date: Thu, 20 Mar 2008 16:38:39 +0000 from: Tim Osborn subject: Re: further CRUTS2.1 vs 3.0 comparisons to: Phil Jones ,Ian Harris 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? 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. 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, Tim Attachment Converted: "c:\eudora\attach\cruts_tmp-0.5dtr_cmp.pdf" Dr 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