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We find the large-scale precipitation (Fig. 2, blue curves) to
have a positive trend in winter, and a negative trend in
summer, much like that found in the observations. In
winter, the large-scale precipitation is mainly due to the
migration of large scale moist air masses from the Atlantic
and Arctic oceans. As the continental air is cold and easily
saturated, there will be an increase in the cloud and
precipitation formation with rising temperatures. In summer,
the warmer air is not as readily saturated, in fact it gets more
difficult to saturate the higher the temperature. Therefore,
the large-scale precipitation, which has a typical time scale
of one day, decreases with increasing temperature as there is
simply not enough water vapor available for cloud
formation. The atmospheric moisture variables are further
studied in Berg et al. (submitted to JGR)
month, thereafter it shows a steep negative trend. The peak
coincides closely with the range for the interruption of the
negative summertime trend in the observational data.
When studying the sum of the convective and large-scale
precipitation we see that the large-scale is generally
dominating, except for in the warm season at the higher
end of the temperature range.
5. Discussion
The results presented here, and further in Berg et al.
(submitted to JGR), shows that the precipitation intensity
is restricted by the C-C relationship in winter, while for
summer, the general trend is toward weaker events with
higher temperature. The convective precipitation shows a
different dependence on temperature compared to the
large-scale precipitation, but it too shows a negative trend
at the higher end of the temperature range. This leads us to
argue that also the convective precipitation is limited by
the availability of moisture and the level to reach for
saturating the atmosphere at the higher temperatures. To
further explore this argument we need to look further into
sub-daily data to properly resolve the convective events.
Furthermore, it would be interesting to study these
phenomena in other regions of the world to assess the
generality of the results.
6. Acknowledgments
We acknowledge the funding of the European Union FP6
project WATCH (contract nr. 036946). We further
acknowledge the model data sets contributed by the
HIRHAM, REMO and HadRM groups, and the
observational data set from the ECA&D project
(http://eca.knmi.nl), through the ENSEMBLES project.
References
Berg, P., J. Haerter, P. Thejll, C. Piani, S. Hagemann, J. H.
Christensen, Moisture availability and the relationship
between daily precipitation intensity and surface
temperature, submitted to JGR.
Figure 2: The 99 th percentile of precipitation intensity larger
than 0.1 mm/day as a function of daily average temperature
for the HIRHAM4 model. The curves indicate the total
(black), convective (red), and large-scale (blue)
precipitation. The dashed line in the January panel shows a
C-C like increase with temperature. Note the logarithmic
vertical axis. The relative contribution to the total
precipitation of the two sub types of precipitation is shown
in the gray inserted curve at the bottom of each plot. The
dotted line in the middle indicates equal contributions, while
above (below) the convective (large-scale) type dominates.
The convective precipitation (Fig. 2, red curves) has a much
shorter time scale, and a more intense convection, so it is not
bound in the same way to the availability of moisture. That
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large-scale cloud formation on the daily scale, a smaller
scale convective event with a shorter time scale can still
collect enough moisture to produce an intense precipitation
event. Furthermore, we find the convective precipitation to
increase at a rate similar to the C-C rate, but that it has peak
intensity at between ten to twenty degrees, depending on the
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