Precipitation-driven typology of storms in the Alps

Georgia Papacharalampous , Eleonora Dallan, Moshe Armon, Joydeb Saha, Colin Price, Marco Borga, Francesco Marra

Numerous advances in precipitation science hinge on our ability to categorize storms into homogeneous classes, for instance to isolate time series with convective- or stratiform-like characteristics. Nonetheless, achieving such classifications remains challenging. Here, we use an Alpine storm typology developed through a straightforward methodology for unsupervised classification based on gauge-based precipitation only. From a vast sub-hourly dataset, we extracted over 790,000 independent storm time series. To categorize these, we employed a resampling-based partitioning algorithm, optimal in clustering big data. The algorithm revealed five dominant storm classes, with distinct characteristics in terms of maximum sub-hourly intensity, total storm volume, total duration and temporal variability, and clear spatial organization. Examination of additional characteristics (month of initiation, solar time at maximum intensity and lightning count) prompts us to suggest one class is likely associated with convective-like events (high intensities, peaks in summer and afternoon, strong association with lightning) and one is likely associated with stratiform-like events (medium intensities, large volumes, long durations, low association with lightning). A third class is characterized by moderate intensities and volumes, and relatively short durations, while the remaining two classes gather minor storms. The proposed typology could support modelling applications, such as class-specific stochastic simulation, class-informed bias adjustment of climate projections or multi-class extreme value analyses. Investigations of its climatological traits revealed, among others, higher activity of the convective-like class in recent years and specific Alpine regions. We provide the historical occurrences of the classes as an open dataset to facilitate further investigation of Alpine storm dynamics.