The Scaling of Short-Term Precipitation Events in a Changing Climate

The scaling of heavy precipitation events in a changing climate is a fundamental question and of key importance for climate change adaptation. Several decades ago it has been postulated that heavy precipitation events should scale with the Clausius-Clapeyron relationship, i.e. their intensity should increase with about 7% per degree of global warming. This is a surprisingly simple relationship, which would be of great practical importance for the planning of future water resource infrastructure. During the last decades, theoretical, observational and modeling studies have largely confirmed this hypothesis for daily events. However, the behavior of short-term (hourly and below) precipitation events is often more significant than that of daily events, due to their paramount role in flash and urban flooding, as well as for debris and mud flows. Several recent studies (e.g. Lenderink and van Meijgaard, 2008) have presented evidence that the observed precipitation scaling for hourly events has about twice the Clausius-Clapeyron rate, and amounts to about 14%/K. This peculiar scaling is also referred to as super-adiabatic scaling. The scaling exhibits pronounced deviations from the Clausius-Clapeyron relationship that would seriously affect the future planning of water resource infrastructure.

Here we present results from decade-long convection-resolving regional climate simulations at Alpine and European scales (Ban et al. 2014, 2015; Leutwyler et al. 2016a, 2016b) with a spatial resolution of 2.2 km using the COSMO model. The European-scale simulations employ a computational domain with 1536x1536x60 grid points. Such simulations have become feasible with a model version that runs entirely on Graphics Processing Units (Fuhrer et al. 2014, Leutwyler et al. 2016a, 2016b). Comparison against simulations with parameterized convection demonstrate that hourly events are much better captured at convection-resolving resolution -- in terms of diurnal cycle, precipitation frequency, and extreme events. Scenario simulations with an RCP8.5 emissions scenario suggest that for the summer season, when mean precipitation is projected to decrease significantly over Southern Europe and the Alps, the intensity of extreme events increases consistently with the Clausius-Clapeyron scaling, i.e. heavy events asymptotically increase with <7%/K, i.e. much slower than the super-adiabatic scaling suggests.

Analysis reveals that some of these differences are related to the different use of precipitation percentiles (Schar et al. 2016). Lenderink and van Meijgaard (2008) used wet-hour percentiles that are conditional upon the occurrence of precipitation. We demonstrate that our results are consistent with their analyses. In particular, observations and simulations exhibit a wet-hour super-adiabatic scaling. Yet, when considering the results in terms of all-hour percentiles, which is more relevant in term of impacts, results demonstrate that the currently observed wet-hour scaling cannot be extrapolated into the future, as changes in precipitation frequency are intimately linked to changes in wet-hour occurrence.