Sunday, June 17, 2012

5239.txt

date: Wed Feb 25 13:09:59 2004
from: Phil Jones <p.jonesatXYZxyz.ac.uk>
subject: Re: Trends in DTR from ERA-40
to: Adrian.SimmonsatXYZxyzwf.int

Adrian,
A couple of things have come up that need rapid attention, so I should get to your
draft on Friday and the weekend. If I go through and put my comments on my copy and
then email this back - will that be OK ? I say this as I've got into the habit of adding
comments to a word version. You might be using a different system as you've sent a
pdf file.
I think hard copy comments should work.
Cheers
Phil
At 09:26 25/02/2004 +0000, you wrote:

Phil
The SSTs in the simulation were the same analyses as used in ERA-40. So the simulation
can be regarded as an ERA-40 analysis without atmospheric, land-surface and ocean-wave
observations, but still, effectively, using observations of SST. Obviously that provides
quite a control.
The analyses were from the Hadley Centre for the period up to November 1981 and from
NCEP thereafter. The switch was made because NCEP provided higher time frequency. There
is a report on this in an advanced stage of preparation.
Best regards
Adrian
Phil Jones wrote:

Adrian,
I've been away, but I've printed off the draft paper and the recent set of figures.
I will
go through these when I've cleared a few pressing things in the next couple of days.
A quick look at the figures seems to indicate that ERA-40 is much better than NCEP
and a paper would be a useful addition to the literature.
I will contact the people at NCDC about the max/min series and possible updates to
nearer the present as well as a gridded dataset.
I have a couple of quick questions:
- the specified SST run, is this the same as ERA-40 but with SSTs just having the same
annual cycle year on year? The proper ERA-40 run used SSTs as they were measured -
a dataset developed at the HC.
Cheers
Phil
At 13:08 24/02/2004 +0000, Adrian Simmons wrote:

Phil
I've now computed a set of monthly means of daily max and min temperatures from ERA-40.
They are not based directly on any observations of Tmax and Tmin. Instead they are
derived from maximum and minimum values computed during each of the six-hour background
forecasts of the data assimilation system.
The first attachment shows trends in the daily temperature range for the periods
1958-2001 and 1979-2001. This shows a reduction over time for North America (1958-2001
at least) and for eastern Europe and much of Asia. Values in the tropics must be treated
with most caution as we know we have spurious trends in some aspects of the hydrological
cycle which could well impact the daily temperature range. The trend over the US may be
biased a bit by a too-dry troposphere from 58-63 caused by wrongly encoded humidity
observations in one of the radiosonde datasets supplied to us - we picked this up too
late to allow correction in ERA-40 production.
The other plots show the trends in maximum and minimum temperatures for 1958-2001 and
1979-2001. Overall the short-range forecast trends are quite consistent with the trends
for the daily mean analyses (Fig 7 in the document attached to yesterday's mail). The
increase in daily temperature range over northern latitudes is seen to be associated
with a larger rate of increase of minimum than maxiumum temperatures - at least we seem
to have got the most basic feature right.
The Tmax and Tmin trends for 1958 to 2001 also show that the almost certainly erroneous
cooling trends seen in the daily mean analyses for Australia and much of tropical South
America arise much more from a decrease over time in maximum temperatures than from a
decrease in minimum temperatures. This suggests that the ERA-40 background forecasts
(and "analyses") of daytime (rather than nightime) temperatures are the ones that are
biased warm in the earlier years in which we assimilated little or no SYNOP data over
these areas.
Western Europe seems to show a slightly larger trend in maxiumum than minimum
temperatures. Hotter summers and drier soils?
It will be interesting to make more quantitative comparisons with the new data from
NCDC.
Best regards
Adrian
--
--------------------------------------------------
Adrian Simmons
Head of Data Division
European Centre for Medium-Range Weather Forecasts
Shinfield Park, Reading, RG2 9AX, UK
Phone: +44 118 949 9700
Fax: +44 118 986 9450
--------------------------------------------------

Prof. Phil Jones
Climatic Research Unit Telephone +44 (0) 1603 592090
School of Environmental Sciences Fax +44 (0) 1603 507784
University of East Anglia
Norwich Email p.jonesatXYZxyz.ac.uk
NR4 7TJ
UK
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--
--------------------------------------------------
Adrian Simmons
Head of Data Division
European Centre for Medium-Range Weather Forecasts
Shinfield Park, Reading, RG2 9AX, UK
Phone: +44 118 949 9700
Fax: +44 118 986 9450
--------------------------------------------------

Prof. Phil Jones
Climatic Research Unit Telephone +44 (0) 1603 592090
School of Environmental Sciences Fax +44 (0) 1603 507784
University of East Anglia
Norwich Email p.jonesatXYZxyz.ac.uk
NR4 7TJ
UK
----------------------------------------------------------------------------

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