A robust shift towards higher intensity convective and orographic rainfall over the Alps in a warmer climate

Recent research with “km-scale” or “convection-permitting” climate models (resolutions with grid-spacing generally < 4km) has described substantial improvements in the representation of precipitation when compared to conventional parameterized models. In particular, the distribution of precipitation is more faithfully reproduced and, in particular, precipitation extremes are more closely aligned with observations in terms of frequency, magnitude and duration. Future changes for many regions largely follow the mantra “the extremes become more extreme”. These results imply serious consequences arising from impacts commonly associated with extreme precipitation such as flash flooding, landslides, as well as  water resources availability. However, questions remain regarding the robustness of these responses as well as which types of precipitation contribute (and how) to the projected changes. Most of the existing literature at convection permitting scale consists of one or two model experiments and characteristics of precipitation are often defined based on arbitrary intensity thresholds. 

Here we employ the coordinated, multi-model ensemble of convection-permitting simulations generated within the WCRP CORDEX Flagship Pilot Study on Convection over Europe and the Mediterranean. While the domain covers the greater Alpine region we focus here on the Alps themselves given its exposure to a  wide variety of storm types and extremes and the importance of representing convection and its interactions with the complex topography for the local climate. This multi-model ensemble is now complete and the present paper investigates the changing characteristics of precipitation over the complex terrain of the Alps.

A physically-based algorithm is employed to categorize precipitation as either convective, stratiform or orographic. The algorithm was specifically designed for use with km-scale modeling and uses commonly available variables on only a few levels of the atmosphere. This algorithm has been shown previously to accurately categorize precipitation types over the Scandanavian mountains as well as the Alps. 

The results show strong decreases in annual convective and orographic precipitation over the greater Alpine region, while stratiform precipitation changes little if at all. Upon closer inspection, using the Analysis of precipitation across scales method (AsoP) and traditional IDF analyses, a more nuanced picture emerges. IDF plots show that the frequency of high intensity events increases, across all durations over all Alpine regions (NW, NE, S). Conversely, frequency decreases for more moderate events, most strongly in the summer season. The AsoP analysis shows that this occurs due to a shifting of the entire distribution of precipitation for all precipitation types. This shift to higher intensities comes at the expense of more moderate intensity events, which decrease. While all seasons show similar patterns of change the change is most pronounced in summer. Convective and orographic precipitation show similar patterns but the magnitude of the change is largest for convective precipitation. Thus, despite an overall drying over the Alps, the extremes indeed become more extreme and more frequent. This behavior is remarkably robust across the entire ensemble.