Turbulent exchange in urban areas in highly complex terrain

The research project Understanding turbulent exchange over urban areas in highly complex terrain aims to improve current understanding of weather and climate in cities located in mountainous regions. Turbulent exchange determines how emissions from urban areas, such as carbon dioxide released by traffic, mix with the atmosphere. On a clear day, the sun warms the surface, which heats the air above, causing the warm air to rise and facilitating the upward transport of pollutants. In winter (or at night), the surface may instead cool the air, suppressing vertical mixing and trapping pollutants close to the surface. Thus, correctly understanding these processes has important implications for human health; not only in terms of air quality, but also for weather forecasting and warnings of heat-waves, cold-spells or floods. Despite the large number of people living in cities (now over half the worlds population) and the fact that most cities are surrounded by non-uniform terrain (mountains, valleys, river basins), the challenges of measuring and modelling these complex environments means they have been little studied. This project aims to address this issue, and will investigate how the city of Innsbruck and the topography of the Inn Valley influence turbulent exchange processes, with a view to better understanding how human activities impact the environment. For example, how might urban development impact air quality in and downwind of the city? Numerical modelling is a very useful tool to investigate such scenarios. However, most models have been developed to represent much simpler environments: landscapes that are flat, uniform and originally without cities. To reliably interpret the model results it is therefore necessary to first compare model output with measured data to assess whether the model can reliably simulate this complex setting. A range of instruments will be used to assess model performance for different seasons and at a range of scales, including a unique two-wavelength scintillometer system, which provides observations of turbulent heat exchange and evaporation at larger scales than is possible with more traditional techniques. Turbulent statistics of some exotic gases (e.g. benzene, toluene) will be analysed to gain new insights into their behaviour compared to more easily measured and therefore more frequently studied quantities (temperature and water vapour). The Weather Research and Forecasting (WRF) model used for this study is widely applied for both research and operational purposes, hence improving understanding of its performance will benefit both the scientific community and the general public. This work would constitute the most comprehensive study of turbulent exchange processes in urban areas in mountainous terrain to date. By conducting careful analyses at the limits of current understanding, this project should lead to progress in several areas of atmospheric science.

Cities in mountainous terrain are subject to extreme conditions such as heat stress, heavy snowfall, flooding, downslope windstorms (foehn) and air quality issues. Despite the considerable number of people living here, these regions have been little studied before now. Understanding how atmospheric processes above cities in mountainous terrain differ from those in flat terrain is key to improving the accuracy of environmental predictions and avoiding inadvertent harmful effects that can result from attempts to mitigate climate issues. Using meteorological data collected above the rooftops of Innsbruck, Austria, this project provides many new insights into atmospheric processes over cities in mountainous terrain. Solar radiation is blocked by the valley sides changing the time of local sunrise and sunset, and the amount of solar radiation reaching the surface in winter is further reduced by the build-up of pollutants in the valley atmosphere. A valley-wind circulation generates strong daily and seasonal patterns in flow and turbulence, and there is extreme spatial variability especially where flows from different valleys meet. Frequent foehn events bring sudden increases in temperature, wind speed and turbulence, impacting the exchange of heat. In winter, the temperature of foehn air can exceed the temperature of the surface contributing to downwards transport of heat (heat transport in cities is usually upwards from the warm surface to the air above). Furthermore, the extremely high turbulence during foehn helps to ventilate the city by mixing pollutants away from the surface, particularly noticeable when foehn persists overnight. Despite the clear influences of the mountains, some similarities were found with studies in flat terrain. Evaporation is limited by the lack of vegetation and most of the energy received from the sun goes into heating the city and near-surface atmosphere. Carbon dioxide emissions are dominated by human activities (mainly traffic and building heating). The direct impact of human behaviour on the environment was further evidenced by a reduction in carbon dioxide emissions observed during COVID-19 lockdown periods. The dataset was also used to evaluate the performance of the Weather Research and Forecasting (WRF) model and showed that, even this complex setting, the atmosphere is reasonably well simulated in fair- weather conditions. The valley-wind circulation and structure of the valley atmosphere is represented fairly well during daytime, although modelled temperatures are too warm at night and the strength of turbulence is almost always underestimated. Detailed evaluation of model performance in Innsbruck will continue with future work involving high-resolution simulations that will show the impact of individual buildings on conditions in the city. Since the type of circulations that occur in mountainous regions also occurs to some extent over less complex terrain, as well as at great distances from the mountains, the findings from this project are widely relevant.