From: Phil Jones
To: "Folland, Chris"
Subject: RE: VARIANCE PROBLEM
Date: Thu, 10 Jun 1999 15:48:05 +0100
Cc: d.parker@meto.gov.uk,t.osborn@uea.ac.uk
Chris,
Sorry to be flooding you with another email, but I was discussing
this with Tim. Tim reminded me of a paper that he'd written
in that well known journal Dendrocronologia ! I've sent down
a copy of the proofs to you both. The paper has been in press for
the last 2 years ! This must be the slowest journal in the
world. This has some more theory in it and some variance
corrections for tree-ring and temperature series.
We are going ahead with the method I've outlined over the
last few emails. Tim and I have modified a couple of things
slightly :
1) Using the present combined dataset ( Jones, 1994 and Parker
et al. 1995) we will calculate monthly rbars for each 5 by 5
box. The grid-box time series will be filtered with a 30-
year Gaussian filter. rbar will be calculated from the residual
grid-box time series. Tim reckons that a longer filter is better
(an analysis in the paper). He suggests 40 years, but this
involves more problems with the ends, so we'll go with 30. I
don't think 20,30,40 will make that much difference to the
rbar values.
We are using the combined dataste for the estimation as this
should produce better rbar values around coasts and islands. If we
used the land only dataset we would have real problems with
isolated islands and with some coasts ( where all neighbouring
boxes will be in one direction from the coastal box).
2) Having got fields of the monthly rbars we'll then apply the
formula to the land-only dataset. As you're doing something
similar with the marine dataset, we can remerge the two
variance corrected datasets using David's merging ( growing
land and neighbour checking) program.
3) We will then write this up as a small paper for GRL, about
the land only results. Both of you can be on this if you want.
We can decide later what to do about the merged dataset.
4) applying the correction in real time in the future will mean
that we will always be slightly changing approximately the last
15 years data - because of the filter end effects. Best would
seem to be to maintain the present version we have and apply
this variance correction every few years ( eg the IPCC cycle !).
Cheers
Phil
Prof. Phil Jones
Climatic Research Unit Telephone +44 (0) 1603 592090
School of Environmental Sciences Fax +44 (0) 1603 507784
University of East Anglia
Norwich Email p.jones@uea.ac.uk
NR4 7TJ
UK
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