date: Fri, 7 Jul 2006 15:28:19 +0100
from: "Tett, Simon" <simon.tettatXYZxyzoffice.gov.uk>
subject: RE: Attaching All250 to Nat500 runs
to: "Tim Osborn" <t.osbornatXYZxyz.ac.uk>, "Jonathan Gregory" <j.m.gregoryatXYZxyzding.ac.uk>
I have data from a run with the correct land-sfc fields. We can
see if there is a difference in the global sea-level in that run
relative to a 100 year period from Natural500.
Dr Simon Tett Managing Scientist, Data development and applications.
Met Office Hadley Centre (Reading Unit)
Meteorology Building, University of Reading, Reading RG6 6BB
Tel: +44 (0)118 378 5614 Fax +44 (0)118 378 5615
Mobex: +44-(0)1392 886886
E-mail: simon.tettatXYZxyzoffice.gov.uk http://www.metoffice.gov.uk
Global climate data sets are available from http://www.hadobs.org
From: Tim Osborn [mailto:t.osbornatXYZxyz.ac.uk]
Sent: Friday, July 07, 2006 2:39 PM
To: Jonathan Gregory; Tett, Simon
Cc: Alex Wright; Orson van de Plassche; Lowe, Jason; Keith Briffa
Subject: Re: Attaching All250 to Nat500 runs
At 17:07 06/07/2006, Jonathan Gregory wrote:
> > Yes! switch from Nat500 to All250 in 1750.
> > Simon
>Yes, I agree.
OK, so we (Alex and I) can now drop the adjustment of means for
comparing/plotting the all250 as a smooth branch away from the nat500
for the local sea level curves that I sent to Alex. It still makes
sense though to use a common reference period for comparing different
locations, so the subtraction of the 1500-1549 mean of nat500 from both
nat500 and all250 would be appropriate (please say if you disagree
Jonathan/Simon or anyone else).
For plotting the global curves an adjustment must still be made to make
all250 branch smoothly off from nat500 in the year 1750 because the
global-mean thermal expansion file that I was sent has values near -4 mm
at the start of all250, but +4 mm around 1750 in nat500. If this
adjustment is not made, then all250 does not branch off smoothly from
nat500 in 1750, but has an 8mm jump. I don't know
*why* this adjustment must be made because, as both Simon and Jonathan
said, all250 begins from nat500 in 1750. But the version of the data
that I was sent has a very clear difference in mean levels between the
two runs in this period.
> > if the "rigid_lid_pressure" local
> > sea level deviations data are at consistent absolute levels (are
> > they, Jason?),
>The global average of those fields should be zero by construction, and
>if not it is meaningless anyway and should be subtracted (as you did).
Unfortunately the global average of these fields is nowhere near zero,
as I queried earlier with Jason. Please see attached global-mean of the
rigid_lid_pressures. It varies quite considerably. The range is about
800 hPa, or 80 mm after conversion, and the mean is about 6000 hPa, or
600 mm. I just subtracted this global-mean series from each grid box
series. I'm pretty sure that this was the right thing to do, but I was
worried by the unexpected magnitude of the mean and variation of this
global-mean series. Can you reassure me that it's ok?
I appreciate that you all have other things on your plates and also
appreciate your time spent helping with this sea level stuff. All I
need is to get to the point where we are confident in going ahead and
using this data without worrying that we will have to redo our analyses
if it turns out something is wrong later. I think we're virtually at
that point, and then I can stop bugging you all!
Cheers and thanks for your help,