cc: ddokken@usgcrp.gov date: Thu, 30 Jan 1997 12:18:00 -0500 from: ddokken@usgcrp.gov (Dave Dokken) subject: Re: Regional Climate Projection Piece to: Mike Hulme Thank you Mike. Since you never saw the figs/tables, I faxed them to your attention. dave >Dave, > >Here are some comments on the IPCC Special Paper on Regional Climate >Impacts. I have copied these to Richard also and have put a manuscript in >the post which you might care to pass on to the authors of the IPCC >manuscript for me. > >Thanks, > >Mike > > >Simulation of Regional Climate Change with Coupled Climate Models and >Regional Modelling Techniques > >Some comments by Mike Hulme >Climatic Research Unit >UEA, Norwich NR4 7TJ >(email: m.hulme@uea.ac.uk) > > >I found the paper clear and well-written and it should form the basis of a >useful introduction for the Special Report. I have not seen the Figures so >I cannot comment on these in detail. My additional comments are below: > >p.1, Regional Simulations .... 1st para: Figure 1: it is not strictly >legitimate to compare change fields from eq. and tr. experiments at the time >of CO2 doubling (because of the difference between equilibrium and realised >change) or between warm-start and cold-start tr. experiments at the time of >CO2 doubling (because realised warming is a function of the historic forcing >which may or may not have been included). I have not seen a list of the >experiments in Figure 1 so I do not know how far this is a problem. But be >careful. Even if it is not a problem, the point should be made that DC at >time of CO2 doubling in a tr. experiment is not equivalent to DC at time of >CO2 doubling in an eq. experiment. People used to dealing with eq. 2xCO2 >experiments may get confused here. > >p.2: the point should be made when comparing regional change patterns from >different GCMs that some (maybe a lot) of the differences between >experiments is due to interdecadal variability in the experiments >(especially true of non-temperature variables). A range of precip. changes >for a region from -20% to +20% is not a robust measure of the true >disagreement amongst the models about the GHG rainfall signal since no >allowance is made for interdecadal variability (i.e., signal to noise >ratios). Several simulations from the same model and same forcing would >also yield a range of regional precip. changes which might be just as >large!! Little work has been published on the analysis of GHG signals from >ensemble experiments, but see Cubasch et al. (1995) for a helpful example >(also a manuscript in preparation by John Mitchell). I think this point is >an important one to make (see also later). > >p.3: final bullet: this difference between the statistical significance of >precip. and temp. biases is most important (see my point above). A regional >rainfall 'signal' of +10% from a single GCM simulation may be meaningless >against the background noise. > >p.3: I do not like your use of the term 'model sensitivity' (and elsewhere >'climate sensitivity', e.g. line 17) to describe the regional DC from a >model simulation. This is dangerously confusing with the proper use of the >term 'climate sensitivity' which is the equilibrium warming from an >instantaneous doubling of CO2-equivalent. 'Biases' is fine for >model-observed differences; perturbed minus control differences should be >called 'regional climate changes', not sensitivities. > >p.3, lines 12/13: the sentence, ' ... the range of simulated scenarios of >the model regional biases were still large ....' is muddled. I cannot >discern your meaning. > >p.4, section on aerosol and GHG signals: I think again the point about >signal to noise ratios should be made. Just because an aerosol simulation >changes the sign of precip. change for a region, does not mean that this is >at all significant. If the rainfall change for a decade (or 30-year period) >switches from +5% to -5% when aerosols are included means nothing if the >natural 10-30 year rainfall variability is, say, 20% (see my manuscript >Hulme & Brown). > >p.5, 'Precipitation' section: here it is stated (twice) about a general >reduction in precip. over southern Europe, but on p.2 all models agreed >about a winter increase in precip. over the same region! Which is correct? > >p.5: I think this point about switching of rainfall change over Asian >monsoon in the aerosol experiments needs handling with some care. While it >may be true for the particular domain chosen here (5-30N, 70-105E), if one >looks within this domain or else looks at surrounding regions, one does not >necessarily get the same result. Here is a case where the headline rainfall >change number is highly sensitive on the region defined. For example, in >HADCM2 experiment over the Indian land area only, both GHG and aerosol >experiments yield a decrease in rainfall (see Hulme & Brown). > >pp.5/6, soil moisture section: I think the point should be made that many >environmental modellers prefer to calculate soil moisture themselves using >GCM output as an input into their (often) more sophisticated models of soil >and the water cycle. GCM soil moisture schemes are very crude! Relying on >GCMs for soil moisture changes is not often a good idea. > >p.7, statistical downscaling: a word of caution is needed here about >downscaling. Just because these methods may yield more spatial detail >and/or actual point data does not necessitate that these scenarios are in >any way 'better' than the GCM-derived ones. Downscaling methods still rely >on GCM output to drive them and, furthermore, most downscaling methods make >the big assumption that synoptic-weather relationships will remain unchanged >between the past and the future. The impression must not be given that >downscaling will solve all scenario difficulties (see below). > >p.7, line -11: change 'Limited applications ...' to 'A few applications ....' > >pp.7/8, regional modelling: very few climate change experiments with >regional models using more than 5-10 years of simulation. I do not know of >a high resolution climate change experiment for Africa (p.8, line 12). >There have been high res. experiments that include aerosols (p.8, lines >14/15) - the Hadley Centre have done this. > >pp. 7/8: time slice experiments should be mentioned here (see Cubasch et >al., 1995). > >p.9, line -5: 'inherent predictability' this statement may well be true, >but it depends what you mean by it. See my earlier comments about >signal/noise ratios. > >p.10, line 3: a good example where 'model sensitivities' should be replaced >by 'regional climate changes'. > >p.10, lines 7/8: dangerous to imply that high resolution models yield high >accuracy scenario!!! They may well yield more faithful control simulations >of regional climate, but their 'accuracy' re. future scenarios still depends >on assumptions about what drives the climate (i.e., what emissions scenario) >and about how good the driving GCM is! If your comment about inherent limts >to climate predictability is true then it doesn't matter what resolution the >climate model is!!! Even 1km resolution can give you appearance of excellent >control climate but says nothing about the accuracy of any scenario. > >These comments above also apply to the last paragraph. It seems you to me >that you are too upbeat about what high resolution models can do for us. >Climate prediction will still be elusive because: > >a) we do not know future forcing >b) we do not know the climate sensitivity (1.5 to 4.5C) >c) there are inherent limitations on climate predictability which are >resolution independent. > > > >Some Other References: > > >Airey,M., Hulme,M. and Johns,T. (1996) Performance of the Hadley Centre >GCM in simulating precipitation over land areas Geophys. Res. Letts., 23, >1657-1660. >Cubasch,U., Waszkewitz,J., Hegerl,G. and Perlwitz,J. (1995) Regional >climate changes as simulated in time-slice experiments Climatic Change, >31, 273-304 >Hewitson,B.C. and Crane,R.G. (1996) Climate downscaling: techniques and >application Climate Research, 7, 85-95. >Hulme,M. and Brown,O. (1997) How likely is tolerable regional climate >change? Unpublished manuscript, UEA, Norwich. (copy mailed to Dave Dokken) > >Risbey,J.S. and Stone,P.H. (1996) A case study of the adequacy of GCM >simulations for input to regional climate change assessments J. Climate, >9, 1441-1467. >Whetton,P., England,M., O'Farrell,S., Watterson,I. and Pittock,B. (1996) >Global comparison of the regional rainfall results of enhanced greenhouse >coupled and mixed layer ocean experiments: implications for climate change >scenario development Climatic Change 33, 497-519. > > > > > >At 12:33 15/01/97 -0500, you wrote: >>Dear Dr. Hulme - >> >>Richard Moss asked that I forward this draft chapter for the IPCC Special >>Report on the Regional Impacts of Climate Change to your attention. Since >>it is now a Special Report, we can bring in post-SAR material, and you may >>have an opinion on how to augment the piece. >> >>Any observations should be forwarded to both Richard and my attention, >>since he's on travel for the next 4 weeks solid! For easy reference, his >>e-mail address is rmoss@usgcrp.gov. >> >>I'm merely going to attach the MSWord 6.0 file as a quick fix. If you >>cannot read it, get back to me and I can pipe it from our server as an >>ASCII, convert it to your preferred word-processing package, or rely on the >>tried-and-true fax. >> >>Thanks for your help, >> >> >> >>Attachment Converted: c:\eudora\attach\Regional.doc >> >>Dave Dokken >>Project Administrator >>IPCC Working Group II TSU >>Code YS-1 >>300 E Street, SW >>Washington, DC 20546 >>+1.202.358.0507 (voice) >>+1.202.358.4104 (fax) >> >> >> >----------------------------------------------------------------------------- >Dr Mike Hulme tel: +44 1603 593162 >Climatic Research Unit fax: +44 1603 507784 >School of Environmental Sciences email: m.hulme@uea.ac.uk >University of East Anglia web site: >http://www.cru.uea.ac.uk/~mikeh/ >Norwich NR4 7TJ > > >Mean temperature in C.England during 1996 was 0.3degC below the 1961-90 >average. >The maximum temperature in Norwich: Tuesday 28 January: 8.1degC.