On convective enhancement of Vb-events in present and warmer climate

An important source of floods in Central Europe are so-called Vb cyclones (i.e., cyclones that move across the Mediterranean Sea and then move northward toward Central Europe along the eastern flank of the European Alps). Embedded convective precipitation contributes to flood risk through Vb precipitation events. This contribution depends on temperature and additional variables.

This study presents km-scale, i.e. convection-permitting, simulations with limited-area model COSMO-CLM using the MedCORDEX domain and quantification of the convective precipitation contribution during a set of extreme Vb events using a Lagrangian method for tracking convective cells [1]. In addition, this set of km-scale Vb event simulations is used to train a diagnostic of the convective fraction (based on simulated vertical velocity and vorticity in the mid-troposphere) following Poujol et al. [2]. This diagnostic method, applied to a coupled MedCORDEX COSMO-CLM/NEMO regional climate simulation with an atmospheric grid spacing of about 12 km, allows us to investigate and quantify the change in the convective fraction of Vb precipitation in an SSP5-8.5 climate scenario. In this simulation, the mean Vb precipitation decreases over the course of the century, but the convective fraction, and thus the flood risk associated with locally intense precipitation, increases.

[1] Purr. C., E. Brisson, and B. Ahrens, Convective rain cell characteristics and scaling in climate projections for Germany, Int. J. of Climatology, 41, 3174-3185 (2021).

[2] Poujol, B., S. Sobolowski, P.A. Mooney, S. Berthou, A physically based precipitation separation algorithm for convection‐permitting models over complex topography, Quarterly Journal of the Royal Meteorological Society, 146, 748-761 (2019).