from: Keith Briffa <k.briffaatXYZxyz.ac.uk>
subject: Re: Dai's PDSI dataset
to: "Femmie en Gerard van der Schrier" <fem-gerardatXYZxyznet.nl>
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)
At 07:21 01/02/2005, you wrote:
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.
Professor Keith Briffa,
Climatic Research Unit
University of East Anglia
Norwich, NR4 7TJ, U.K.