date: Fri, 21 Nov 2008 15:46:03 -0000 from: "Nigel Arnell" subject: RE: ClimGen vap to: "Tim Osborn" Tim, Thanks - I'll digest this on the way to China. Thanks for your input to the meeting on Wednesday - the meeting was extremely helpful, and I'm really pleased with progress and direction. One immediate question - when do you think you could have the extra scenario data (or code+pattern)? It influences the timing of the rest of the project. Regards Nigel Professor Nigel Arnell Director Walker Institute for Climate System Research University of Reading Earley Gate Reading RG6 6BB UK +44-118-378-7392 www.walker-institute.ac.uk -----Original Message----- From: Tim Osborn [mailto:t.osborn@uea.ac.uk] Sent: 21 November 2008 15:42 To: Nigel Arnell Subject: ClimGen vap Hi Nigel, sorry I had to leave Wednesday's meeting early, but please let me know anything that arose after I left (or at the stakeholder meeting) that needs some input from me. Regarding the VAP scenarios for those GCMs that haven't provided surface humidity output... Harry has now completed his calculations, using specific humidity on pressure levels in the atmosphere, together with the surface pressure (note: different from sea level pressure over the land, of course). Surface pressure is used to find the two lowest pressure levels that are above the land surface, and then to extrapolate from the spec. hum. at these two levels down to the pressure at the land surface, and then convert this estimated surface spec. hum. to surface vapour pressure. This then gets put into the pattern estimator (pooling across multiple ensemble members, averaging into 30-year running means, and regressing against 30-year running mean global temperature change). You probably don't need to know all this detail! Instead, please look at the attached PDF which compares our results for the cccma_cgcm3_1 model where we have both surface and pressure level humidity data -- so we can compare the estimated patterns with the correct surface patterns. One page per season. Top-left is the correct VAP change pattern (hPa per degC of warmning) calculated from model surface humidity (called "v2p"). Top-right is "vaplev" (our estimate from the pressure level humidity). Bottom-left is the difference (vaplev-v2p). Green is essentially zero difference, light blue and grey are small differences. The browns and pinks are those of most concern. Bottom-right is a scatter plot showing v2p vs. vaplev values for all grid boxes, plus the correlation and slope of a best-fit (least squares regression) line. The black line is the perfect y=x line, while the blue line is the best-fit line. I've only shown the land -- the fit is even better over the oceans. Over the land, the pattern correlations are 0.95, 0.93, 0.90, and 0.93 for the four seasons. Very good, but the scatter plots and the difference maps show some significant variations. Most problems are where vaplev has underestimated the correct changes (points above the line in the scatter plot). I've been trying to think why the extrapolation from levels would fail, but can't seem to get an explanation that fits the pattern. For example, some of most extensive differences are over the Mediterranean in JJA. Here this model shows strong summer drying, which leads to decreased soil moisture. If anything I'd expect that the drier soils would mean that near-surface humidity would increase less than away from the surface at the pressure levels, yet the surface VAP patterns shows more increase than the pattern estimated from the pressure level data! What is your opinion about this? Can we use vaplev estimates on the basis of the >0.9 pattern correlations? Are they better than the alternative(s), e.g. the constant RH assumption? Best regards Tim No virus found in this incoming message. Checked by AVG - http://www.avg.com Version: 8.0.175 / Virus Database: 270.9.8/1800 - Release Date: 11/19/2008 6:55 PM