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Fakultät für Biologie, Chemie und Geowissenschaften

Mikrometeorologie - Prof. Christoph Thomas

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Masterarbeit

Simulating an extreme heat event in a mid-sized city in Europe with Large Eddy Simulation: investigating the impact of spatial resolution and validation with an observation network

Eva Späte

Betreuer: Christoph Thomas, Wolfgang Babel

Urban citizens are known to be exposed to higher temperatures than rural citizens making urban citizens particularly vulnerable to extreme heat events. Large Eddy Simulation (LES) models can simulate micrometeorological heat transport and mixing processes by directly resolving large-scale turbulence. These models can also be used to evaluate urban development strategies aiming at mitigating the adverse effects of heat waves in cities by analyzing their influence on the urban microclimate. Despite their use in formulating recommendations for city planning, these models are often not validated with observed meteorological data. We here present results from conducting a model-observation comparison for a mid-sized city in Germany. Model simulations were computed with the LES model PALM-4U run at two different isotropic spatial resolutions ( x,y,z = 5 and 20 m) and evaluated against observations from a network of microweather stations for a heat wave in 2019 reaching maximum near-surface air temperatures of 37 °C. During daytime, differences between observed and modeled near-surface air temperatures were small (−3.8 to 1.1 K, mean = 0.9 K), but much larger during nighttime and the early morning transition. The latter findings can be explained by an overestimated modeled ground heat flux resupplying too much energy to the surface. This offset the radiative cooling and led to overestimated modeled air temperatures of up to +9 K (mean = 5.3 K). For wind speeds, the results showed that in areas where the actual urban surface structure was reproduced well by the model resolution, differences between observed and modeled wind speeds were lower. Our findings indicate that a spatial resolution smaller than the height of most buildings produce more accurate model results for wind speeds. That the simulation was run without the spin-up mechanism of PALM-4U constitutes an uncertainty factor for the modeled output. We therefore recommend including a spin-up period for future runs to ensure a sufficient initialization of the surface layers’ temperature and moisture content. To improve the simulation output, we further recommend reviewing the thermal diffusivity values of the surface layer, adjusting the boundary input of incoming shortwave radiation, and increasing the fixed water temperature in the model setup. To further explore model-observation differences and the impact of spatial resolution, we recommend analyzing the modeled advection which was out of the scope of this thesis.

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