption (& emission)• proportional to condensate volume (mass)condensate volume (mass)
– no rno ree (droplet size) dependence! (droplet size) dependence!– typically ~2 times greater than SW scattering extinction
Maxwell E,B for .
ensemble
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Angle integralMaxwelleqs.
particle ensemble
angleintegral
Robin Hogan ECMWF Seminar 2008
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Wavelength integral can be done
• Complicated for gases but – Yields to precision
laboratory (controlled) empiricism
• leveraged with physics– Captured/ simplified in
clever bundling• ‘bands’ of abs. coeff. k
– Tuned up with final broadband empirical calibrations
• clouds mercifully gray
Maxwelleqs.
particle ensemble
angleintegral
wavelengthintegral
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“Unreasonable” assumptionsMaxwell
eqs.particle
ensembleangle
integral
Robin Hogan ECMWF Seminar 2008
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Unbiased vs. accurate
• The vastness vastness of our integral can be useful
– don’t need the integrand accurate and complete– merely need a sufficiently large and unbiased
sample, of an unbiased estimator of it!
– Example: McICA radiation • Independent Column Approximation (ICA)• Monte Carlo (Mc) treatment of wavelength integral
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Locally wrong, but unbiased• ICA:
– Neglect hor. photon flux Fhor (3D effects)– Wrong alm. ev. in inhomogeneous clouds– But unbiased unbiased since
• MC:– Send different wavelength bands through each
subgrid cloud overlap realization– Unbiased, and large-enough subsample of vast 2D
space • (even for weather forecasts)
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Hooray for atmospheric physics!
∫∫∫∫∫∫∫ R (longlived GHGs,
T, qv, aerosol,
qcond, phase, re)
Maxwelleqs.
particle ensemble
angleintegral
wavelengthintegral
small-mesostructure
geo-space(lat, lon)
seasonsENSO...
z up to TOA(overlaps)
Now for the problem of space-time integration...
(x,y,z,t)(x,y,z,t)
(x,y,z,t)(x,y,z,t)
(x,y,z,t)(x,y,z,t)
Nice solid rules and tools!Nice solid rules and tools!(who remembers “anomalous absorption” ?)(who remembers “anomalous absorption” ?)
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Outline
• Preamble: clouds as a climate feedback• A step backward: stating the problem flatly• Triumphs of atm. RT column physics• Now about cloudiness (x,y,z,t)...
– Statistical descriptions from observations– Formulating GCMs to be constrainable from above– Tuning = right mean answer for nonright reasons
• Prospects for understanding cloud changes– in ensembles of runs of ensembles of GCMs– via conceptualizations
• a lot is learned even as uncertainty fails to shrink
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THE PROBLEM OF SCALES THE PROBLEM OF SCALES in practice, (x,y,t) is really in practice, (x,y,t) is really
(x,y,t,(x,y,t,scalesscales))
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Almost without limit...
but remember the independent
column approximation!
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For a cloudy column, 2 things matter2 things matter: Emission temperature, and albedo
the ISCCP 2D space for
characterizing cloudy columns
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net CRF in that space
• Kubar et al. (2007)
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Project any cloud population (joint histogram in this space), and sum to get total CRF...presto!
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Can do for any set of cloudy pixels – like these ‘cloud population regimes’
• from a cluster
analysis (aka self-
organizing maps) of
daily 5 degree joint histograms in the ISCCP
2-space
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Outline
• Preamble: clouds as a climate feedback• A step backward: stating the problem flatly• Triumphs of atm. RT column physics• Now about cloudiness (x,y,z,t)...
– Statistical descriptions from observations– Formulating GCMs
• Prospects for understanding cloud changes– in ensembles of runs of ensembles of GCMs– via conceptualizations
• a lot is being learned even as uncertainty fails to shrink
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Stuck with 3D models
• The vastness vastness of our integral can be a pain pain too
• We only have laws to predict cloudiness in We only have laws to predict cloudiness in 3D3D– where air saturates, fundamentallywhere air saturates, fundamentally– where air almost-saturates, for scale-truncated where air almost-saturates, for scale-truncated
fluid dynamics/ thermodynamicsfluid dynamics/ thermodynamics• implying cloud in unresolved smaller-scale fluctuationsimplying cloud in unresolved smaller-scale fluctuations
• GCMs are stuck integrating partly-cloudy GCMs are stuck integrating partly-cloudy radiation over zradiation over z
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First off, cloud water is a precision nightmare
• only a few % of the water is condensed
0.3 60mm = kg/m2
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Problems with sums and integrals • The radiative impact of a local volume of cloudiness
is highly nonlinear• so it matters what’s above/below
condensate path/re
LW
SWopacity
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courtesy Robin Hogan
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courtesy Robin Hogan
Magic number
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courtesy Robin Hogan
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courtesy Robin Hogan
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courtesy Robin Hogan
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courtesy Robin Hogan
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Yet all these effects are secondary!
• (cloud fraction at each model level, condensed water at each model level)
courtesy Robin Hogan
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Outline
• Preamble: clouds as a climate feedback• A step backward: stating the problem flatly• Triumphs of atm. RT column physics• Now about cloudiness (x,y,z,t)...
– Statistical descriptions from observations– Formulating GCMs – tuning
• Prospects for understanding cloud changes– in ensembles of runs of ensembles of GCMs– in observations– via conceptualizations
• a lot is being learned even as uncertainty fails to shrink
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Tuning
• GCM cloudy radiation is tuned– by several or 10s of Watts, I think– to have net flux =0 for preindustrial control climate– in each latitude belt– to have right SW and LW individually?
(somebody correct me if wrong?)
Does this constrain sensitivity? no such luck
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Back to the integral -- tolerances
Global warming is driven by the imbalance [∫∫∫∫∫∫swR - ∫∫∫∫∫∫LwR] <1 Wm<1 Wm-2-2 out of 235.
Hansen et al. 2004Science Express
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Outline
• Preamble: clouds as a climate feedback• A step backward: stating the problem flatly• Triumphs of atm. RT column physics• Now about cloudiness (x,y,z,t)...
– Statistical descriptions from observations– Formulating GCMs to be constrainable from above– Tuning
• Prospects for understanding cloud changes– in models– in obs– via conceptualizations
• a lot is being learned even as uncertainty fails to shrink
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What could change systematically with climate?
• Large-scale cloud coverage?– say low cloud incr. due to static stability increase?
– Miller 1997 negative feedback» but see Wood-Bretherton 2006 EIS recasting
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What could change with climate?
• Small-scale cloud fraction at a given altitude? – if vigor/ variance of w fluctuations changed?
• in a globally systematic way– say via increased static stability
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What could change with climate?• Large-scale cloud coverage
– say low cloud incr. due to static stability increase– Miller 1997 negative feedback
» but see Wood-Bretherton 2006 EIS recasting– or say high cloud changes (anvil T or thickness)
• Small-scale cloud fraction at a given altitude? – if vigor/ variance of w fluctuations changed?
• in a globally systematic way– say via increased static stability
• Effective radius– aerosol indirect effects – inadvertent or engineered!
• Overlap issues– say by shear...
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All connected
• Dreaming up individual “possible effects” is not terribly fruitful
• Local effects tend to fade in the global average– systematic or random cancellation?
• There is a hunger for whole-system results
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Careful with “hunger”
• Global mean well defined but how meaningful?• What is the phys/phil status of a math. average?
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Conceptual connection/ closure
• example
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Begat quantitative work (n-box model)(Bony and Dufresne, Wyant et al.)
500
long time
means
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SW effects of low clouds under subsidence
are the key to climate sensitivity uncertainty
Bony and Dufresne 2006
in GCMs
high sens models
low sens models
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But is stratocu subsidence
really so linked to the
ascending tropics?
• field impressions: connected to midlatitude influences more than tropical?
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Need whole-planet understandings
(General Circulation), not
just slice cartoons
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very recent obs constraint attempt
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decadal, in a box off california
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tied to subsidence and stability
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consistent satellite & surface obs
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analogue to global warming?
• The hard part of GW is the area average, not the time scale
• Patchy decadal variability seems to me no better than patchy interannual variability (or seasonal, or...) – all infinitely slow relative to cloud time scales– connected by circulation to compensating (or at
least complicating) changes elsewhere
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One special model
• NICAM – a 7km mesh globally– and 50+ vertical levels
• Verrrrry expensive
• Climate sensitivity estimated by SST+2 run
• Result:
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positive cloud feedback... but dominated by LW effects of high-thin cloud incr., unlike GCMs!
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Summary• Cloud changes are a positive feedback in GCMs
– LW: reduced emission in all (via high clouds?)• like in NICAM 7km mesh model but weaker?
– SW: variable among models• low clouds in subsiding regions are key (Bony and Dufresne)
• Prospects for fundamental GCM accuracy seem dubious• RT physics is good, but 3D cloudiness and overlap seems a quagmire
– still lots of effort is being expended!
• Prospects for understanding are better– slicing and dicing GCMs is actually informative
• if not “reducing uncertainty” exactly– especially outside model-attuned science community!
– conceptualization is still important and not cemented yet• an interesting time in any science
– if it turns out to be a science
• Reducing Uncertainty in GW as a Big Problem: I sure hope the paleo/ holistic constraints are stronger!