The diurnal cycle of Alpine summer convection in a convection-resolving model: evaluation with satellite data and sensitivity to atmospheric forcing

Moist convection over Central Europe and the Alps is characterized by a pronounced diurnal cycle. Convective plumes are triggered between the late morning and sunset. These can be followed by the generation of major convective cells and thunderstorms that typically peak in the late afternoon or early evening. Convection is poorly represented in current convectionparameterizing models (CPMs), often leading to large biases in the diurnal cycle of precipitation, in particular a too early onset and peak of precipitation. These biases raise questions regarding the robustness of important aspects of European climate change scenarios for the summer season. Preliminary experience with convection-resolving models (CRMs) has shown promising results. In this thesis, the diurnal cycle of convection is investigated during 11 days in June 2007 with a pronounced diurnal cycle. For this purpose, the numerical model COSMO is used with horizontal resolutions of 12 km (CPM) and 2 km (CRM). The specific model setup used is explained in chapter 2. In chapter 3, the simulations are evaluated with three satellite and two ground-based observational products. Additionally to the standard one-moment microphysics scheme (1M), a two-moment microphysics scheme (2M) is used with the CRM. For the CPM, an overestimation of high cloud cover is found, which is reduced with the CRM with 1M, and further reduced with 2M. The main reason for the latter improvement can be attributed to the neglect of cloud-ice sedimentation in the 1M. Further, a negative mid-level cloud bias is found for all simulations. The overestimation of high cloud cover in the CPM leads to a compensation effect in the top of the atmosphere energy budget, with an underestimation of outgoing longwave radiation (OLR) and an overestimation of reflected solar radiation (RSR). For the CRM with 1M, no compensation effect is found due to an underestimation of both the OLR and RSR. This leads to an increased absolute bias in the top of the atmosphere energy budget compared to the CPM, despite the improvements found in high cloud cover. Although differences in high cloud cover are also found between the CRMs, the diurnal cycles of precipitation for 1M and 2M are virtually the same. In chapter 4, the study of chapter 3 is expanded to surrogate climate change experiments of the same period. The goal is to assess the sensitivity of the diurnal cycle of moist convection with respect to characteristic changes expected in a future climate. In the surrogate experiments, an increase of temperature and absolute humidity is applied at the lateral boundaries, compared to the control simulations, while relative humidity and the large-scale circulation are maintained. This mimics a possible future climate with the same synoptic forcing. Following a previous study, two types of warming are applied: a homogenous warming (HW) and a vertically-dependent warming (VW). For the investigated period, an increase of heavy precipitation events is found for HW and VW compared to control. This intensification in heavy precipitation is consistent with the Clausius-Clapeyron relation for the CRM simulations with 1M and 2M, but is beyond this relation for the CPM simulations. For clouds, similar amounts are found for control and HW, but a distinct reduction in high cloud cover is found for VW. In terms of the diurnal cycle of convection, low precipitation intensities, and the top of the at-