date: Thu, 20 Nov 2008 14:49:25 +0000 from: Ian Harris subject: Re: vap and vaplev to: t.osborn@uea.ac.uk Hi Tim, On 20 Nov 2008, at 14:28, Tim Osborn wrote: > Hi Harry, > > I'll be back in on Friday and hopefully we can chat about the QUEST > meeting that I half went to on Wednesday... Er, sorry - I'm out on Friday (tomorrow). Thought I'd mentioned it, obviously not. Can chat next week (M/T/W/T/F), on the phone later today, or drop me a one-para summary to chew on over the weekend? > In the meantime, thanks for calculating the vaplev for cccma_cgcmNNN. > I've plotted maps for the four seasons, comparing vap and vaplev > patterns. > > Please see the attached files. > > 1 page per season. Top-left is vap pattern. Top-right is vaplev. > Bottom-left is the difference (vaplev-vap). Bottom-right is a scatter > plot showing vap 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. > > Look at ...landandsea.pdf first. Pattern correlations 0.98, 0.97, > 0.94, > 0.97. Clearly very good. The difference plots however (here green > is a > good match, grey and pale blue are ok, anything else is not so > good) show > problems over the land, especially in the subtropics, moving north- > south > with the seasons. > > ...land.pdf shows the same but just the land. The scatter plot is > now not > so good, correlations 0.94, 0.92, 0.88, 0.93. They're still not bad > though, and on the scatter plots there are very many red circles > superimposed near the line y=x. But still there are quite a few > above the > line, indicating vaplev is underestimating the increase in vap, > often by > 50% or more. > > I presume there must be some change in soil moisture in these > regions that > makes the real near-surface vap change rather differently from the > vap in > the lowest two levels of the model from which vaplev is calculated. Sounds plausible. It's worth remembering that the original vap was erroneously calculated using sea-level pressure, (not surface pressure), so is that a better explanation of the land differences? > Can I just check with you, did you extrapolate hus from the two lowest > levels to approximate huss (surface hum) and then calculate surface > vap. > Or did you calculate vap from the two lowest hus levels and then > extrapolate from these vaps to get surface vap? The former is > probably > the best one to do, and I think that's what we discussed, but can you > confirm. Yup, derived a surface level specific humidity, then calculated vap using that and the surface pressure value. Extrapolation was from the two layers 'above' the surface pressure value, ie: Surface P Levels Used 1010 1000,925 988 925,850 910 850,700 (etc) I'm glad to see such good correlations, actually. And the 'drift' over land is, as I say, what we should be looking for (as the wrong pressure was used for the huss calculations). Cheers Harry > > Cheers > > Tim > > Ian "Harry" Harris Climatic Research Unit School of Environmental Sciences University of East Anglia Norwich NR4 7TJ United Kingdom