Fourth Study Conference on BALTEX Scala Cinema Gudhjem

scale parameters the correlation between the anomalies
derived from ECMWF and NCEP reanalyses significantly
decreases during summer, being insignificant in most
locations in Southern Europe. Analysis of the commonly
shared leading modes in the statistical characteristics of
precipitation shows that the temporal variability of the
leading mode of the shape parameter in ERA40 differs
significantly from the other three reanalysis products in
summer.
Finally we discuss the problems of the comparisons between
the precipitation statistics derived from the station data and
from NWP products. Such a comparison should involve the
analysis of subgrid-scale precipitation, which is to a
different extent accounted for in NWP and station
collections. Station data in comparison to the reanalyses
show significantly higher shape parameter and, thus, higher
occurrence of heavy and extreme precipitation. Temporal
variability in the station data and reanalyses has been found
to be consistent for the extreme precipitation in winter and
shows little comparability in summer. The main problem of
the comparability of statistics derived from the station data
and NWP products is the real orography, which forces a
much stronger spatial inhomogeneity in the mesoscale
precipitation patterns, than in reanalyses. On the other hand,
inhomogeneous arrangement of stations does not adequately
sample the continental orography, while NWP products do
(if only a smoothed one). To partly account for these
problems we analysed the impact of subgrid-scale
precipitation on the variability patterns of extreme
precipitation statistics. Special analysis has been performed
in order to take into account the continuity of the extreme
precipitation. Our approach employs the analysis of 2dimensional
probability density functions in coordinates
“precipitation intensity-precipitation duration”. This allows
for a better quantification of the extremes with long
duration, which may lead to dramatic floods on the major
European rivers.
5. Conclusions
We conclude that different data sets exhibit quantitatively
different variability patterns of extreme precipitation. The
diagnosed differences in the statistical characteristics of
precipitation between different NWP products and between
reanalyses and station data vary within 30-40% on average.
This is larger than the differences in these characteristics
simulated by climate models in greenhouse gas experiments,
which report normally the largest changes between the
greenhouse gas experiments and the present climate to be
within 10 to 20%. Thus, one has to be careful when
choosing reanalysis data set for the description of what we
term the present climate.
6. Acknowledgements
This study is supported by the North Rhine-Westphalia
Academy of Science under the project “Large Scale Climate
Changes and their Environmental Relevance” and Russian
Foundation for Basic Research (grant 02-56331).
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