cc: John Kennedy <john.kennedy@metoffice.gov.uk>, philip.brohan@metoffice.gov.uk, david.parker@metoffice.gov.uk
date: Wed Apr  9 12:48:40 2008
from: Phil Jones <p.jones@uea.ac.uk>
subject: Fwd: problem with trends in Europe in HadCRUT3
to: Geert Jan van Oldenborgh <oldenbor@knmi.nl>

    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
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