date: Fri, 19 Jan 2007 12:16:58 +0100
from: "Klein Tank, Albert" <Albert.Klein.Tank@knmi.nl>
subject: RE: Precipitation trends statement IPCC 4AR SPM
to: <David.Easterling@noaa.gov>, "Phil Jones" <p.jones@uea.ac.uk>

Hi Dave and Phil,

For your information:

My KNMI-colleague Geert Jan van Oldenborgh has sent this to the both of
you. He figured the Ch.3 authors from the NCDC and CRU datasets
portrayed in Figure 3.3.3 will be able to explain.

Cheers,

Albert.






 

-----Original Message-----
From: Oldenborgh van, Geert Jan 
Sent: Friday, January 19, 2007 12:05 PM
To: David.R.Easterling@noaa.gov
Cc: Klein Tank, Albert; Ulden van, Aad
Subject: Precipitation trends statement IPCC 4AR SPM

Dear David Easterling,

as climate researcher at the Royal Dutch Meteorological Institute (KNMI)
I had been asked to contribute to the government review of the SPM.  One
of my points, which made its way into the Dutch review, was that I could
not find back many of the trends in precipitation stated in the SPM.  I
checked all of these in a few datasets that I have available on the KNMI
Climate Explorer (http://climexp.knmi.nl).  Based on my view of the
data, only two of the nine trends mentioned are clearly visible and
significant in the observations, and these are slightly mislabelled. 
Two other highly significant trends are not mentioned.  Could you
comment why the SPM is so different from my trend maps?  I have attached

a very rough analysis for internal use, with lots of figures.   The main

points are listed briefly below, in the order of the SPM final draft
(SPM-6 line 5-10).

1) The eastern North America trend seems weak, confined to a small area
in Canada, so labelling it "eastern North America" is misleading.

2) The South American trends are poorly specified; if the trends in
Argentina are meant, why use the phrase "eastern"?  It is also absent in
the GPCC datasets.

3) The trend in Northern Europe is in the winter only, this should be
mentioned.

4) The North Asian trend is not a trend but a discontinuity in 1940,
which looks suspiciously like a change in the observing system.

5) I see no significant trends in Central Asia except for 3 stations in
the far west of China.

6) The trend in the Sahel is only significant when you start late and
finish early; rainfall has increased substantially again since 1995. 
Given the large decadal variability in the first half of the century,
and the attribution to aerosols of the drought in the 1970s and 1980s, I
would hesitate to call the remaining trend "significant".  Also, it is
only the western Sahel that has a trend, not the eastern Sahel.

7) In the Mediteranean there is only a significant trend in North
Africa, there is no significant trend on the northern shores.  Labelling
it "Mediterranean" is therefore misleading.

8) I see a drying trend in southern Africa only in the Zambia, I do not
know the quality of the data there.  Averaged over all of southern
Africa as implied in the text there is no trend.

9) Parts of southern Asia.  Which parts?

Two trends that are not included, but highly significant in all datasets
are an increase in precipitation in western Australia, and a decrease in
western coastal Africa, see the maps in the attachment.

Could you shed some light on this discrepancy arises, and what can be
done to close the gap?

Greetings from calm & sunny Holland (after a big storm),

     Geert Jan van Oldenborgh

--
Geert Jan van Oldenborgh
Global Climate Division
Royal Dutch Meteorological Institute (KNMI)
oldenborgh@knmi.nl          http://www.knmi.nl/~oldenbor
