Scale-dependency of extreme precipitation processes in regional climate simulations of the greater Alpine region

Abstract

Abstract. Heavy Precipitation Events (HPEs) are a challenging atmospheric phenomenon with a high impact on human lives and infrastructures. The achievement of high-resolution simulations for Convection Permitting Modelling (CPM) has brought relevant advancements in the representation of HPEs in climate simulations compared to coarser resolution Regional Climate Models (RCM). However, further insight is needed on the scale-dependency of mesoscale precipitation processes. In this study, we aim at evaluating reanalysis-driven climate simulations of the greater Alpine area in recent climate conditions and assessing the scale-dependency of thermodynamical processes influencing extreme precipitation. We evaluate COSMO-CLM simulations of the period 1971–2015, at resolutions of 25 km (RCM) and 3 km (CPM) downscaled from ERA-40 and ERA-interim. We validate our simulations against high-resolution observations (EOBS, HYRAS, MSWEP, and UWYO). In the methodology, we present a revisited version of the Precipitation Severity Index (PSI) useful for extremes detection. Furthermore, we obtain the main modes of precipitation variance and synoptic Weather Types (WTs) associated with extreme precipitation using Principal Component Analysis (PCA). PCA is also used to derive composites of model variables associated with the thermodynamical processes of heavy precipitation. The results indicate a good detection capability of the PSI for precipitation extremes. We identified four WTs as precursors of extreme precipitation in winter, associated with stationary fronts or a zonal flow regimes. In summer, 5 WTs bring heavy precipitation, associated with upper-level elongated troughs over western Europe, sometimes evolving into cut-off lows, or by winter-like situations of strong zonal circulation. The model evaluation showed that CPM (3 km) represents higher precipitation intensities, better rank correlation, better hit rates for extremes detection, and an improved representation of heavy precipitation amount and structure for selected HPEs compared to RCM (25 km). CPM overestimates grid point precipitation rates especially over elevated terrain fostered by the scale-dependency of convective dynamic processes such as stronger updrafts and more triggering of convective cells. However, at low altitudes, precipitation differences due to resolution are explained through the scale-dependency of thermodynamic variables, where the largest impact is caused by differences in surface moisture up to 1 g kg-1. These differences show a predominant north-south gradient where locations north of the Alps show larger (lower) surface moisture and precipitation in CPM (RCM) and locations south of the Alps show larger (lower) humidity and precipitation in RCM (CPM). The humidity differences are caused by an uneven partition of latent and sensible heat fluxes between RCM and CPM. RCM simulates larger emissions of latent heat flux over the Sea (25 W m-2 more), and CPM emits larger latent heat over land (15 W m-2 more). In turn, RCM emits larger surface sensible heat fluxes over land (30 W m-2 more), showing a warmer surface (0.5 to 1 °C) than CPM. These results provide evidence that CPM is a powerful tool for obtaining accurate high-resolution climate information also pointing at the different scale-dependency of dynamic and thermodynamical precipitation processes at high and low terrain.

Identification of regions with a robust increase of heavy precipitation events

“…It should be noted, however, that both the resolution and the convection parameterization used by the models may hinder an adequate reproduction of convective precipitation events, as discussed by Giorgi et al (2019). Furthermore, simulations over regions with highly structured topography such as the Alps are generally problematic at relatively coarse resolution, and can still pose problems at higher resolutions, as described by Caldas-Alvarez et al (2022).…”

A multi-disciplinary analysis of the exceptional flood event of July 2021 in central Europe – Part 2: Historical context and relation to climate change

“…3. This intensification can often be related to embedded convection initiated by orography (e.g., Fuhrer and Schär, 2005;Cannon et al, 2012) or frontal systems (e.g., Trier et al, 1991;Weckwerth and Parsons, 2006).…”

A multi-disciplinary analysis of the exceptional flood event of July 2021 in central Europe. Part 2: Historical context and relation to climate change

“…The second measure used is the Precipitation Severity Index (PSI; Caldas-Alvarez et al, 2022b). It identifies extreme precipitation events based on three characteristics: grid point intensity, the affected area, and persistence.…”

“…Finally, a temporal summation was applied to grid points exceeding a ratio of one for up to two days prior to the considered event day. The PSI has been successfully implemented in a recent study about heavy precipitation (Piper et al, 2016;Caldas-Alvarez et al, 2022b). Due to the temporal summation, the PSI tends to shift the day of maximum severity by one day compared to HPE crit .…”

“…For example, for the historical period, the LAERTES-EU data set (Ehmele et al, 2020(Ehmele et al, , 2022 with over 12 000 years at 25 km horizontal resolution provides an excellent basis to estimate how uncommon precipitation values are in the scope of recent climate conditions. Moreover, the examination of convectionpermitting ensemble simulations (Prein et al, 2015;Ban et al, 2021) further improves the basis for such an evaluation, given that such models exhibit a largely reduced bias in terms of precipitation intensities compared to lower resolution (12 to 25 km) climate models (Prein et al, 2015;Caldas-Alvarez et al, 2022b).…”