date: Thu, 20 Nov 2008 14:49:25 +0000
from: Ian Harris <i.Harris@uea.ac.uk>
subject: Re: vap and vaplev
to: t.osborn@uea.ac.uk

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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
>
> <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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