Tuesday, December 13, 2011

1078236401.txt

From: Phil Jones <p.jones@uea.ac.uk>
To: Ben Santer <santer1@llnl.gov>
Subject: Re: [Fwd: More PCM-ERA40 comparisons]
Date: Tue Mar 2 09:06:41 2004

Ben,
Thanks for the plots and keeping me up to date. The ERA-40/CRU comparisons
are quite interesting. I'm hopeful Adrian will write up a summary for publication in
addition
to an ECMWF report.
This sort of thing is important wrt IPCC and also papers such as Kalnay and Cai.
I'm also working with Russ Vose and others at NCDC to get a comparison of CRU/GHCN
and NASA datasets in GRL. NCDC have used their first difference technique with CRU
data. Differences are very, very small due to data and the technique doesn't matter much
either. All seems to boil down to how the global average is defined. Calculated as one
domain as NCDC (and until recently the HC as well) want to do it, it is biased to the NH.
If you do it the CRU way (G=0.5(NH+SH)) then it looks much more like an OA version
of HadCRUT2v that the HC have just produced. Been saying this for years as has Tom,
so no surprises. Finally got the HC to realise it, now just need to convince NCDC.
NCDC will also have a new 5 by 5 deg gridded dataset of Tx and Tn soon, right up to
the present. Need to compare this with ERA-40.
Cheers
Phil

At 18:46 01/03/2004 -0800, you wrote:

Dear Phil,
Here are the PCM/ERA-40 2m temperature comparisons that I mentioned in my email
to Adrian....
Cheers,
Ben
--
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Benjamin D. Santer
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Lawrence Livermore National Laboratory
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Livermore, CA 94550, U.S.A.
Tel: (925) 422-7638
FAX: (925) 422-7675
email: santer1@llnl.gov
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Date: Thu, 26 Feb 2004 18:03:00 -0800
From: Ben Santer <santer1@llnl.gov>
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To: Adrian.Simmons@ecmwf.int, wmw@ucar.edu, meehl@ucar.edu, wigley@ucar.edu,
ammann@ucar.edu
Subject: More PCM-ERA40 comparisons
References: <403B1219.4060905@ecmwf.int>
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Dear Adrian,
Thanks very much for sending me your comparison of surface air temperature
changes in CRU and ERA-40. I've been looking at a related issue - the
correspondence between 2m temperature changes in ERA-40 and PCM.
Here's the background to this work. Increasingly, there is some interest in the
problem of identifying anthropogenic climate change at regional scales. I have
to give a brief talk on this subject tomorrow. In preparing for this talk, I
decided that it would be useful to show how signal and noise change as a
function of spatial scale. I looked at the behavior of 2m temperature in the
four individual realizations of the PCM "ALL forcings" experiment (the same
experiment that we analysed in our joint Nature paper). For each realization, I
computed spatial averages over the globe, the Northern Hemisphere, and the
western United States (30-50N, 126W-114W). These spatial averages were then
expressed as anomalies relative to climatological monthly means over 1979-1999.
The orange shading in the three panels of the figure entitled "tas_tseries3.ps"
is a measure of the between-realization variability in PCM. The envelope is
simply the range (during any given month) between the maximum and minimum values
of the four realizations. This range was then low-pass filtered. The solid red
is the low-pass filtered ensemble mean.
To facilitate comparison with PCM data, I've defined 2m temperature anomalies in
ERA-40 in the same way (i.e., relative to climatological monthly means over
1979-1999), and have used the same low-pass filter. One can then ask whether the
2m temperature changes in ERA-40 are consistent with those in PCM - in other
words, are they encompassed by PCM's envelope of possible climate responses to
combined anthropogenic and natural forcing?
They are. Surprisingly, this consistency occurs not only at the global-mean
level, but also for the NH and western U.S. For the global-mean and the NH, the
ERA-40 2m temperature changes are outside PCM's envelope of 2m temperature
changes during the first 5-10 years of the reanalysis. After the late 1960s,
however, the ERA-40 2m temperature changes are entirely consistent with those in
PCM. Over the western U.S., 2m temperature changes in PCM and ERA-40 are
consistent throughout the reanalysis period.
Such qualitative consistency, while interesting, is no substitute for formal,
pattern-based fingerprint detection studies at global, hemispheric, and regional
scales. For example, an overestimate of the regional-scale variability of 2m
temperature by PCM could explain why PCM's 2m temperature changes over the
western U.S. fully encompass the ERA-40 result (see panel C). On the other hand,
there is some real similarity in the low-frequency component of the 2m
temperature changes in ERA-40 and PCM (look at the similar responses to Agung,
Chichon, and Pinatubo in panel B!)
The bottom line is that PCM's 2m temperature changes are reasonably consistent
with those in ERA-40, even at sub-global spatial scales. This suggests that
formal regional-scale detection work might be useful. If you are interested,
perhaps we could collaborate on such work. A collaboration would also involve
the PCM group at NCAR (to whom I'm copying this email).
The second figure that I've appended shows the global-mean changes in synthetic
MSU channel 2 temperatures in PCM and ERA-40. The message is pretty much the
same as for 2m temperatures: PCM's "envelope" of possible changes in
tropospheric temperatures largely encompasses the ERA-40 results, except during
a few large El Nino and La Nina events. Once again, there is surprising
similarity in the low-frequency component of the model and reanalysis T2
changes.
It would be fun to take these simple comparisons a little further!
With best regards,
Ben
--
----------------------------------------------------------------------------
PCMDI HAS MOVED TO A NEW BUILDING. NOTE CHANGE OF MAIL CODE!
Benjamin D. Santer
Program for Climate Model Diagnosis and Intercomparison
Lawrence Livermore National Laboratory
P.O. Box 808, Mail Stop L-103
Livermore, CA 94550, U.S.A.
Tel: (925) 422-7638
FAX: (925) 422-7675
email: santer1@llnl.gov
----------------------------------------------------------------------------

Prof. Phil Jones
Climatic Research Unit Telephone +44 (0) 1603 592090
School of Environmental Sciences Fax +44 (0) 1603 507784
University of East Anglia
Norwich Email p.jones@uea.ac.uk
NR4 7TJ
UK
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