date: Tue Feb 1 10:45:41 2005 from: Keith Briffa subject: Re: Dai's PDSI dataset to: "Femmie en Gerard van der Schrier" OK- just talked to Phil and he says the calibration period is probably 1951-80 - hence the changing range (and means at zero) but it is still apparent that there is great asymmetry in the extreme values, much wetter than drier, and perhaps this is a regional bias (poor data in some ares for calibration?) . These extreme values may all lie in ares of poor data coverage .The comparisons with our European (and the US) grid will probably be much better behaved. Apparently Dai (and Trenberth) are making much of the declining trend in recent years, but this Figure shows that this is the result of less wet rather than more dry - probably an important point in terms of impact. Keith Gerard I agree that this looks very much like a post-analysis truncation . What geographic area is represented by these numbers? What seems even more strange though, is the inhomogeneity at around 1956 , and perhaps between 1956 an 1980. This surely is not the result of a natural circulation related phenomenon. Have you tried comparing any of his previuos extreme maps against latest ones (and yours in overlap region)? You could plot PDF for particular dry and wet years to see if there is a sharp truncation at pdsi values of +or _ 15. Give me a ring (and I wil call you back) Keith At 07:21 01/02/2005, you wrote: Keith, It seems that Dai et al. have managed to get rid of the 'spikes' in their latest PDSI dataset (the one published in J. Hydrometeorology, 2004). At a cost though. Attached is a figure of the mean, the minimum value and the maximum value of the PDSI for each of their maps (horizontal in the figure is time in months, starting in January 1870). It seems to me that the max. and min. values are artificially bounded between +15 and -15. What do you think? My guess is that they, either during the PDSI calculations or as a post-processing routine, reset the largest PDSI values to be within +/-15. Cheers, Gerard -- Professor Keith Briffa, Climatic Research Unit University of East Anglia Norwich, NR4 7TJ, U.K. Phone: +44-1603-593909 Fax: +44-1603-507784 [1]http://www.cru.uea.ac.uk/cru/people/briffa/