## 1 Introduction to Urban Flows
### Influence of an Urban Obstacle  
The buildings for the urban environment investigated here are represented by wall-mounted cylinders with a square cross-section. A wide range of computational-fluid-dynamics (CFD) and experimental studies have been conducted around such an obstacle in order to describe the flow topology. The flow around a cubical obstacle under a laminar inflow condition was simulated by [PERSON] and [PERSON] [18]. [PERSON] et al. [7] investigated the influence of the inflow conditions by performing DNS and several experimental results report on this, such as the one from [PERSON] and [PERSON] [8]. The vortical structures of the flow have been investigated experimentally in the following works: [PERSON] [19], by using digital particle image velocimetry (DPIV) and [PERSON] and [PERSON] [20] by using laser Doppler anemometry (LDA). [PERSON] et al. [21] studied the influence of the angle of incidence of the wind on the structure of the flow around a wall-mounted square cylinder. Similarly, [PERSON] et al. [22] studied the influence of a small angle of incidence and later the influence of a larger angle of incidence [9] on the flow through a simplified urban environment. The urban environment creates a relatively large drag force: The flow is significantly slowed down by the obstacle. [PERSON] and [PERSON] [23] performed numerical studies with low-to-moderate Reynolds number to describe the vortical structures of the flow around a wall-mounted square cylinder. They highlighted a stagnation point in the flow. The region very close to the cylinder is significantly disturbed, with large areas of reversed flow: A low velocity region upstream the obstacle and an acceleration region at both side of the obstacle. Within the low-velocity region upstream the obstacle, the flow stagnates and raises to get over the obstacle. This was also discussed in the context of climate models by [PERSON] and [PERSON] [24]. A wide horseshoe vortex is formed around the cylinder due to the rollup of the incoming boundary layers spanwise vorticity and its interaction with the obstacle. This has been described by several studies and observed experimentally by [PERSON] and [PERSON] [8] and numerically by [PERSON] et al. [7]. [PERSON] and [PERSON] [13] identified a separation saddle point upstream the obstacle. This point marksthe upstream limit of this horseshoe vortex, which then curls around the cube. Inside the region downstream of the obstacle, the streamwise velocity slows down because of the drag force created by the obstacle. The adverse pressure gradient formed at the obstacle because the regions of different velocities are responsible for a recirculation bubble downstream of the obstacle near the wall. High turbulent kinetic energy (TKE) regions are also noticeable on both sides of the obstacle ([PERSON] et al. [7] and [PERSON] et al. [21]) because of the interactions at the wake layer with the shear layer on both sides of the urban obstacle. This increased TKE, mostly due to the streamwise velocity fluctuations, is responsible for the emergence of coherent vortical structures. Many experimental studies investigate the flow around a wall-mounted obstacle characterized by a small aspect ratio: [PERSON] et al. [21], [PERSON] [19] or [PERSON] and [PERSON] [20] used laser Doppler anemometry (LDA) to focus on the energy balance within the turbulent flow. Many studies proposed a description of the flow topology around the obstacle based on experimental measurements or computational data. They highlighted the two large vortices that are formed on both sides of the obstacle and a roof vortex is formed at the top of the cube. Due to a large recirculation region downstream the obstacle, these three separated coherent vortical structures move closer and form the so-called arch vortex behind the back side of the obstacle. These vortical structures are increasing momentum exchanges between the urban canopy layer and the outer flow and are therefore of great interest for the study of pollutant dispersion in a street network.