cc: "Phil Jones" , "Tim Osborn" date: Mon, 24 Mar 2008 14:28:54 -0000 (GMT) from: P.Jones@uea.ac.uk subject: Re: further CRUTS2.1 vs 3.0 comparisons to: "Ian Harris" Harry (and Tim), OK. We now know what to do. There are two reasons why we need Tmp to be the average of Tx and Tn. People expect it, and it makes use of better data. 1. Tx and Tn have had hardly any long-term homogeneity checks. Tmp has in work going back to the 1980s. 2. Tmp series are much longer than Tx and Tn especially outside Europe, N. America and Australia. If we'd wanted to improve Tmp we should have added series of (Tx+Tn)/2 for places where Tmp wasn't available. Anyway, this has given me a few ideas for diagrams for the paper. Things aren't always better through using more data. Cheers Phil > Hi, > > I did indeed follow a different process from 2.10. This was remiss, but > can be explained. > > Firstly, I don't really think the published papers reflect the > actualité of how the dataset was produced. Neither do the read me > files. They contain elements of truth but there are glaring > inaccuracies (including the gridding method, possibly the most > important single issue). For this reason I've not been as slavish to > them as I should have been. > > Secondly, the tmin and tmax databases are very good. I've removed many > duplicates and 'badly augmented' stations and the two databases I used > are completely aligned. I think that if stations are measuring a > parameter then that's the truth of the matter (conceding the > complications arising from measurement times). > > There are climatologies for DTR, TMN and TMX. This also pushed me > towards treating them as primary parameters. > > I derived a DTR database from the TMN and TMX databases, and gridded > all three. That's what you're playing with. > > If you want TMN and TMX to be derived from TMP and DTR, so be it > (though it seems superfluous since it's simply derived). > > The problem comes when one considers this process - when we are talking > about TMN and TMX as theoretical constructs that may not have existed > at all. > > The TMN, TMX (and therefore DTR) databases each have 13654 stations. > Even if they require further cleaning, and I'm sure that they do, > that's still a lot more than TMP. > > Incidentally, sorry but I don't really follow this deduction: > >>> Since TMN and TMX are both correlated with TMP, the v2.1 method is >>> clearly the right way to go. > > When either are correlated with TMP, then the strength of that > correlation is of great interest, isn't it? But the approach of TMN = > TMP - DTR/2, TMX = TMP + DTR/2 just creates dummies with a correlation > of 1.0. And although the correlation between TMP and DTR is very > important, DTR is not the only information contained in the TMN and TMX > databases. > > Cheers > > Harry > > On 21 Mar 2008, at 18:17, P.Jones@uea.ac.uk wrote: > >> >> Tim, >> As far as I'm concerned, the T fields that get gridded >> are Tmean and DTR. It might be that the climatology >> is for Tmean, Tx and Tn. If this is the case then we >> need a DTR climatology. >> >> Then what you say below is how I would calculate as you say. >> >> Hopefully this is what Harry has done. >> >> It might be that Harry has done a Tx and Tn gridding as the >> two datasets aren't exactly the same - so you lose a bit of >> data getting DTR, but it is very, very small. >> >> Cheers >> Phil >> >> >> Phil -- if Harry says "yes", then will I get exactly the "correct" >>> result if I ignore the TMN and TMX files that Harry's made and >>> instead make my own from TMP and DTR, using: >>> >>> TMN=TMP-0.5DTR and TMX=TMP+0.5DTR? >>> >>> I tested this and now get much better correlations with v2.1 for TMN, >>> and presumably (not checked yet) for TMX. The standard deviations >>> are now much more similar too. >>> Here's the comparison when I make my own TMN from TMP and DTR for >>> v3.0 and compare with TMN from v2.1. >>> >>> Tim >>> >>> -------------------------- >>> Phil & Harry, >>> >>> Next problem! : >>> >>> I've now made comparisons for TMN, TMX and DTR for v3.0 vs. v2.1. >>> >>> Attached is the result for TMN. You'll see we now have 4 plots per >>> month, for Jan-Apr-Jul-Oct. Plot 1 is the temporal >>> correlation. Plot 2 is the SD of v2.1. Plot 3 is the ratio of v2.1 >>> SD to v3.0 SD. Plot 4 is the SD of v3.0. >>> >>> Clearly the correlation is rather weak in many areas and the ratio of >>> standard deviations exceeds 1.5 across most of S America, Africa and >>> India, plus Greenland and N. Russia. >>> >>> I think that this may be due to the way you've made the data. I've >>> checked Mitchell and Jones (2003) and it says very clearly that TMN >>> (and TMX) are derived variables, taken entirely from the grids of TMP >>> and DTR (presumably TMN = TMP - 0.5*DTR and TMX = TMP + 0.5*DTR???). >>> >>> Now if you haven't followed this approach for making v3.0 but have >>> instead "independently" made gridded fields of TMP, DTR, TMN and TMP, >>> relaxing to climatology where there are no nearby observations, so >>> that they are all primary variables and none are derived, then if you >>> have TMP data but not DTR, TMN or TMX values, the latter 3 will be >>> relaxed to climatology. But the v2.1 methodology would relaxed DTR >>> to climatology, but TMN would be TMPactual - 0.5*DTRclimatology, and >>> hence would still have variations that paralleled the observed >>> variations in TMPactual. >>> >>> Since TMN and TMX are both correlated with TMP, the v2.1 method is >>> clearly the right way to go. The only time when something different >>> to both approaches might be useful is if you have lots of >>> stations/months with only TMN or TMX but not both. But Harry says >>> that you generally have both or none. In which case v2.1 will be the >>> best we can do. >>> >>> Harry -- now that I've confirmed what Mitchell & Jones did to make >>> v2.1, can you confirmed that you have made v3.0 in the way I described >>> above? >>> >>> I'll attach the plot with the next >>> email (too big for this one!). >>> >>> Given the time and effort I've put in to CRU TS 3.0, I shall expect >>> to be a co-author when the paper describing CRU TS 3.0 is written! >>> >>> Cheers and happy Easter, >>> >>> TimDr Timothy J Osborn, Academic Fellow >>> Climatic Research Unit >>> School of Environmental Sciences >>> University of East Anglia >>> Norwich NR4 7TJ, UK >>> >>> e-mail: t.osborn@uea.ac.uk >>> phone: +44 1603 592089 >>> fax: +44 1603 507784 >>> web: http://www.cru.uea.ac.uk/~timo/ >>> sunclock: http://www.cru.uea.ac.uk/~timo/sunclock.htm >>> >>> >> >> > >