Abstract
This study represents the first detailed analysis of multi-year near-surface turbulence observations for an urban area located in highly complex terrain. Using four years of eddy covariance measurements over the Alpine city of Innsbruck, Austria, the effects of the urban surface, orographic setting and mountain weather on energy and mass exchange are investigated. In terms of surface controls, findings for Innsbruck are in accordance with previous studies at city-centre sites. The available energy is partitioned mainly into net storage heat flux and sensible heat flux (each comprising about 40 % of the net radiation, Q*, during summer daytimes). The latent heat flux is small by comparison (only about 10 % of Q*) due to the small amount of vegetation present but increases for short periods (6–12 h) following rainfall. Additional energy supplied by anthropogenic activities and heat released from the large thermal mass of the urban surface helps to support positive sensible heat fluxes in the city all year round. Annual observed CO2 fluxes (5.1 kg C m−2 y−1) correspond well to both modelled emissions and expectations based on findings at other sites with a similar proportion of vegetation. The net CO2 exchange is dominated by anthropogenic emissions from traffic in summer and building heating in winter. In contrast to previous urban observational studies, the effect of the orography is examined here. Innsbruck’s location in a steep-sided valley results in marked diurnal and seasonal patterns in flow conditions. A typical valley-wind circulation is observed (in the absence of strong synoptic forcing) with moderate up-valley winds during daytime, weaker down-valley winds at night (and in winter) and near-zero wind speeds around the times of the twice-daily wind reversal. Due to Innsbruck’s location north of the main Alpine crest, south foehn events frequently have a marked effect on temperature, wind speed, turbulence and pollutant concentration. Warm, dry foehn air advected over the surface can lead to negative sensible heat fluxes both inside and outside the city. Increased wind speeds and intense mixing during foehn (turbulent kinetic energy often exceeds 5 m2 s−2) help to ventilate the city, illustrated here by low CO2 mixing ratio. Radiative exchange is also affected by the orography, for example incoming shortwave radiation is blocked by the terrain at low solar elevation. Interpretation of the dataset is complicated by distinct temporal patterns in flow conditions and the combined influences of the urban environment, terrain and atmospheric conditions. The analysis presented here reveals how Innsbruck’s mountainous setting impacts the near-surface conditions in multiple ways, highlighting the similarities with previous studies in much flatter terrain and examining the differences, in order to begin to understand interactions between urban and orographic processes.
Highlights
Urban eddy covariance measurements have been made across a range of surface types, including urban parks (e.g. Kordowski and Kuttler, 2010; Lee et al, 2021), vegetated suburban neighbourhoods (e.g. Grimmond and Oke, 1995; Crawford et al, 2011; Ward et al, 2013), densely-built city centres (e.g. Grimmond et al, 2004; Gioli et al, 2012; Kotthaus and Grimmond, 2014a) and high-rise districts (Ao et al, 2016)
Orography gives rise to various phenomena which interact across a range of spatial and temporal scales, including the mountain-plain circulation, valley winds, slope winds, gap flows, downslope windstorms, mountain waves and cold-air pools (Whiteman, 2000)
For urban climatology and urban planners, many of the findings here are in accordance with previous urban studies, for example:
Summary
Store and release a significant amount of heat, and the lack of vegetation and pervious surfaces limits water availability, all of which impact surface-atmosphere exchange and boundary layer characteristics (Oke et al., 2017). Most previous urban-related studies in or near complex terrain focused on dispersion of pollutants (e.g. Allwine et al, 2002; Doran et al, 2002; Velasco et al, 2007) or used routinely measured variables such as air temperature and near-surface wind speed to demonstrate the presence of an urban heat island and/or regional circulations (e.g. Miao et al, 2009; Giovannini et al, 2014). The scarcity of turbulence observations in complex terrain, especially urban complex terrain, means there is very little information available on how the orographic setting of a city affects surface-atmosphere exchange. The two main research goals are to investigate how surface-atmosphere exchange of energy and mass for a city in a complex orographic setting compares to other sites in the literature which are in much less complex terrain, and to examine the effect of the orographic setting on near-surface conditions in the city. Most of the built-up area is confined to the reasonably flat
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