date: Tue Feb 1 10:45:41 2005
from: Keith Briffa <k.briffaatXYZxyz.ac.uk>
subject: Re: Dai's PDSI dataset
to: "Femmie en Gerard van der Schrier" <fem-gerardatXYZxyznet.nl>
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.
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.