Thursday, June 7, 2012


cc: Dick Dee <>
date: Fri, 19 Dec 2008 18:38:00 +0000
from: Adrian Simmons <>
subject: CRUTEM3v and HadCRUH comparisons with ERA-40 and ERA-Interim
to: Phil Jones <>, Kate Willett <>, Peter <>

Dear Phil, Kate and Peter

I finally managed to spend some time over the past few days assembling
some provisional figures (attached) for a comparison of our reanalyses
with your data sets. Era-Interim is now in January 2008, and when it
completes 2008 (probably late next month or early February), we'll have
20 years of this reanalysis, and thirty years of combined
ERA-40/ERA-Interim reanalysis from the FGGE year onwards. My idea is to
start writing something soon, with a view to finalising the figures and
text once we have results up to the end of 2008.

Here are some comments on the figures (all of which are land-only).

(i) Figs. 1-3 These cover the temperature comparisons, and simply update
the picture from Simmons, Jones et al. (2004). In these and other
figures I have normalised everything to give zero anomaly for the
ten-year mean 1989-1998, which is convenient as I can then plot maps of
the anomalies for the decades 1979-1988 and 1999-2008. (Note that for
now the maps labelled 1999-2008 are actually for 1998-2007, and those
for 2004-2008 are actually for 2003-2007 - this will be fixed when we
actually have reanalysis data for 2008). For the maps I show all CRU
grid squares for which less than 5% of months are missing.

Basically we see excellent continuation of the ERA-40 results when we
sample ERA-Interim and ERA-40 in the same way as CRUTEM3v samples in
space and time. A point that might be emphasized is that the coverage of
CRUTEM3v is much poorer for the last ten years than the first one. Any
comment on this? Are there many late-arriving CLIMATs that will change
the picture when I download a new version of CRUTEM3v? It is noteworthy
that CRUTEM3v samples poorly the Arctic region where ERA indicates the
strongest warming (and ERA also shows strong warming over sea-ice - or
where sea-ice used to be!). Fig 1 shows a stronger trend in ERA that in
CRUTEM3v when we use the complete ERA record rather than sampling it as
CRUTEM3v. In other words ERA-Interim shows less recent cooling than
CRUTEM3v. Here we are treading on thin ice (sorry!) from a PR viewpoint,
so I'm interested in your reaction to all this.

(ii) Fig. 4 shows excellent agreement between HadCRUH and ERA-40 time
series for q. After 2003, the "sampled as HadCRUH" means I sample
ERA-Interim for the grid squares for which HadCRUH gives values for
December 2003. Slightly less trend for the full ERA dataset.

(iii) Fig. 5 and 6 are time series for RH. Generally good agreement
also. Two points. One is shift in ERA-40 around 1990 for Europe. I think
this is a reanalysis problem. Hard to be sure, but could be explained by
increased dry bias of sondes. No time to write more now, but Figs 10 and
11 relate to this, and also to second point - the relative dryness of
recent years. I cannot find a reanalysis problem to explain this, and am
inclined to think for now it could be real - there is no fundamental
physical reason why relative humidity should not show a trend in a
climate that is shifting. But it is really frustrating that HadCRUH
stops at the end of 2003. What are the prospects for extending it? Could
something quick be done as a check on the reanalysis result - omitting
homogenization and subtle QC checks for example.

(iv) Fig 7 shows ten-year anomaly maps. Don't have HadCRUH for 1999-2008
of course. RH map for 1999-2008 show good temporal continuity -
generally drying (relatively) in tropics and mid latitudes, and
moistening (relatively) at high latitudes. Perhaps not implausible - for
RH to stay uniform as T increases the water has to come from somewhere.
That could be difficult over dry soils. And easier over melting soils??
Is anything like this seen in the Hadley Centre climate runs?

(v) Fig 8 shows consistency across boundary layer (level 49 is close to
850hPa, where analysis in influenced more strongly by sondes rather than

(vi) Fig 9 shows some 5-year means, comparing ERA-INT and HadCRUH for

(vii) Figs 10 and 11 are fits of background and analysis to assimilated
observations. There are subtleties I don't have time to explain right
now, but basically a flat line is a good thing from the viewpoint of
trend analysis. It is the drop in the background RH curves and rise in
the background Q curves for the northern hemisphere between 1988 and
1993 that makes me think the RH shift between the 1980s and 1990s in ERA
relative to HadCRUH is a reanalysis problem. Equally though, there is
nothing in these curves (or some others I've looked at) that points to
the lowering of RH in the reanalyses for the last few years as being

I look forward to your reaction to all this.

And best wishes for Christmas and 2009.

Adrian Simmons
European Centre for Medium-Range Weather Forecasts
Shinfield Park, Reading, RG2 9AX, UK
Phone: +44 118 949 9700
Fax: +44 118 986 9450

Attachment Converted: "c:\eudora\attach\Hum_figs.doc"

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