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A case-study is presented of a south foehn emanating from the Wipp Valley, Austria, which encountered a cold-air pool (CAP) in the Inn Valley near the city of Innsbruck. The analysis is based on data collected during the second Intensive Observation Period of the Penetration and Interruption of Alpine Foehn (PIANO) field experiment. Foehn was initiated on 3 November 2017 by an eastward moving trough and terminated in the afternoon of 5 November 2017 by a cold front passage. On two occasions, reversed foehn flow deflected at the mountain ridge north of Innsbruck penetrated to the bottom of the Inn Valley. The first breakthrough occurred in the afternoon of 4 November 2017. It was transient and locally limited to the northwest of the city. The second (final) breakthrough occurred in the morning of 5 November 2017 and was recorded by all surface stations in the vicinity of Innsbruck. It started with a foehn air intrusion to the northeast of Innsbruck and continued with the westward propagation of the foehn-CAP boundary along the valley. Subsequently observed northerly winds above the city were caused by an atmospheric rotor. A few hours later and prior to the cold front passage, the CAP pushed back and lifted the foehn air from the ground. During both nights, shear flow instabilities formed at the foehn-CAP interface, which resulted in turbulent heating of the CAP and cooling of the foehn. However, this turbulent heating/cooling was partly compensated by other mechanisms. Especially in the presence of strong spatial CAP heterogeneity during the second night, heating in the CAP was most likely overcompensated by negative horizontal temperature advection. K E Y W O R D S cold-air pool, foehn, heat budget, multiple Doppler wind lidars, shear flow instabilties, turbulent erosion This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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Abstract. Eddy covariance measurements of the turbulent sensible heat, latent heat and carbon dioxide fluxes for 12 months (2011–2012) are reported for the first time for a suburban area in the UK. The results from Swindon are comparable to suburban studies of similar surface cover elsewhere but reveal large seasonal variability. Energy partitioning favours turbulent sensible heat during summer (midday Bowen ratio 1.4–1.6) and latent heat in winter (0.05–0.7). A significant proportion of energy is stored (and released) by the urban fabric and the estimated anthropogenic heat flux is small but non-negligible (0.5–0.9 MJ m−2 day−1). The sensible heat flux is negative at night and for much of winter daytimes, reflecting the suburban nature of the site (44% vegetation) and relatively low built fraction (16%). Latent heat fluxes appear to be water limited during a dry spring in both 2011 and 2012, when the response of the surface to moisture availability can be seen on a daily timescale. Energy and other factors are more relevant controls at other times; at night the wind speed is important. On average, surface conductance follows a smooth, asymmetrical diurnal course peaking at around 6–9 mm s−1, but values are larger and highly variable in wet conditions. The combination of natural (vegetative) and anthropogenic (emission) processes is most evident in the temporal variation of the carbon flux: significant photosynthetic uptake is seen during summer, whilst traffic and building emissions explain peak release in winter (9.5 g C m−2 day−1). The area is a net source of CO2 annually. Analysis by wind direction highlights the role of urban vegetation in promoting evapotranspiration and offsetting CO2 emissions, especially when contrasted against peak traffic emissions from sectors with more roads. Given the extent of suburban land use, these results have important implications for understanding urban energy, water and carbon dynamics.