A long‐lived supercell over mountainous terrain

• Published in: Quarterly journal of the Royal Meteorological Society Vol. 143; no. 709; pp. 2973 - 2986
• Main Authors: Scheffknecht, Phillip, Serafin, Stefano, Grubišić, Vanda
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.10.2017; Wiley Subscription Services, Inc
• Subjects: Alpine environments; Alps; Banks (topography); case‐study; Cold front; Convection; Environmental impact; Life cycle; Life cycles; Moist convection; numerical modelling; Ridges; Slope; Storm development; Storms; supercell; Topography; Topography (geology); Weather forecasting; wind shear; WRF
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.3127

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. On 2 August 2007, a supercell formed over eastern Switzerland and moved east through Austria along the northern side of the Alps for more than 8 h while covering over 400 km. The storm is examined using a high‐resolution hindcast simulation with the WRF model. It is shown how an approaching cold front with prefrontal southwesterly mid‐level wind and a northerly low‐level orographic circulation provided favourable wind shear for the formation and maintenance of a supercell.