Editorial Type:
Article
Article Type:
Research Article

The Missing Link between Terrain-Induced Potential Vorticity Banners and Banded Convection

Simon K. Siedersleben Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innsbruck, Austria

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Alexander Gohm Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innsbruck, Austria

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Print Publication:
01 Nov 2016
DOI:
https://doi.org/10.1175/MWR-D-16-0042.1
Page(s):
4063–4080
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Abstract

On 1 February 2014, the southern side of the Alps was affected by a severe snowstorm that forced authorities to issue the highest level of avalanche danger in southern parts of Austria. The northern side of the Alps was mostly dry. Nevertheless, radar imagery captured the evolution of quasi-steady convective cloud bands over the northern Alpine foreland with a remarkable length of up to 300 km. This study illuminates the processes that generated these cloud bands based on numerical simulations. The storm was associated with a deep large-scale trough that caused strong southwesterly cross-Alpine flow, orographic precipitation on the southern side, and foehnlike subsidence on the northern side of the Alps. Orographic potential vorticity (PV) banners developed at small-scale topographic features embedded in the Alps and extended downstream over the northern Alpine foreland. Convective cloud bands were aligned parallel to these PV banners. They formed in an environment of inertial instability (negative absolute vorticity) and conditional instability. Sensitivity experiments reveal that the structure and size of these cloud bands are strongly sensitive to the small-scale terrain roughness. Removing small-scale topographic features suppresses the formation of orographic vorticity banners, which in turn suppresses the development of cloud bands. These results suggest that the release of inertial instability at negative orographic vorticity banners was crucial for establishing circulations and associated uplift that triggered conditional instability. To summarize, inertial instability was most likely responsible for the banded structure and conditional instability for the convective nature of these cloud bands.

Denotes Open Access content.

Corresponding author address: Simon K. Siedersleben, Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innrain 52f, Innsbruck 6020, Austria. E-mail: simon.siedersleben@gmail.com

Abstract

On 1 February 2014, the southern side of the Alps was affected by a severe snowstorm that forced authorities to issue the highest level of avalanche danger in southern parts of Austria. The northern side of the Alps was mostly dry. Nevertheless, radar imagery captured the evolution of quasi-steady convective cloud bands over the northern Alpine foreland with a remarkable length of up to 300 km. This study illuminates the processes that generated these cloud bands based on numerical simulations. The storm was associated with a deep large-scale trough that caused strong southwesterly cross-Alpine flow, orographic precipitation on the southern side, and foehnlike subsidence on the northern side of the Alps. Orographic potential vorticity (PV) banners developed at small-scale topographic features embedded in the Alps and extended downstream over the northern Alpine foreland. Convective cloud bands were aligned parallel to these PV banners. They formed in an environment of inertial instability (negative absolute vorticity) and conditional instability. Sensitivity experiments reveal that the structure and size of these cloud bands are strongly sensitive to the small-scale terrain roughness. Removing small-scale topographic features suppresses the formation of orographic vorticity banners, which in turn suppresses the development of cloud bands. These results suggest that the release of inertial instability at negative orographic vorticity banners was crucial for establishing circulations and associated uplift that triggered conditional instability. To summarize, inertial instability was most likely responsible for the banded structure and conditional instability for the convective nature of these cloud bands.

Denotes Open Access content.

Corresponding author address: Simon K. Siedersleben, Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innrain 52f, Innsbruck 6020, Austria. E-mail: simon.siedersleben@gmail.com
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