Sunday, June 10, 2012


date: Wed, 6 Aug 2008 16:58:32 +0100 (BST)
from: David Lister <>
subject: Draft report (fwd)


---------- Forwarded message ----------
Date: Fri, 25 Jul 2008 10:23:08 +0200
From: "Buishand, Adri (KNMI)" <>
To: "David Lister, env (School of Environmental Sciences) " <>
Subject: Draft report

Dear David,

I read the draft report on your work for WP5.4 last weekend. I also
consulted two of my colleagues about a number of points this week.

The meaning of the values for the decadal trends in Figs. 3 - 7 is not
entirely clear to me. Do these values refer to the slope of a linear
regression fit? Does the brown colour in e.g. the upper right panel of
Fig. 5 indicate that there is an upward trend of 1 to 1.5 degrees per
decade (or 4.5 to 6.7 degrees over the 45 year period) in parts of
northern and eastern Europe during spring?

I think it may be good to say something about the statistical
significance of the trends. Further, some references could be given to
other results on trends in extremes. I wonder what is already known and
what is new. A striking point in Fig. 5 is the negative trend over
Turkey. One of our Greek partners in ENSEMBLES mailed me once a
Climate Research publication (Vol. 30, 161-174, 2006) in which negative
trends in Tn_10 were found over parts of Greece.

My colleague Albert Klein Tank compared your results with his trends in
the number of days > Tx_90 and number of days < Tn_10 in the ECA data
set. Looks good. The only difference is that the large positive trends
in Tn_05 over Finland, the Baltic states, Russia and the Ukraine in
spring (rose , red and brown colours in the upper right panel of Fig. 5)
are not found in the number of days < Tn_10 (most station trends are
not significant in thist area). These relatively large values do also
not show up in your Fig. 6 for the trends in Tn in the CRUTS dataset.

Figures 8 - 11 show considerable differences between the trends in the
WP5.1 gridded observed dataset and those in the ERA40 driven output. I
wonder how far these difference could be explained. I showed Fig. 8 to
Erik van Meijgaard who is involved in the RACMO2 simulations. The trend
differences are sometimes more than 1 degree per decade (in parts of
northern Europe in spring, and in southern Norway and small parts of
central Europe in autumn). Does this imply that there is no trend, or
even a decreasing trend, in the RACMO2 simulation? Is there a bias in
Tn_-05 that changes over time? This may occur in spring in northern
Scandinavia if the model is unable to reproduce snow melt. Another point
is that RACMO2 has difficulties with low temperatures in stable boundary
layers. This likely leads to a positive bias in Tn_05 (RACMO2 warmer
than observed), especially during winter. But it is unclear how this
should lead to a trend in Tn-_05. Furthermore, unlike the other regional
climate models, RACMO2 has the same physics as ERA40 reanalysis and
biases in the RACMO2 simulation may therefore also show up in the ERA40
reanalysis. Because the ERA40 reanalysis may suffer from the same
shortcomings as the RACMO2 simulation, it might be interesting to see
whether the trends in Tn-05 in the ERA40 reanalysis differ from those in
the WP5.1 gridded observations. But please keep in mind the difference
between the trends in Tn-05 and those in other Tn indices noted above
for Finland, the Baltic states, Russia and the Ukraine in spring.

Concluding, it might be useful to think a bit about the causes of trend
differences. Maybe we can have some discussion on it at an ENSEMBLES
meeting. Are you going to attend the Santander General Assembly?

Kind regards,



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