date: Thu, 20 Nov 2008 14:49:25 +0000
from: Ian Harris <i.HarrisatXYZxyz.ac.uk>
subject: Re: vap and vaplev
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
> 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-
> Look at ...landandsea.pdf first. Pattern correlations 0.98, 0.97,
> 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-
> 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
> Or did you calculate vap from the two lowest hus levels and then
> extrapolate from these vaps to get surface vap? The former is
> the best one to do, and I think that's what we discussed, but can you
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
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).
Ian "Harry" Harris
Climatic Research Unit
School of Environmental Sciences
University of East Anglia
Norwich NR4 7TJ