Conclusions

What did we learn in this project, what are the limitations, and what should be further explored?

1. For hourly precipitation intensity on a local scale, we found evidence for a 10-14 % increase per degree warming. In recent years, the intensity of summer showers appears to be 15 % higher than before 1990, which is likely attributed a one-degree warming of the Netherlands. Extrapolating this relation to the future, increases of up to 100 % in intensity appear possible by the end of this century (with a warming of 5 degrees, which is approximately the upper range in the Dutch KNMI’06 climate scenarios). Extrapolating observed relations to such high temperatures is obviously dangerous – although results from Hong Kong suggest that it may work reasonably well – and therefore we also looked for evidence in regional climate models. Unfortunately, models appear to be very uncertain at predicting changes in hourly extremes with changes between close to zero and up to 60-80%. What is even worse is that the models fail to reproduce the observed relations for high temperatures, casting serious doubt on their ability to predict future changes. A major cause of these model deficiencies is related to the fact that these model do not resolve the physics of convective clouds that give rise to extreme precipitation intensities; instead they use simplified prescriptions, so-called parameterizations.
2. For large scale precipitation extremes connected to synoptic scale (>500 km) low pressure systems which occur mostly in the winter season, it was shown that the role of natural variability is large even on a 30-year time scale. When the natural variability was averaged out, it turned out that extremes increased at a rate similar to the mean precipitation change in winter. Typical increases are 3-7 % per degree warming, which are consistent with the KNMI’06 scenarios.
3. For the present-day climate, It was shown in that the probability of a NNW storm surge is larger after a period of 5-20 days of extreme precipitation in the Rhine catchment than climatology. The probability of a simultaneous occurrence of a high river discharge and NNW storm surge could be a factor 4 higher than in the case that these events are independent (as commonly assumed). This could have considerable implications for the safety norms. Yet, we would also like to note two limitations of this study. First, these results are obtained in a model with relatively coarse resolution. Second, in order to have sufficient statistics we could only look at moderate extremes occurring approximately once a year.
4. We studied precipitation extremes on two totally different scales: local intensity on an hourly time scale and multi-day precipitation on a scale of hundreds of km. In between there is a whole range of time and spatial scales, which are relevant for different users such as, for instance, the water boards. The event on the 26-27 August 2006, as discussed in here, underlined the importance of these intermediate scales. We do not have sufficient insight into how these intermediate-scale (mesoscale) precipitation extremes could change. They are affected by processes acting on different scales. Convective processes could give rise to increases in the order of 10-14 % per degree warming, yet large scale processes give rise to changes that are 2-3 times smaller. In addition, the influence of non-local processes, such as the occurrence of atmospheric rivers channeling moist air from the sub-tropics to our regions, are poorly understood and could give rise to unforeseen changes.

Much of the research will continue in the KfC research program “Theme 6: High quality climate projections”  which runs from
2011 to 2015.

References

Attema, J.J. & G. Lenderink, 2011. Mean precipitation changes and coastal effects in the ENSEMBLES regional climate model simulations. KNMI Scientific Report: WR-11-03, 30/8/2011.

Kew, S. F., F. M. Selten, G. Lenderink & W. Hazeleger, 2010. Robust assessment of future changes in extreme precipitation over the Rhine basin using a GCM.  Hydrology and Earth System Sciences, 7, 9043-9066, doi:10.5194/hessd-7-9043-2010.

Kew, S. F., F. M. Selten, G. Lenderink, 2011. Storm surges and high discharge:  a joint probbilities study. KNMI Scientific Report, WR 2011-05.

Lenderink, G. & E. van Meijgaard, 2010. Linking increases in hourly precipitation extremes to atmospheric temperature and moisture changes.

Environmental Research Letters, 2, 5, 025208, doi:10.1088/1748-9326/5/2/025208.

Lenderink, G., G. J. van Oldenborgh, E. van Meijgaard & J. Attema, 2011: Intensiteit van extreme neerslag in een veranderend klimaat. Meteorologica, 20, 2, 17-20.

Lenderink, G., H. Y. Mok,  T. C. Lee,  & G. J. van Oldenborgh, 2011. Scaling and trends of hourly precipitation extremes in two different climate zones – Hong Kong and the Netherlands. Hydrol. Earth Syst. Sci., 15, 3033-3041, doi:10.5194/hess-15-3033-2011.