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We present a comprehensive analysis of four south föhn events observed during the Penetration and Interruption of Alpine Foehn (PIANO) field campaign in the Inn Valley, Austria, in the vicinity of Innsbruck. The goal is to detect and quantify processes of cold-air pool (CAP) erosion by föhn as well as processes of föhn breakdown. Despite differences in föhn breakthrough and strength, the four cases exhibit similarities in CAP evolution: initially, the CAP experienced strongest warming in the centre of Innsbruck, where the föhn jet from the Wipp Valley interacted with the CAP in the Inn Valley. The resulting shear-flow instability at the föhn–CAP interface caused turbulent CAP erosion and, together with vertical warm-air advection, led to CAP depression over the city centre. This depression drove pre-föhn westerlies near the surface that caused cold-air advection inside the CAP west of the city centre and warm-air advection in the east. Ultimately, the latter contributed to stronger CAP erosion in the east than in the west. This stronger heating also explains the preferential initial föhn breakthrough at the valley floor east of Innsbruck. In two of the cases, subsequent westward propagation of the föhn–CAP boundary across the city accompanied by northerly (deflected) föhn winds led to a complete föhn breakthrough. Föhn breakdown occurred either by a backflow of the CAP remnant or by a cold-frontal passage. This study emphasizes the importance of both turbulence and advection in the CAP heat budget and reveal their large spatio–temporal variability. [ABSTRACT FROM AUTHOR]

Exchange of momentum and scalars in the mountain boundary layer is achieved through an interaction of meso‐to‐microscale motions, occurring to varying extents depending on the combined effect of thermally driven as well as dynamically driven forcings. One such motion, known as a secondary circulation, results from a horizontal force imbalance across a curved valley segment, wherein the centrifugal force towards the outside of the valley bend can create a pressure gradient force in the opposite direction. The lack of adequate measurement strategies capable of sampling such motions in curved mountain valleys explains the near‐absence of any observational evidence of secondary circulations there. The goal of the CROSSINN (Cross‐valley flow in the Inn valley investigated by dual‐Doppler lidar measurements) campaign, conducted in a curved segment of the Inn valley, Austria, was to determine the character of the cross‐valley flow by means of a coplanar retrieval applied to a multi‐Doppler wind lidar configuration. A signature of a secondary circulation, hereafter referred to as a cross‐valley vortex, stood out particularly during intense daytime upvalley flow episodes. Vortices were detected on 23 upvalley wind days, with a declining frequency of occurrence from August to October. Nearly all identified vortices were marked by a low‐level upvalley jet, a clockwise wind direction turning with height, and a cessation of upvalley flow at the local ridgeline level. The routinely sampled coplanar‐retrieved cross‐valley wind field enabled the quantification of more advanced parameters based on vorticity, revealing a faster spin rate of the vortex around its streamwise axis given a stronger upvalley flow, and a period of revolution on the order of several tens of minutes. A detailed inspection of the lateral momentum budget and associated uncertainties confirmed the importance of the relationship between the centrifugal and the pressure gradient force for the cross‐valley vortex occurrence in a curved valley. [ABSTRACT FROM AUTHOR]

While the exchange of mass, momentum, moisture, and energy over horizontally homogeneous, flat terrain is mostly driven by vertical turbulent mixing, thermally and dynamically driven mesoscale flows substantially contribute to the Earth--atmosphere exchange in the atmospheric boundary layer over mountainous terrain (MoBL). The interaction of these processes acting on multiple scales leads to a large spatial variability in the MoBL, whose observational detection requires comprehensive instrumentation and a sophisticated measurement strategy. We designed a field campaign that targets the three-dimensional flow structure and its impact on the MoBL in a major Alpine valley. Taking advantage of an existing network of surface flux towers and remote sensing instrumentation in the Inn Valley, Austria, we added a set of ground-based remote sensing instruments, consisting of Doppler lidars, a ceilometer, a Raman lidar, and a microwave radiometer, and performed radio soundings and aircraft measurements. The objective of the Cross-Valley Flow in the Inn Valley Investigated by Dual-Doppler Lidar Measurements (CROSSINN) experiment is to determine the mean and turbulent characteristics of the flow in the MoBL under different synoptic conditions and to provide an intensive dataset for the future validation of mesoscale and large-eddy simulations. A particular challenge is capturing the two-dimensional kinematic flow in a vertical plane across the whole valley using coplanar synchronized Doppler lidar scans, which allows the detection of cross-valley circulation cells. This article outlines the scientific objectives, instrument setup, measurement strategy, and available data; summarizes the synoptic conditions during the measurement period of 2.5 months; and presents first results. [ABSTRACT FROM AUTHOR]

The Innsbruck Atmospheric Observatory (IAO) aims to investigate atmospheric chemistry, micrometeorology, and mountain meteorology in a synergistic fashion within an urban setting. A new measurement supersite has been established in order to study processes affecting the exchange of momentum, energy, trace gases, and aerosols in an Alpine urban environment. Various long-term continuous measurements are augmented by frequent focused research campaigns with state-of-the-art instrumentation, linking different classes of data and addressing significant gaps in scientific data availability for urban environments. Current activities seek to address research objectives related to the urban heat island, trace gas emissions, the influence of foehn on air quality, and the atmospheric distribution of trace gases and aerosols in a mountainous city. We present initial results from long-term operations and first highlights from two intensive operational phases, showing that 1) the exchange of greenhouse gas emissions is dominated by anthropogenic activities and is driven by location-specific venting of street canyon air; 2) foehn events significantly perturb the photostationary state indicative for an extensive and rapid airmass exchange of the valley atmosphere; 3) the temporal distribution of pollutants is often decoupled from their emissions and primarily modulated by mountain boundary layer dynamics; 4) we can detect a large number of volatile chemical products in the urban atmosphere, which can be used to fingerprint anthropogenic emission sources; and 5) the first urban carbonyl sulfide (COS) flux measurements point toward anthropogenic emission sources. [ABSTRACT FROM AUTHOR]