To: "Folland, Chris" <ckfolland@meto.gov.uk>

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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