Background
The authors write that "estimates of future changes in extremes of multi-day precipitation sums are critical for estimates of future discharge extremes of large river basins and changes in [the] frequency of major flooding events," citing Kew et al. (2010); and they indicate, in this regard, that "a correct representation of past changes is an important condition to have confidence in projections for the future."

What was done
In an attempt to achieve some of that all-important confidence, van Haren et al. investigated changes in multi-day precipitation extremes in late winter in Europe and the Rhine river basin over the last 60 years using (1) daily precipitation data and (2) "the state-of-the-art gridded high resolution (0.5°) precipitation fields of the European ENSEMBLES project version 7.0 (Haylock et al. 2008)," where "observations [were] averaged to the same regular 1.5° grid when compared directly with the model results."

What was learned
The four researchers determined that the climate models "underestimate the trend in extreme precipitation in the northern half of Europe" because they "underestimate the change in circulation over the past century and as a result have a much smaller (extreme) precipitation response." More specifically, they state that "a dipole in the sea-level pressure trend over continental Europe causes positive trends in extremes in northern Europe and negative trends in the Iberian Peninsula," while "climate models have a much weaker pressure trend dipole and as a result a much weaker (extreme) precipitation response."

What it means
Van Haren et al. conclude their report by declaring that "it is important that we improve our understanding of circulation changes, in particular related to the cause of the apparent mismatch between observed and modeled circulation trends over the past century," citing Haarsma et al. (2013); for if the models don't improve in this regard, neither will their precipitation predictions improve.

References
Haarsma, R., Selten, F. and van Oldenborgh, G. 2013. Anthropogenic changes of the thermal and zonal flow structure over Western Europe and Eastern North Atlantic in CMIP3 and CMIP5 models. Climate Dynamics 10.1007/s00382-013-1734-8.

Haylock, M.R., Hofstra, N., Tank, A.M.G.K., Klok, E.J., Jones, P.D. and New, M. 2008. A European daily high-resolution gridded data set of surface temperature and precipitation for 1950-2006. Journal of Geophysical Research 113: 10.1029/2008JD010201.

Kew, S.F., Selten, F.M., Lenderink, G. and Hazeleger, W. 2010. Robust assessment of future changes in extreme precipitation over the Rhine basin using a GCM. Hydrology and Earth Systems Science Discussions 7: 9043-9066.