Spatial heterogeneity of the Inn Valley Cold Air Pool during south foehn: Observations from an array of temperature loggers during PIANO

Abstract

Interactions between foehn and mountain-valley cold air pools (CAPs) strongly influence severe weather and air quality at the valley bottom, but have seen limited research compared to the fully established foehn phase. The Penetration and Interruption of Alpine Foehn (PIANO) campaign was conducted in the Inn Valley near Innsbruck (Austria) during fall and early winter 2017 to investigate these interactions. The focus of this paper is evaluating spatial heterogeneity of the Inn Valley CAP near Innsbruck during south foehn using observations from the six PIANO Intensive Observation Periods (IOPs). In particular, the analysis is concentrated on an array of 50 temperature loggers deployed throughout the greater Innsbruck area. A selection of these loggers were used for slope profiles above the valley floor at four different locations (north, south, east and west of the city center). To obtain a single quantitative measure combining information of both CAP strength and depth, temperature observations from the slope stations were used to approximate mean potential temperatures for each profile. It was found that foehn can lead to strong CAP heterogeneity in both south-north and west-east directions. Stronger and/or thicker CAPs in the west can partially explain the well known pre-foehn westerlies in Innsbruck by enhancing the along-valley pressure gradient at the valley bottom. Typically observed initial foehn breakthroughs east of the city mirrored the spatial heterogeneity within the pre-foehn CAP. West-east differences in profile-mean potential temperatures were compared to west-east valley bottom pressure differences to investigate the cause of the enhanced pressure gradient forcing pre-foehn westerlies. For IOPs 2, 4 and 6, the relationship was approximately linear. The exception was the strongest foehn event IOP 7. The interpretation is that for weaker foehn cases, valley bottom pressure differences are closely related to spatial differences in CAP depth or strength, whereas during stronger foehn, valley-bottom pressure differences mainly result from gravity wave asymmetry within the foehn flow. Differences in profile-mean potential temperatures between north and south were less in magnitude than between east and west. They were found to have a relationship with Froude numbers calculated from the observed upstream foehn wind speed and column integrated buoyancy within the CAP.

Keywords

• foehn
• cold air pool
• froude number
• complex terrain
• valley