cc: Tim Osborn date: Fri, 23 Mar 2001 16:38:58 +0000 from: "Tim Osborn" subject: Re: quick question to: srutherford@virginia.edu >>Sorry to bother you again, I know you're swamped at the moment. I'm >>still having trouble reproducing your comparisons; even with the >>corrected reference period. Maybe the difference is the time >>dependant mask. In any case, I don't think we need anything else for >>the EGS talk, I just want to be clear on a couple of things. In your >>results when you compare your reconstruction, say 0-90 and our >>0-70masked. Can you tell me exactly what goes into both? What's the >>difference between 0-70masked, treeboxes, and treeboxes masked? Scott, I'm away from the office today (meeting in London) so I still can't get the time dependent mask to you. But I can explain the differences: We have two distinct reconstructions. First is our regional and quasi-hemispheric reconstructions, where we average (with some appropriate weighting) our tree-ring density series into regional averages (in the case of the quasi-hemispheric series, all density series are averaged), and *then* we calibrated against regional-mean warm-season temperatures. We prefer to do this because the temperature signal is stronger at the regional scale than at the grid box scale. However, we have only done this for certain pre-defined regions and thus it is not useful if you want to know the past variation in temperature averaged over some region that we've never used! For the quasi-hemispheric series, where we average all the trees together, we have then calibrated this against two different observed temperature series. The first is the temperature obtained by averaging all instrumental temperatures from grid boxes that contain tree-ring sites. In our papers we called this reconstruction "ALL". The second is the temperature obtained by averaging warm-season temperatures from all land grid boxes north of 20N. In our papers we called this reconstruction "NH" and this is the time series that I sent you I think. The second reconstructions we have are the gridded fields of warm-season temperatures that we've reconstructed by a combination of local linear regression and principal components regression. But the coverage is incomplete because we only reconstruct those boxes that had a reasonable independent verification correlation. And this coverage varies through time because the number of predictors (trees) decreases, and with less predictors we can reconstruct less boxes with skillful verification. Using the gridded fields, we can of course reconstruct many different regions etc. When we reconstruct the exact same regions for which we already have regional averages, then we reproduce them reasonably well from the gridded data. So, for *our* reconstruction, the black lines give the average of our gridded fields over various regions. 0-90 is an average over all our grid boxes (since we don't try to reconstruct any southern hemisphere boxes), but it isn't complete coverage since we only have some boxes to average together, and as mentioned above the coverage also varies with time. For the REG-EM case, 0-70 is an average of all boxes (which has virtually complete coverage - all but 6 boxes). So this is interesting, but isn't a like-with-like comparison. 0-70masked is an average of the REG-EM reconstructions over all boxes, but after the same time-dependent mask is applied to the REG-EM data as we have for our gridded fields. So this is a like-with-like comparison (because we have very few boxes north of 70N; if we had more we'd have to truncate our fields at 70N, to avoid more bias). land20-90 is then an average of all land boxes north of 20 in our fields, with time-varying incomplete coverage. land20-70 is average of all land >20N from REG-EM, with complete and time-invariant coverage - not a like-with-like comparison. On this I think I overlaid the blue curve which is our regional "NH" series. A like-with-like comparison is possible by applying our implicit time-varying mask to the REG-EM data and then sampling land boxes north of 20N (i.e. land20-70masked cf. our land20-90). Finally, treeboxes is when we average only those boxes that contain trees. But in our reconstruction this coverage still varies in time, whereas the REG-EM treeboxes uses the time-invariant mask I sent to you - again, not a fair comparison. I overlaid in blue our "ALL" regional reconstruction here. treeboxesmasked is when I sample REG-EM at only the boxes with trees *and* make the mask time-varying, so we can compare with our treeboxes series (which already has the time-varying mask in it implicitly). I think maybe the latter point is the confusing one - because our fields have time-varying coverage anyway, there's no need to apply the mask to them, so I didn't add the word "masked" to those series. Perhaps I should have called these series "masked", because the time-varying mask is there implicitly. Hope this long explanation is now completely clear! Tim