Kunz, M., Kottmeier, Ch.

Brombach, Joris

The aim of this study was the systematic identification of flow effects in southwestern Germany, which may favor the initiation or enhancement of convection. The investigation area covered most parts of Baden-Wuerttemberg with the Black Forest and the Swabian Jura as orographic decisive low mountain ranges. To analyse the flow effects, semi-idealized model simulations were carried out with the atmospheric prediction model COSMO. The simulations were initialized with a mean vertical profile of the atmospheric conditions on hail days. The better understanding of these intense events that can lead to large losses, is of great interest. Sensitivity studies with slightly modified boundary conditions in each simulation, especially of the incoming flow, the stability of the atmospheric stratification and the orography should show in which parameter range the identified flow effects occur. Like this, three orographic flow effects were identified in Baden-Wuerttemberg that occur systematically at typical atmospheric conditions on hail days.
First, there is a flow around the southern Black Forest with a ground-level convergence of the flow in the lee northeast of it. In addition, an overflow of the Swabian Jura from the south can be observed, which leads to the formation of ground-level flow convergence in a large area north of the Swabian Jura. Furthermore gravity waves are triggered here, which amplify the near-surface convergence. At the elevations of the northern and southern Black Forest gravity waves with even higher vertical velocities are triggered. In addition to their effects on ground-level divergence for example in the Kinzig valley between the northern and southern Black Forest, the updraft regions of these waves may trigger or enhance convection in higher atmospheric layers.