Surface-atmosphere interactions at an urban site in highly complex terrain

We present the first detailed analysis of multi-seasonal near-surface turbulence observations for an urban area in highly complex terrain. Using four years of eddy covariance data collected over the Alpine city of Innsbruck, Austria, we assess the impact of the urban surface, orographic setting and mountain weather on the exchange of energy, momentum and mass. In terms of urban surface controls, findings indicate several similarities with previous studies at city-centre sites (in much flatter terrain). The available energy is used mainly for the net storage heat flux and sensible heat flux, while the lack of vegetation in the source area means latent heat fluxes are small. Observed carbon dioxide fluxes are dominated by anthropogenic emissions from building heating in winter and traffic in summer. The measured annual total carbon dioxide flux corresponds well to both modelled emissions and observations from other sites with a similar proportion of vegetation, but interpretation of seasonal and diurnal patterns is complicated by spatial heterogeneity in the source area combined with distinct temporal trends in flow conditions.

Innsbruck’s mountainous setting impacts atmospheric conditions and surface-atmosphere exchange in multiple ways. Steep valley sides block solar radiation at low sun angles, resulting in a shift in the times of local sunrise and sunset compared to over flat terrain. In the absence of strong synoptic forcing, a thermally driven valley-wind circulation develops with characteristic daily and seasonal flow patterns. Moderate up-valley winds are observed during the afternoon (these are strongest during summer), while weak down-valley winds prevail overnight, in the early morning and during winter. During spring and autumn, downslope windstorms (foehn) can lead to marked increases in temperature, wind speed and turbulence. Sensible heat fluxes in the city are almost always positive (even at night and during winter), however the presence of warm air above cooler surfaces can result in negative sensible heat fluxes during foehn. Furthermore, very low carbon dioxide mixing ratios observed during foehn events illustrate how the intense mixing helps to ventilate the city and reduce pollutant concentrations.

For the first time, the combined influences of the urban environment, complex orography and atmospheric conditions on surface-atmosphere exchange are analysed in order to begin to understand interactions between urban and topographic processes. These results are thus relevant not only for other urbanised Alpine valleys, but for the numerous cities across the globe which are located in some kind of topographic complexity, such as in river valleys or basins, on hilltops or plateaus or along coastlines.