Saturday, June 9, 2012


date: Thu, 20 Nov 2008 14:49:25 +0000
from: Ian Harris <>
subject: Re: vap and vaplev

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.
> 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

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
> Tim
> <vap_vs_vaplev_landandsea.pdf><vap_vs_vaplev_land.pdf>

Ian "Harry" Harris
Climatic Research Unit
School of Environmental Sciences
University of East Anglia
Norwich NR4 7TJ
United Kingdom


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