Turbulent Exchange over the Alpine City of Innsbruck

Understanding turbulent exchange in urban areas is of great importance to human health and well-being, since accurate representation of urban surface processes in numerical models is necessary for weather forecasting and pollutant dispersion modelling. Urban areas in mountainous terrain represent regions of high population density that are frequently exposed to adverse weather conditions (e.g. downslope windstorms, heavy snowfall) and poor air quality. Although the field of urban climate research has grown substantially over the last twenty years, there are very few observational studies focusing on turbulent exchange in cities in mountainous terrain.

Thus, most of our knowledge, and therefore most of the physics behind mesoscale models, currently relies on theory that has been developed over simpler surfaces (i.e. those that are flat and horizontally homogeneous). In this study, we investigate the extent to which complex environments conform (or do not conform) to current theory.

Situated in the Austrian Alps, the city of Innsbruck is an ideal location for investigating urban areas in highly complex terrain. Whilst Innsbruck is about 600 m above sea level, mountains on either side of the city reach to over 2000 m with a peak-to-peak distance (across the Inn Valley) of about 20 km. Measurements of the turbulent exchange of heat, water and carbon dioxide have been conducted at the Innsbruck Atmospheric Observatory (IAO) on the roof of the university building close to the centre of Innsbruck since 2014. Eddy covariance observations at two levels offer insight into possible variation of turbulence characteristics with height and the impact of local building effects on the measurements. Comparison with flux stations outside the city, established as part of the PIANO project (Penetration and Interruption of Alpine foehn), highlights the influence of urban characteristics and human behaviour on surface exchange processes, and reveals substantial spatial variation in radiative input, air flow and turbulent exchange. This variation is due to both the presence of the city itself and its complex topographical setting. For example, shading by the surrounding topography modifies the diurnal cycle of incoming shortwave radiation, thus affecting the available energy particularly during morning and evening. Outflow from side valleys interacts with the main valley wind circulation, giving rise to spatial variability not only at the surface but also in the atmosphere above. Limited vegetation within the city restricts the evaporation rate compared to the surrounding countryside. To examine the interplay between urban and topographical effects, both long-term measurements and several case-studies are analysed: clear-sky days with typical valley wind circulation; stable periods with a persistent cold pool; and foehn events. In each case, the focus is on ascertaining the relevant physical processes and understanding how these influence turbulence characteristics and turbulent exchange. During foehn conditions, for example, major differences are seen in the timing, magnitude and even sign of the sensible heat flux, depending on the location of the site relative to the extent of the foehn breakthrough. The applicability of similarity scaling under various conditions is also assessed.

This work provides new insight into surface exchange processes in extremely complex environments, as well as a deeper understanding of the limits of current micrometeorological theory.