A long-lived supercell over mountainous terrain

The life cycle of a convective storm forming over highly complex topography on the northern side of the Alps is studied. Moist convection began ahead of a cold front in the late morning of 2 August 2007 in eastern Switzerland. It developed into a supercell storm that moved along the Alpine main crest for more than 8 h, passed over several 2000 m high ridges and ultimately dissipated over eastern Austria. This study analyzes the impact of topography on the pre-storm environment and on the storm development using several simulations with the Weather Research and Forecasting (WRF) model at a minimum horizontal grid spacing of 833 m. A hindcast simulation of the event features a convective system evolving in good agreement with observations. Variations in the simulated storm intensity appear to be related to the topography below the storm. Two idealized simulations help understand how the Alpine topography affected storm initiation and development. North of the Alps, relatively strong shear between southwesterly synoptic flow aloft and thermally induced plain-to-mountain flow near the ground created favourable conditions for supercell development. Small-scale terrain features supported the upward transport of moisture via slope circulations, locally reducing CIN and increasing CAPE, and ultimately enhancing the storm longevity.