Multi-platform Observations of Nocturnal Exit-Jet Events in the Inn Valley Exit Area: Insights from the TEAMx Summer 2025 Campaign

The Inn Valley in the European Alps frequently exhibits thermally driven along-valley flows under clear, high-pressure conditions. During nighttime, a pronounced down-valley jet can develop near the valley exit and extend into the Bavarian foreland, commonly referred to as the Inn Valley exit jet. While its occurrence has been documented in previous studies, detailed observations of its structure, variability, and governing dynamics remain limited.

During the summer 2025 Intensive Observation Period of the TEAMx field program, three exit-jet events were captured using a coordinated multi-platform observational strategy. Measurements included a network of five automatic weather stations, a Doppler wind lidar, repeated radiosonde launches, and observations from meteorological drones. This combination of platforms provides a comprehensive view of both the temporal evolution and vertical structure of the flow across the valley exit region and into the adjacent foreland.

The exit-jet events were identified and analyzed with respect to their onset characteristics, synoptic forcing, and spatial and temporal extent. The events exhibit substantial variability, particularly in the timing and intensity of near-surface wind acceleration. Exit jet wind speed maxima ranged between 13 and 17 m s⁻¹ at heights between 150 and 200 m above ground. Despite this variability, all events share a consistent nocturnal intensification and a coherent low-level jet structure in the valley exit region.

Turbulence characteristics were examined using bulk estimates of the gradient Richardson number and Froude number, providing constraints on flow stability and dynamical regime. Across the observed cases, the diagnosed flow conditions indicate the absence of a supercritical flow regime. This finding is robust across events despite their variability in intensity and spatial extent.

These observations provide new insights on the dynamics of valley-exit flows and highlight the importance of coordinated multi-platform measurements for capturing their temporal and vertical structure. The results offer a valuable benchmark for evaluating high-resolution numerical simulations and improving the representation of thermally driven flows in complex terrain.