Observations of the Temperature and Humidity Structure Parameter Over Heterogeneous Terrain by Airborne Measurements During the LITFASS-2003 Campaign

We used numerical simulations to investigate the general relationship between urban morphology and the intensity of wind gusts in built-up areas at the pedestrian level. The simulated urban boundary layer developed over a 19.2 km (length) \(\times \) 4.8 km (width) \(\times \) 1.0 km (height) simulation domain, with 2-m resolution in all directions, to explicitly resolve the detailed shapes of buildings and the flow at the pedestrian level. This complex computation was accomplished using the lattice Boltzmann method and by implementing a large-eddy simulation model. To generalize the results, a new parameter that expresses the intensity of gusts (the gust index, \({\tilde{U}}_{ max})\) was defined as the local maximum wind speed divided by the freestream velocity. In addition, this parameter was decomposed into the mean wind-speed ratio, \({\tilde{U}} \) and turbulent gust ratio, \({\tilde{U}}^{{\prime }}\) to evaluate the qualities of gusts. These parameters were useful for quantitatively comparing the gust intensities within urban canopies at different locations or even among different experiments. In addition, the entire horizontal domain was subdivided into homogeneous square patches, in which both the simulated gust parameters and the morphological characteristics of building geometries were averaged. This procedure masked the detailed structure of individual buildings but retained the bulk characteristics of the urban morphology. At the pedestrian level, the gust index decreased with increasing building cover. Compared to \({\tilde{U}} \), the quantity \({\tilde{U}}^{{\prime }}\) notably contributed to the index throughout the range of plan area index \((\lambda _p)\) values. The dependences of all normalized wind-speed ratios transiently changed at \(\lambda _p =~0.28\). In cases where \(\lambda _p < 0.28, {\tilde{U}} \) decreased with increasing \(\lambda _p \), although \({\tilde{U}}^{{\prime }}\) was almost constant. In cases where \(\lambda _p > 0.28, {\tilde{U}}\) was almost constant and \({\tilde{U}}^{{\prime }}\) decreased with increasing \(\lambda _p \). This was explained by the change in flow regimes within the building canyon. At a higher elevation above the canopy layer, \(\lambda _p \) becomes less relevant to normalized wind-speed ratios, and instead the aerodynamic roughness length became important.     

A Two-Day Case Study: Comparison of Turbulence Data from an Unmanned Aircraft System with a Model Chain for Complex Terrain

The airborne measurement platform MASC-3 (Multi-Purpose Airborne Sensor Carrier) is used for measurements over a forested escarpment in the Swabian Alps to evaluate the wind field. Data from flight legs between 20 and 200 m above the ground on two consecutive days with uphill (westerly) flow in September 2018 are analyzed. In the lowest 140 m above the ground a speed-up is found with increased turbulence and changes in wind direction directly over the escarpment, whereas in the lowest 20 to 50 m above the ground a deceleration of the flow is measured. Additionally, simulation results from a numerical model chain based on the Weather Research and Forecasting (WRF) model and an OpenFOAM (Open Source Field Operation and Manipulation) model, developed for complex terrain, are compared to the data captured by MASC-3. The models and measurements compare well for the mean wind speed and inclination angle.