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Will future summer time temperatures go berserk?
Jens H. Christensen, Fredrik Boberg, Philippe Lucas-Picher, and Ole B. Christensen
Danish Meteorological Institute, Lyngbyvej 100, DK-2100 Copenhagen Ø, Denmark; jhc@dmi.dk
1. Temperature bias in perfect boundary
experiments
In a recent analysis of monthly mean model temperature bias
over Europe of the complete ensemble of regional climate
models (RCMs) participating in the EU FP6 project
ENSEMBLES, it was demonstrated that models tend to have
a common behavior when temperatures are getting very high
Christensen et al. (2008). Monthly mean temperature bias in
most RCMs tends to increase (in a positive direction) as the
simulated temperature increase, e.g. typically, summers are
simulated warmer than they are in reality.
Figure 1a depicts the essentials of the work presented in
Christensen et al. (2008). The figure shows monthly mean
model bias with respect to modeled monthly mean
temperature for a region covering most of the Mediterranean
region (see Christensen & Christensen (2007) for details)
covering the period 1961-2000. Here we have chosen to
show bias wrt. model temperature, whereas Christensen et
al. showed bias wrt. observed temperature. Obviously these
are equivalent, but the former is chosen for reasons that will
become obvious. The figure indicates the bias for the
individual months as simulated by three models, REMO,
RACMO and RCA when nested in ERA40, shown by dots
(more models will be included when the paper is presented).
The full lines represent the best fitted second order
polynomial fit, while the dashed lines represent the
extension of this functional relation into a warmer regime
not yet realized by the simulations.
2. Climate change simulations
Currently, the ENSEMBLES RCMs are providing their
results from climate change experiments taking boundary
conditions from a suite of GCMs. These are being archived
and are publicly available. Only three models were fully
completed and fully available in time for this manuscript.
However, the three models all provide information until the
end of the 21 st century.
Figure 1b illustrates how the monthly mean temperatures for
different 30 year periods change with respect to the
simulated mean temperature in the particular cases of 1961-
1990, 2011-2040, and 2071-2100. Figure 1c shows how the
bias as estimated by the best fit curve deduced in Figure 1a
can be used to adjust the simulated values as it is clear that
the warmer the month in concern is, the larger also the bias
is. Thus all temperatures are shifted to a somewhat lower
level depending on the actual temperature (see Christensen
et al. 2008 for details).
Figure 1d shows the adjusted (non-linearly) bias corrected
monthly mean climate change temperatures. It is seen that
for two of the models the bias correction almost entirely
eliminates the seasonal warming signal. The third model did
not show this behavior. This is explained by the almost
temperature independent behavior of the bias, while the two
other models apparently exhibit quite a strong temperature
dependent bias.
3. Conclusion and discussion
There appears to be a non-linear bias behavior in the ability
of some RCMs to simulate the annual cycle of monthly
mean temperatures. This behavior indicates the possibility
that when these models are used to simulate climate
change due to an increase in greenhouse gas
concentrations, they are likely to overestimate the
warming, particularly in the warmest months. Here we
1951-1980
2071-2100
2011-2040
Figure 1. Monthly mean temperature bias (top) and
warming for three ENSEMBLES RCMs. For details
see text
have illustrated that for two models participating in
ENSEMBLES, a correction for this bias leads to a more
uniform temperature increase throughout the year. The last
model, which apparently does not exhibit a large
temperature dependent bias still shows a marked warming