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date: Wed, 02 Jan 2008 10:08:31 +0000
from: Peter Thorne <peter.thorneatXYZxyzoffice.gov.uk>
subject: Re: Douglass et al. paper
to: Susan Solomon <Susan.SolomonatXYZxyza.gov>
Susan et al.,
I had also seen the Forster et al paper and was glad to see he had
followed up on work and ideas we had discussed some years ago when he
was at Reading and from the Exeter workshop. At the time I had done some
simple research on whether the stratosphere could affect the tropical
troposphere - possibly through convection modification or radiative
cooling. I'd done a simple timeseries regression of T2LT=a*Tsurf+b*T4+c
and got some regression coefficients out that suggested an influence.
Now, this was with old and now discredited data and the Fu et al.
technique has since superseded it to some extent (or at least cast
considerable doubt upon its efficacy) ... it would certainly be hard to
prove in a regression what was cause and effect with such broad
weighting functions even using T2LT which still isn't *really*
independent from T4.
But one thing I did do to try to "prove" the regression result was real
is take the composite differences between QBO phases on 45 years of
detrended (can't remember exactly how but I think I took differences
from decadally filtered data) data from radiosondes (HadAT1 at the
time). This showed a really very interesting result and suggested that
this communication if it was real went quite far down in to the
troposphere and was statistically significant, particularly in those
seasons when the ITCZ and QBO were geographically coincident. I attach
the slide for interest. I think this is the only scientifically valid
part of the analysis that I would stand by today given the rather
massive developments since. I doubt that raobs inhomogeneities could
explain the plot result as they project much more onto the trend than
they would onto this type of analysis.
The cooling stratosphere may really have an influence even quite low
down if this QBO composite technique is a good analogue for a cooling
startosphere's impact, and timeseries regression analysis supports it in
some obs (it would be interesting to repeat such an analysis with the
newer obs but I don't have time). A counter, however, is that surely the
models do radiation so those with ozone loss should do a good job of
this effect. This could be checked in Ben's ensemble in a poor man's
sense at least because some have ozone depletion and some don't.
The only way this could be a real factor not picked by the models, I
concluded at the time, is if models are far too keen to trigger
convection and that any real-world increased radiative cooling
efficiency effect is masked in the models because they convect far too
often and regain CAPE closure as a condition.
On another matter, we seem to be concentrating entirely on layer-average
temperatures. This is fine, but we know from CCSP these show little in
the way of differences. The key, and much harder test is to capture the
differences in behaviour between layers / levels - the "amplification"
behaviour. This was the focus of Santer et al. and I still believe is
the key scientific question given that each model realisation is
inherently so different but that we believe the physics determining the
temperature profile to be the key test that has to be answered. Maybe we
need to step back and rephrase the question in terms of the physics
rather than aiming solely to rebutt Douglass et al? In this case the key
physical questions in my view would be:
1. Why is there such strong evidence from sondes for a minima at c. 500?
Is this because it is near the triple point of water in the tropics? Or
at the top of the shallow convection? Or simply an artefact? [I don't
have any good ideas how we would answer the first two of these
2. Is there really a stratospheric radiative influence? If so, how low
does it go? What is the cause? Are the numbers consistent with the
underlying governing physics or simply an artefact of residual obs
3. Can any models show trend behaviour that deviates from a SALR on
multi-decadal timescales? If so, what is it about the model that causes
this effect? Physics? Forcings? Phasing of natural variability? Is it
also true on shorter timescales in this model?
It seems to me that trying to do an analysis based upon such physical
understanding / questions will clarify things far better than simply
doing another set of statistical analysis. I'm still particularly
interested if #2 is really true in the raobs (its not possible to do
with satellites I suspect, but if it is true it means we need to
massively rethink Fu et al. type analysis at least in the tropics) and
would be interested in helping someone follow up on that ... I think in
the future the Forster et al paper may be seen as the more
scientifically significant result when Douglass et al is no longer cared
Happy new year to you all.
Peter Thorne Climate Research Scientist
Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB
tel. +44 1392 886552 fax +44 1392 885681
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