The Study of Thermally Driven Circulations during the Formation of a Cold Air Pool in the Inn Valley using High-resolution Modelling

Forecasting in mountainous regions is particularly challenging due to the influence of their complex topography and the associated weather events that occur on different temporal and spatial scales. Interactions within the Mountainous Boundary Layer (MoBL) are governed by a variety of thermodynamic and dynamic processes. High-resolution models offer the ability to investigate how these complex atmospheric circulations evolve and how they impact the weather situation in complex terrain. However, a fundamental question pertains to whether current NWP models do adequately represent small-scale thermally driven circulations, given that the underlying turbulence parametrizations were not developed with such high-resolution applications in mind.

Cold air pools are a common occurrence in complex orographic terrain, where cold air accumulates in the valleys and causes a temperature inversion. The PIANO (Penetration and Interruption of Alpine Föhn) campaign examined how Föhn events erode cold air pools in the Innsbruck region of Austria in autumn and early winter of 2017. This case study focuses on the night between 15 and 16 October 2017, when the combination of a clear sky, weak south-westerly synoptic flow and the absence of a Föhn interruption favored the development of a persistent cold air pool in the Inn Valley.

We investigate how the representation of this event is affected by model resolution and refined initial conditions. For this purpose, we compare three experiments with different initial and boundary conditions: the IFS forecast, ICON-DREAM, our global reanalysis, and ICON-FORCE, our regional reanalysis. All of the experiments use a one-way nesting approach, beginning with the operational ICON-D2 domain, which has a horizontal grid spacing of 2 km and covers the entire Alpine region. Within this domain, three nests are configured with horizontal grid spacings of 1 km, 0.5 km, and 0.25 km. We find that all simulations consistently benefit from increased resolution, and that a grid spacing of 0.25 km already yields a reasonably well-represented cold air pool and intermittent jet. As expected, ICON-FORCE produces the closest agreement with the observations. However, the differences in boundary conditions are smaller than those resulting from changes to the horizontal grid spacing. Finally, the ICON model in its NWP configuration is able to capture the dynamics of the cold air pool when initial conditions are refined and the simulation is conducted at sub-kilometre resolution.