cc: Mike Hulme date: Wed, 31 Mar 2004 14:52:24 +0300 from: Timothy Carter subject: Re: ATEAM climate data to: Phil Jones , Tim Mitchell Dear Phil, I agree with the thrust of your argument here. Yes indeed, impacts people rarely have much of a conception about the limitations of climate data, but I would say that this is particularly true where they wish to apply scenario information. They quite often expect to be provided with scenario information at the same spatial and temporal scale as the climate inputs for their models. I usually suggest to impact modellers who ask that they should first get their observational data in order before worrying about the scenarios. That is a obvious prerequisite for effective impact studies. The impacts observed in the recent past should be reproducable based on the climate observed during the same period. Some impact studies fall short even of this basic validation step. If they have reasonable quality high resolution observed data (which may be the case for individual sites or even limited regions) then how they perturb this for developing scenarios of the future climate is as much an art as a science. It is also worth noting that analysts sometimes use weather generators to represent present and future climate. Often, close inspection reveals a poor representation by WGs of the observed climate, in which case baseline impacts are estimated erroneously, even before considering future climate. So, again, the baseline climate is key. I recognise that one of the ATEAM ambitions was to have European coverage in the climate time series. This is fairly straightforward for future scenarios from GCM outputs. Unfortunately, it is not straightforward for representing the historical climate, as you at CRU can attest after 30 years of working with such data. I am not criticising the climatological data that Tim et al. have prepared - it is the best one could have hoped for. The problem has been that the drawbacks of the data were not effectively communicated (I take equal responsibility for this). The data set was presented as a package - very convenient to download and apply, but not very easy at all to modify by non-climatologists according to user needs. The ATEAM impacts people knew that their models are sensitive to climate variability, so they were delighted to be offered this feature in the data set. What they didn't realise, was that the data sets are actually incomplete, although they appear not to be, by having values allocated at all grid boxes. So how are they to adjust their analysis to cover only the more reliable regions and time periods in the record? Some of the models being used in ATEAM are transient, so the effects of climate variability are cumulative over time - trees grow; species succeed one another according to tree mortality and ambient climatic variability. It isn't possible to run these models for 20 years in the 20th century and compare with the same period in the 21st century as it might be for e.g. hydrological models. Other methods are required to create a realistic time series over periods of hundreds of years. It is these issues that we have regrettably overlooked in providing these data. In this case, I do not blame the impacts people. In fact, I am grateful to them for highlighting some obvious difficulties in providing climate data for application. Yes, the problems are documented somewhere (as Tim points out), but how many ecologists have time or expertise to find the relevant climate journals and to interpret the subtleties of the many methods used to generate these observed data? I think the lesson to be learnt is that these data sets need to have up front (at the site of downloading) documentation that provides basic information on applications for which the data are or are not appropriate. This requires second guessing some of the potential applications, and though we already tried to do that in ATEAM we only partly succeeded. With only T and P, I doubt if anyone would have noticed any weaknesses in the data sets (they are reasonably complete). It was only because we were ambitious in introducing other variables, that the problems emerged. I suppose this dialogue process takes time, and we learn from our mistakes. I have seen Tom's (amazingly conciliatory) review. You should frame it in the Unit! Best regards, Tim C. At 12:18 31/03/04, Phil Jones wrote: > Tim C. > Quickly reading your response to Tim. M. I think you're defending > impacts analysts > far too much. Whenever I meet some of these people, I have to bite my > lip to avoid > saying something I'll regret. Impacts people need to be made aware of > the limitations > of observed data and even more of model data. What Tim has done is > likely the best that > can be done given the limitations of what we can get hold off, yet still > trying to maintain > the weak correlations between variables. > At many meetings impacts people ask for model futures for variables > and time intervals > we just don't have in the real world. How then do they test their > models? Chris Kilsby > is working to derive 5 minute rainfall scenarios for an EPSRC project, > because the > hydrologists on one project want this. There is one raingauge in the UK > with 5 minute > rainfall for 20 years. They want it for urban catchments in northern > England, the long > record is from Farnborough. When pushed on this they gave us one year's > data for site > near Bradford. They said they had techniques for making 1000 years of > records from > one year of data. Despite this being a climate change project they just > thought that > high-frequency rainfall variations will change according to the mean. > To show them at our next meeting, we're going through HadAM3P/H and > HadRM3P/H > looking at convective/total precip and large-scale/total precip ratios > and A2 scenario > changes. I've never seen these sorts of plots before. The results are > frightening. In winter > over the Mediterranean, 90% of the rainfall over the sea is convective, > but on land less > than 10% is convective. I've never seen a variable delineate the > coastline so well. How does > large-scale rainfall which falls on the land not fall into the sea. > > Tim may not have said, but we already have one review of the J. > Climate paper > (from Tom WIgley) which is by Tom's standards good. I'm dreading getting > the reviews back > as I think it will be me who has to respond to them. I know I'm not > going to have much > time to respond, so the first thing I'll do will be to ask for an > extension of the likely 1 month > that we'll be given - if the other reviews are as favourable as Tom's. > > Cheers > Phil