cc: "Kennedy, John" date: Wed, 09 Apr 2008 13:35:16 +0100 from: David Parker subject: Re: Fwd: problem with trends in Europe in HadCRUT3 to: "Jones, Phil" 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 > >Organization: KNMI > >User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.13) > >Gecko/20080313 SeaMonkey/1.1.9 > >To: Phil Jones > >Subject: problem with trends in Europe in HadCRUT3 > >X-OriginalArrivalTime: 09 Apr 2008 08:27:46.0989 (UTC) > >FILETIME=[971995D0:01C89A1B] > >X-Spam-Score: undef - message too big (size: 597818, limit: 153600) > >X-CanItPRO-Stream: UEA:f028 (inherits from > >UEA:10_Tag_Only,UEA:default,base:default) > >X-Canit-Stats-ID: Bayes signature not available > >X-Scanned-By: CanIt (www . roaringpenguin . com) on 139.222.131.184 > >X-UEA-Spam-Score: 0.0 > >X-UEA-Spam-Level: / > >X-UEA-Spam-Flag: NO > > > >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