Summer extreme flooding in Central Europe is often associated with Vb-cyclones which travel through the Mediterranean, then northwards east of the Alps towards Central Europe. Extreme convective precipitation intensities scale with the Clausius-Clapeyron relation under global warming. This study quantifies the importance of convective precipitation during Vb-events in present and in warmer climate by simulating selected Vb-events with convection-permitting grid-spacing. A simple convective precipitation diagnostic is compared against Lagrangian convective cell tracking. The simple method shows skill identifying convective precipitation in coarser simulations with parameterized convection. On average, 30\% of precipitation is classified as convective in reanalysis and historical EC-Earth3 driven simulations. This fraction increases to 52\% in a warmer climate under SSP5-8.5 scenario. The increase is explained by a frequency increase of the convectively active cut-off low-pressure systems and a doubling of the convective fraction in the less active trough-like Vb-cyclones, suggesting amplified flood risk in a warmer climate.

A document by Larisa Tarasova. Click on the document to view its contents.

The total meridional heat transport (MHT) is relatively stable across different climates. Nevertheless, the strength of individual processes contributing to the total transport are not stable. Here we investigate the MHT and its main components especially in the atmosphere, in five coupled climate model simulations from the Deep-Time Model Intercomparison Project (DeepMIP). These simulations target the Early Eocene Climatic Optimum (EECO), a geological time period with high CO2 concentrations, analogous to the upper range of end-of-century CO2 projections. Preindustrial and early Eocene simulations at a range of CO2 levels (1x, 3x and 6x preindustrial values) are used to quantify the MHT changes in response to both CO2 and non-CO2 related forcings. We found that atmospheric poleward heat transport increases with CO2, while the effect of non-CO2 boundary conditions (e.g., paleogeography, land ice, vegetation) is causing more poleward atmospheric heat transport on the Northern and less on the Southern Hemisphere. The changes in paleogeography increase the heat transport via transient eddies at the mid-latitudes in the Eocene. The Hadley cells have an asymmetric response to both the CO2 and non-CO2 constraints. The poleward latent heat transport of monsoon systems increases with rising CO2 concentrations, but this effect is offset by the Eocene topography. Our results show that the changes in the monsoon systems’ latent heat transport is a robust feature of CO2 warming, which is in line with the currently observed precipitation increase of present day monsoon systems.

Limited-area convection-permitting climate models (CPMs) with horizontal grid-spacing less than $4$\,km are being used more and more frequently. CPMs represent small-scale features such as deep convection more realistically than coarser regional climate models (RCMs), and thus do not apply deep convection parameterisations (CPs). Because of computational costs CPMs tend to use smaller horizontal domains than RCMs. As all limited-area models (LAMs), CPMs suffer issues with lateral boundary conditions (LBCs) and nesting. We investigated these issues using idealised so-called Big-Brother (BB) experiments with the LAM COSMO-CLM ($\approx$ $ 2.4$\,km). Deep convection was triggered by idealised hills with driving data from simulations with different spatial resolutions, with/without a deep CP, and with different nesting frequencies and LBC formulations. All our nested idealised $2.4$\,km Little-Brother (LB) experiments performed worse than a coarser CPM simulation ($4.9$\,km) using a four times larger computational domain, but with only 50\% computational cost. A boundary zone of $>100$ grid-points of the LB could not be interpreted meteorologically because of spin-up of convection and boundary inconsistencies. A host with grid-spacing in the so-called grey zone of convection (ca. $4$ - $20$\,km) was not advantageous to the LB performance compared to an even coarser host. The LB performance was insensitive to the applied LBC formulation and updating (3-hourly or better). Therefore, our CPM experiments suggested opting for a larger domain instead of a higher resolution even if coarser than usual (i.e., $> 4$\,km). Better preconditioning the convectivity at the CPM inflow boundaries might decrease the spin-up zone’s depth.