cc: 'Mike Hulme' date: Fri, 26 Mar 2004 11:30:25 -0800 (PST) from: Nathan Gillett subject: RE: gu23wld0098.dat dataset to: Mark New 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 > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >