The orographic impact on patterns of embedded convection during the August 2005 Alpine flood

Convective precipitation structures during an Alpine heavy precipitation event in August 2005 are investigated utilising a mesoscale non‐hydrostatic numerical model and observational data. The focus is on the mechanisms of convective initiation during the very beginning of the event, when organised cellular updraughts enhanced the precipitation over the southern German Alpine foreland. A set of sensitivity experiments with systematically modified topography is conducted in order to investigate the role of single topographic obstacles in initiating and arranging convection. A reference simulation proves to be capable of qualitatively reproducing the development of two distinct, flow‐parallel aligned, non‐stationary, convective precipitation bands, which are observed by radar and surface measurements. The simulations reveal the presence of two low‐level convergence lines that provide a primary mechanism for both initiating and aligning convective cells during the analysed period. Mountains embedded in the Alpine foreland are not decisive for triggering and aligning the convective cells in this case, since the primary mechanism provides a consistent forcing for the convective precipitation bands. However, the experiments demonstrate a dependence of the alignment and intensity of one of the precipitation bands upon the position of the pre‐existing convergence line relative to a flow‐parallel aligned mountain ridge. Enhancement (weakening) of the existing banded structure and convective intensity is achieved if the convergence line is located upstream (downstream) of the ridge with respect to a cross‐ridge low‐level inflow. The ‘weakening’ mode is characterised by a subsiding cross‐ridge inflow into the convergence line that stabilises the atmosphere and therefore inhibits the formation of a continuous convective precipitation band.