cc: m.hulme@uea.ac.uk date: Thu Mar 11 13:26:36 2004 from: Phil Jones subject: Re: Vapour pressure scenarios to: Timothy Carter Tim and Mike, I've sent an email to Tim Mitchell for his thoughts (and asked him what the new job is like). I'm not surprised by what you've found - i.e. the large inter-model differences. In the EU-project SWURVE, we've gone back to calculating PET (assuming this is why you want a humidity type variable) with Thornthwaite and Blaney/Criddle as they only depend on temperature. This is being written into project final report and the special issue of HESS (Hyd. and Earth System Science). Project run by Chris Kilsby and he's arranged this issue. Even with HadCM3 with small changes in vapour pressure (well in HadAM3P/HadRM3P - same there also), the increasing temperature means that vapour pressure deficit becomes very large, so PET calculated with Penman formula is ridiculous. If this is why you want vapour pressure I would suggest you go down this route also. Happy for you to forward this to Nigel as he'll understand what I'm on about. Hydrologists know that Penman should be best, but not with models. Even for 1961-90 the problem can be seen in the warmer summers. Basic problem is that all models are wrong - not got enough middle and low level clouds. Problem will be with us for years, according to Richard Jones. Chris has talked to him about it at length. It looks as though CSIRO2 may be the best one. CGCM2 looks most odd. The HC think their variable tile parameterization may help. This can keep some small portion of open water in each box, so the whole thing doesn't dry out. There was a paper in Science a year or so ago, that showed PET (from evaporimeters) going down recently in many regions ! I'll let you know what Tim thinks. Omitted the two pdfs as they were large. The ppt plot gives the essence of the message. I'm assuming here that Tim hasn't made a mistake - the HadCM3 plots look like the ones Declan produced for SWURVE a while ago and similar to ones Marie has produced for RM3P and AM3P. Cheers Phil At 13:30 11/03/2004 +0200, Timothy Carter wrote: Dear Phil, I understand from Mike that responsibility for the submitted J. Climate paper on the global 10 minute scenario data that Tim M. prepared along with the data sets have been passed over to you. I don't know if you still have contact with Tim, but if you do it might be worth asking him about a potential problem that has arisen concerning the vapour pressure scenarios included in the data set. I noticed the problem this week and co-incidentally so did Dave Wilson from Nigel Arnell's group in Southampton. Both of us queried it with Markus Erhart who has co-ordinated the data distribution for ATEAM and he wasn't aware of the problem. Here are our mails to Markus plus some attachments. For more on the methods of deriving vapour pressure, see Tim's documentation for each model run (do you have access to this?) Did he discuss these mathods with people in the Unit at the time? Whatever the conclusion, we may need to re-examine making these VP data available in the global data set until they have been properly evaluated. The methods and data set are also, of course, discussed in the paper. Best regards, Tim *********** My mail (one PPT attachment) Dear Markus, I have a query about the scenario change fields that Tim M. prepared. I have been looking at some of the mapped results that he presented in his summary notes for each model (PDF files made available on the ATEAM password controlled web site). These seem fine, except for the vapour pressure changes, which differ dramatically between HadCM3 (small changes) and the other three models, which show much larger changes (order of magnitude larger) and, perhaps more disturbingly, very large gradients over relatively small distances for some months/models. It is stated in the documentation that special methods were needed to derive vapour pressure change because the models each had a different way of representing humidity. However, these different methods seem to have yielded big differences in the derived values of vapour pressure which make me suspect that one or more of the equations were not appropriate for the whole of Europe or there were errors in applying them. My questions to you are: 1. Are you aware of any discussion about these change patterns for humidity? 2. Do you know who has applied vapour pressure changes in their impact model simulations for ATEAM? If so, how sensitive are the impact model results to these big differences in scenario changes? I realise that I should have queried this up much earlier in the process (I rather left Tim to handle the data processing side) and perhaps the issue has already been dealt with. Do you have any thoughts on this? I attach a sample model intercomparison for selected months (A2-forced) cut and pasted from Tim's documents to illustrate the apparent problem. Best regards, Tim ***************** Dave Wilson's mail (two PDF attachments) ...... Are you aware of any groups having problems with the vapour pressure data produced for ATEAM? Does any other group use this data? Our models have produced some unusual results and Nigel has identified that the vapour pressure pattern supplied by ATEAM is not that which should be expected. We have used the 30 year timeslice data. I have checked my input with the 30 year timeslice data provided and there appears to be a one to one correspondence - i.e. our input is the data supplied by ATEAM. I have included some maps that Nigel has produced for vapour pressure and relative humidity. We will continue to check that the problem isn't with my input. Thanks Markus. Dave Wilson (Soton). 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 ----------------------------------------------------------------------------