cc: "Kennedy, John" <john.kennedy@metoffice.gov.uk>
date: Wed, 09 Apr 2008 13:35:16 +0100
from: David Parker <david.parker@metoffice.gov.uk>
subject: Re: Fwd: problem with trends in Europe in HadCRUT3
to: "Jones, Phil" <p.jones@uea.ac.uk>

Phil

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.

David


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.
> 
>   Cheers
>   Phil
> 
> 
> 
> 
> >Date: Wed, 09 Apr 2008 10:27:46 +0200
> >From: Geert Jan van Oldenborgh <oldenbor@knmi.nl>
> >Organization: KNMI
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> >To: Phil Jones <p.jones@uea.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.jones@uea.ac.uk
> NR4 7TJ
> UK 
> ----------------------------------------------------------------------------                                                                                 
-- 
David Parker   Met Office Hadley Centre   FitzRoy Road EXETER EX1 3PB UK
E-mail: david.parker@metoffice.gov.uk
Tel: +44-1392-886649  Fax: +44-1392-885681  http:www.metoffice.gov.uk
