cc: m.hulme@uea.ac.uk
date: Thu Mar 11 13:26:36 2004
from: Phil Jones <p.jones@uea.ac.uk>
subject: Re: Vapour pressure scenarios
to: Timothy Carter <tim.carter@ymparisto.fi>

    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
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