Long-Term Simulations of Thermally Driven Flows and Orographic Convection at Convection-Parameterizing and Cloud-Resolving Resolutions

The purpose of this paper is to validate the representation of topographic flows and moist convection over the European Alps in a convection-parameterizing simulation (CPM; Delta x = 6.6 km) and two cloud-resolving simulations (CRM; Delta x = 1.1 and 2.2 km). All simulations and further sensitivity experiments are validated against a large set of observations for an 18-day fair-weather summer period. The episode considered is characterized by pronounced plain-valley pressure gradients, strong daytime upvalley flows, and weak nighttime down-valley flows. In addition, convective precipitation is recorded during the late afternoon and is preceded by a phase of shallow convection. The observed transition from shallow to deep convection occurs within a 3-h period. The results indicate good agreement between both CRMs and the observed diurnal evolution in terms of near-surface winds, cloud formation, and precipitation. The differences between the two CRMs are surprisingly small. In contrast, the CPM produces too-early peaks of cloud cover and precipitation that are due to a too-early activation of deep convection. Detailed sensitivity experiments show that the convection scheme, rather than the underresolved small-scale topography, is responsible for the poor performance of the CPM. In addition, observations and simulations show that late-morning mass convergence does not correlate with afternoon precipitation. Rather, it is found that enhanced convective activity is related to increased conditional instability.