High-resolution turbulence modelling over complex landscapes

Climate change is leading to an increasing number of severe weather events. To help mitigate the effects of such events we need to be able to accurately predict atmospheric conditions (such as temperature and wind speed) at fine spatial scales. While most weather models can provide reasonably reliable predictions over simple landscapes, they are not well suited to more complex environments such as hilly or mountainous terrain, or regions with a mixture of surface cover (such as farmland or urban areas). Yet these more complex environments are precisely the areas we are most interested in since they are the most relevant to our everyday life. In this project we focus on cities in mountainous terrain densely populated areas that are often exposed to dangerous weather conditions (e.g. strong winds, storms and poor air quality). One of the main features of complex landscapes is extreme spatial variability. For example, temperature can be very different from one side of a valley to the other, on the valley floor compared to the mountain tops, or in the city centre compared to the rural surroundings. Wind speed and direction also vary dramatically from place to place as airflow is blocked by buildings and mountains but channelled along valleys and streets. In complex landscapes there is a wide range of processes occurring and interacting across very small to very large scales. How to represent these processes accurately in weather prediction models is a major challenge. Thanks to recent advances in technology, it is now possible to run very high-resolution simulations (called large-eddy simulations, or LES) for real cities. Unlike current weather models, which represent cities by a few grid boxes with typical properties of an urban area, the LES model used here will include the real urban structure (i.e. individual buildings, streets and trees) for the alpine city of Innsbruck. These simulations will provide a three-dimensional picture of atmospheric conditions in the city at each time step, allowing detailed investigation of the turbulent processes that affect air quality, weather and climate. The model will be set up to allow the influence of buildings and mountains on the atmosphere to be studied together for the first time. Therefore, it will be very important to check how the model compares to measured data. Then, the LES will be used as a virtual laboratory to investigate how and why the characteristics of turbulence change horizontally from place to place and vertically from street level to above the rooftops (this region is highly complex and not well understood). The model will show whether measurements at individual points are representative, and whether our instruments are able to measure all the relevant processes. The results will provide new insight into the structure of the urban atmosphere and new understanding of complex landscapes, equipping mankind to better cope with extreme weather and future climate.