cc: "Kennedy, John" <john.kennedyatXYZxyzoffice.gov.uk>
date: Wed, 09 Apr 2008 13:35:16 +0100
from: David Parker <david.parkeratXYZxyzoffice.gov.uk>
subject: Re: Fwd: problem with trends in Europe in HadCRUT3
to: "Jones, Phil" <p.jonesatXYZxyz.ac.uk>
The same problem made our NHem Jan 2008 blended anomaly lower than both
the land-only and the SST-only! It is unlikely to cause problems on
large scales on longer timescales e.g. annual.
On Wed, 2008-04-09 at 12:48 +0100, Phil Jones wrote:
> Geert Jan,
> This stems from the way we combine the land/ocean datasets
> around coastlines.
> In HadCRUT2 we did it according to the area of land/ocean, but not
> letting one
> dominate, so if land or ocean were < 25% it was made at least 25%, with
> the other changed accordingly.
> In HadCRUT3 we did it according to the errors of estimation (see the
> Borhan et al paper). This tends to bias the coastal areas to the
> SSTs as their
> errors of estimation are smaller. This results from the way we deal with
> errors in the land and ocean components.
> Probably neither of these is right all the time, but we will
> reconsider this
> when we think about HadCRUT4 - which is someway off! We might be doing
> something sooner if some improvements to HadSST2 (incorporating new
> SST data from WW1 and WW2) get completed soon.
> Doing what you've done is essentially going back to what was done in
> the earlier version. Combination around coasts has always been a problem.
> >Date: Wed, 09 Apr 2008 10:27:46 +0200
> >From: Geert Jan van Oldenborgh <oldenboratXYZxyzi.nl>
> >Organization: KNMI
> >User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:188.8.131.52)
> >Gecko/20080313 SeaMonkey/1.1.9
> >To: Phil Jones <p.jonesatXYZxyz.ac.uk>
> >Subject: problem with trends in Europe in HadCRUT3
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> >Dear Phil,
> >as you may know I am busy verifying climate models on the observed
> >trend in Europe so far. We submitted a GRL about this a few weeks
> >ago. When cross-checking against station data I found a very
> >curious problem in the HadCRUT3 daatset that we used to represent
> >the real world. In summer, in northern Europe, the trends in
> >HadCRUT3 grid boxes are much higher than the station data (and
> >CRUTEM3) indicate, whereas in southern Europe the reverse happens.
> >Looking at the CRUTEM3 and HadSST2 trends I found large positive
> >trends in HadSST2 in grid boxes that are >90% land, e.g., western
> >France (0-5E, 45-50N) and northern Germany (10-15E,
> >50-55N). Apparently these get a similar or larger weight to the
> >CRUTEM3 data in the same grid box, and thus in HadCRUT3 these grid
> >boxes have large positive trends, which are due to a few SST
> >observations near the beach and down-weigh the large amount of good
> >station data of CRUTEM3. The opposite happens along teh
> >Mediterranean coasts. A weird trend in winter in Finland (20-25E,
> >60-65N) is also due to this effect.
> >As a stop-gap measure I defined my own merged dataset by weighing
> >CRUTEM3 and HadSST2 by the fraction of the grid box covered by land
> >and sea respectively (derived from CRU TS 2.1, so overestimating
> >land a bit). I used this to redo all the plots in the GRL, and will
> >substitute them when I revise it. Fortunately, the main conclusions
> >are not affected.
> >The problem is illustrated in the attached page from my log book, in
> >which I show trends in HadCRUT3 (left), CRUTEM3, HadSST2 and my
> >home-brewn CRUTEM3+HadSST2 combination (right column). The first
> >row shows the trend in the annual mean, next DJF, MAM, JJA and SON
> >on the bottom row.
> >I can understand that you give a disproportional weight to island
> >stations to characterize SST around it, but the opposite seems to
> >give unphysical results in areas with a convoluted coast line. If
> >anyone is working on the weighing for HadCRUT4 this information may
> >be of interest.
> >Greetings from sunny & chilly Holland,
> > Geert Jan
> 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.jonesatXYZxyz.ac.uk
> NR4 7TJ
David Parker Met Office Hadley Centre FitzRoy Road EXETER EX1 3PB UK
Tel: +44-1392-886649 Fax: +44-1392-885681 http:www.metoffice.gov.uk