cc: Phil Jones <p.jonesatXYZxyz.ac.uk>
date: Wed, 24 Sep 2008 12:33:56 +0100
from: Adrian Simmons <Adrian.SimmonsatXYZxyzwf.int>
subject: Re: Downward trend in relative humidity over land?
Thanks for this, Kate.
This will need some digging into, as for Europe I see a higher RH
anomaly in the first half of the period for ERA than for HadCRUH, when I
do the comparison by upscaling and sampling ERA to match HadCRUH. This
needs to be understood. The agreement for RH is not as good as for q.
What remains a solid result is that RH is notably low in the last few
years of the ERA period (which currently ends in July 2006 - two more
years to go).
First challenge (after finding some spare time) will be to work out
exactly how RH has been calculated. I take the archived 2m T and Td for
ERA and put it into a library routine to produce RH. That routine was
written more than thirty years ago, and I have to find the documentation.
Don't hold your breath, but I'll let you know when I've managed to make
> Over land I found non-significant and very small decadal trends, with
> the Northern Hemisphere trend actually being slightly positive. (G =
> -0.03, NH = 0.07, T = -0.10 and SH = -0.34 - SH data is very sparse and
> likely of low quality). In contrast, the Marine data showed very
> significant negative trends but I'm highly suspicious about the pre-1982
> data which has a strong positive bias relative to the rest of the
> The climatology period and possibly the way that anomalies are obtained
> differ for HadCRUH (1974 to 2003) although I'm guessing this should have
> little effect. The scale on my plots is large and so its perhaps
> difficult to see any trends and I agree that the additional 3 years in
> the ERA data bring make the negative trend clearer. I have had problems
> with different trend fitting techniques giving more/less attention to
> end points of the series and so giving very different trends. Ideally it
> would be nice if ERA and HadCRUH were in good agreement but I think it
> may be realistic that they are not given the variability in RH over land
> and the likely very different ways that RH is derived for both.
> I like the idea of a rough sanity check to see if the q and RH changes
> in ERA are consistent. For HadCRUH I did a very rough version of
> percentage change in q for 1K increase in T using the idea that for
> constant RH a ~7% increase in q would be expected. Results suggest
> global land q increases are consistent with constant RH (~7%), NH land q
> increases suggest a slight increase in RH (~8 %) and Tropics land q
> increase suggest a decreasing RH (~5.5 %).
> Not sure if all the above is helpful or just rambling. I think
> presenting the plots at Boulder is a good idea though.
> Dr Kate Willett Climate Research Scientist
> Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB
> tel. +44 1392 884288 fax +44 1392 885681 (mark for my attention)
European Centre for Medium-Range Weather Forecasts
Shinfield Park, Reading, RG2 9AX, UK
Phone: +44 118 949 9700
Fax: +44 118 986 9450