Thermal Wind Circulations in a Small Alpine Sinkhole

The Grünloch basin is an approximately 1-km wide and 150-m deep basin on the Hetzkogel Plateau in the eastern Alps of Austria. It is completely enclosed by the surrounding topography in the lowest 50 m, below the height of the Lechner Saddle, which connects the Grünloch with the Lechner Gorge to the northwest of the basin. In this presentation, we will show results from numerical simulations of thermal wind circulations under semi-idealized, quiescent conditions, with the goal of further explaining observed vertical temperature and wind profiles and surface measurements from several sites throughout the basin and along the crest.

Strong nocturnal inversions can form in the basin, particularly below the height of the Lechner Saddle. Downslope winds along the basin sidewalls are not able to penetrate the strong inversion and thus separate from the sidewall near the top of the inversion to flow toward the basin center. The flow continues through the Lechner Saddle, where it produces a steady outflow of air. We will present a detailed analysis of the structure and mass flux of the downslope flows, the flow above the basin-floor inversion, and the outflow through the gap in the surrounding topography. We will also show how changes in surface characteristics can impact the strength of nocturnal cooling and subsequently the strength of the outflow.