cc: "Parker David (Met Office)" , David Easterling date: Thu Oct 7 10:46:07 2004 from: Phil Jones subject: Re: IPCC base period question to: "Russell Vose" , Kevin Trenberth Russ, We did have a criterion for calculating 61-90 normals (in the paper in J. Climate in 2003 by Jones and Moberg). I don't think this is crucial but we need a background reference for you method if you've made any important changes since the last publication on the subject. With the HC we are updating our dataset (will go to HadCRUT3) - some new stations, but mainly some more work on outliers, checking all normals and importantly producing errors on all grid boxes as well as on the NH/SH and global series. Incorporating errors due to homogeneity checks, errors in normals and errors from the bucket/intake adjustments as well as the sampling errors done before. Paper on all this in the New Year, so forget this for the first draft. So, answer, do what you think is reasonable and have it documented - either in a paper, or we can add something in the Appendix we have on many of these error issues. The paper I mentioned above will give a first attempt at some of the errors we've not considered before. It will be a first attempt though as many of the estimates we use are a little ad hoc. We will have a methodology to see what will happen with a range of estimates. Getting as many of the time series with error bars is important as Kevin says. We will also have spatial patterns as well, but these will be less crucial as we only plan to look at patterns over 1979-2003 (5 or 6 eventually) and also 1901-2003-5 as patterns won't change much from previous reports. I briefly talked to Dave about colour maps of trends (and not the dot format). We can look at this later. Need to be able to be clear where the missing areas are and the dots don't always make this obvious. Cheers Phil At 22:23 06/10/2004, Russell Vose wrote: Thanks, Kevin. I'm pretty sure that Phil used a few specific criteria in some of his papers (e.g., a minimum of 20 years of data, 4 years in each decade). I just didn't know if these details had already been ironed out for IPCC (and if not, then I guess we're the ones to decide). Kevin Trenberth wrote: Russell, No doubt Phil will comment more. The baseline is to establish a common period for anomalies and thus it anables maps of anomalies to be more coherent if the same procedures are used everywhere. If there is missing data, this will clearly upset this consistency spatially. In principle this should still not be a problem as long as uncertainties: error bars, are appropriately calculated that fully account for the missing data. But while those details can perhaps be gone into in a paper it is difficult to do it well in IPCC. I think there are fairly standard sorts or requirements for % of data required in a month, months in season, and years etc for the result to have some credibility and it should really be linked to error bars. But most decisions have been ad hoc. This is where things like reanalysis can help fill spatial and temporal gaps if done right. But maybe that's too ambitious here. All this is by way of saying I don't know the answer. My guess is that yes it should have data in each decade and that it is OK to estimate a base value. But I would let Phil rule on this: if he is available (may be next week). But please do track the error bars. Kevin Russell Vose wrote: Hi guys... Dave Easterling thought I should drop the three of you an e-mail with a "base period" question. As I understand it, the idea is to use 1961-90 as the baseline for IPCC. But have any other subcriteria been discussed or otherwise set in stone? For instance, how many years of data must a station have during that period? Must it have at least some data in each decade? Is it okay to estimate a normal if the station lacks sufficient data during the base period? Thoughts/feedback appreciated. -- Russell S. Vose, Chief Climate Analysis Branch National Climatic Data Center 151 Patton Avenue Asheville, North Carolina 28801 Phone: (828) 271-4311 Fax: (828) 271-4328 E-mail: Russell.Vose@noaa.gov 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 ----------------------------------------------------------------------------