cc: 'Mike Hulme' <m.hulme@uea.ac.uk>
date: Fri, 26 Mar 2004 11:30:25 -0800 (PST)
from: Nathan Gillett <gillett@ocean.seos.uvic.ca>
subject: RE: gu23wld0098.dat dataset
to: Mark New <mark.new@geography.oxford.ac.uk>

Dear Mark,
Thanks very much for your reply regarding the precip data - and thanks
again for providing the data! I am indeed using an optimal fingerprinting
methodology, and as you suggest, the statistical model does not currently
include an estimate of observational uncertainty. However, I'll certainly
bear this issue in mind - I'm going to talking about some results of this
work at a meeting with Phil Jones, and other people who have worked on the
issue of observational error in detection next month. So if I decide to
take this further I'll get back in contact.

For the record - the precip data contains a detectable volcanic signal,
but not detectable anthropogenic signal, at least using the methodology I
chose. I now have a paper submitted to GRL on the topic - if it's
published I'll send you a reprint (or you can get a preprint from
http://climate.uvic.ca/people/gillett/publications/).

Cheers,

Nathan


> Hi Nathan,
>
> These data are really Mike's baby (that is different to the CRU 0.5
> degree data that I was involved in).
>
> I do however have a comment on using these Mike's gridded data in
> detection/attribution work.
>
> It is important to realise that gridded precipitation data is far more
> sensitive to poor station coverage than temperature and precip.  Many of
> the grid points "with data" in Mike's dataset arise from a single
> station; many others from averaging of only two or three stations; and
> still others from a gradual increase in the number of stations thru
> time.  Thus the variance of the grid-point time-series can be biased by
> these sampling issues, with gridpoints with low station numbers having
> an apparently higher variance.  Similarly, gridpoints with few stations
> are associated with larger standard errors.
>
> Ideally one would want to use an estimate of the sampling error, and to
> adjust for station density dependent variance biases, in the statistical
> model you use for detection/attribution.  I did some preliminary work on
> error estimates a few years ago, based on the methodology used by Phil
> Jones for temperature [Jones, P.D., T.J. Osborn, and K.R. Briffa,
> Estimating sampling errors in large scale temperature averages, 10,
> 2548-2568, 1997.], but never pursued it beyond some prelimiary analysis.
> It seems that is is possible to estimate SE of grid point precip
> averages, and you may want to look into this if getting a handle of
> observational error if you can inlcude this in the optimal
> fingerprinting (assuming you are using OF) statistical model.  I seem to
> remember that most OF schemes to date do not include observational error
> in the statistical model, but concentrate on using climate model control
> run data to estimate climate-noise (probably the larger error
> component).
>
> Let me know if you want to pursue this further...this may be the
> stimulus I need to continue with the observational error estimates!
>
> Mark
>
> > -----Original Message-----
> > From: Nathan Gillett [mailto:gillett@ocean.seos.uvic.ca]
> > Sent: Wednesday, March 10, 2004 8:09 PM
> > To: m.hulme@uea.ac.uk; mark.new@geog.ox.ac.uk
> > Subject: gu23wld0098.dat dataset
> >
> >
> > Dear Mike, Mark,
> > I obtained a copy of your gu23wld0098.dat precipitation
> > dataset from Peter Stott at the Hadley Centre (he had a
> > version in pp format, which was easy for me to read). I'm
> > currently using this data for a detection and attribution
> > study. I've added the appropriate acknowldegement, and I'll
> > send you a preprint of the paper once its ready to submit.
> > Any comments or suggestions on the use of this data for
> > detection and attribution would be welcome.
> >
> > Cheers,
> >
> > Nathan
> >
> > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > ++++++++++
> > Mailing address:                     Courier address:
> >
> > Nathan Gillett 			     Nathan Gillett
> > School of Earth and Ocean Sciences   School of Earth and
> > Ocean Sciences
> > University of Victoria                 at Gordon Head
> > Complex, Room 377a
> > PO Box 3055                          University of Victoria
> > Victoria, BC, V8W 3P6  		     3964 Gordon Head Road
> > Canada                               Victoria, BC, V8N 3X3, Canada
> >
> > Tel: +1 250 472 4013		     Email: gillett@uvic.ca
> > Fax: +1 250 472 4004
> http://climate.uvic.ca/people/gillett
>
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>
