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

Abstract

Convective precipitation structures during an Alpine heavy precipitation event inAugust 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.

Observation and modeling analyses of the macro- and microphysical characteristics of a heavy rain storm in Beijing

“…The long axis of storms is usually parallel to the direction of wind on the leeside. Langhans et al (2011) revealed that the intensity of one of the rainbands they studied was related to the location of the topography.…”

A long‐lived supercell over mountainous terrain

“…Also, sensible heat fluxes in mountainous regions induce slope and valley breezes which transport heat and moisture both vertically and horizontally (Whiteman, ), modulating CIN and affecting low‐level shear. Langhans et al () demonstrated that squall lines can be weakened or suppressed by increased CIN due to descending flow in the lee of a ridge.…”

Effect of Spatial Resolution and Cumulus Parameterization on Simulated Precipitation over South Asia

“…In contrast, a microphysical process and a triggering of cloud convection are explicitly calculated in experiments using a fine mesh size (e.g., Prein et al 2015), resulting in an improvement in the thermodynamical processes associated with cumulus convection. A finer mesh size also improves the computation of orographic precipitation because it can resolve the dynamical effect of mountains on heavy precipitation (Leung and Qian 2003;Langhans et al 2011). For these reasons, improved performance is expected from numerical models in experiments using a higher spatial resolution without the CP, relative to experiments using a coarse mesh size but with the CP.…”