  Studying and interpreting the different flow patterns present in urban areas
is becoming essential since they help develop new approaches to fight climate
change through an improved understanding of the dynamics of the pollutants in
urban environments. This study uses higher order dynamic mode decomposition
(HODMD) to analyze a high-fidelity database of the turbulent flow in various
simplified urban environments. The geometry simulated consists of two buildings
separated by a certain distance. Three different cases have been studied,
corresponding to the three different regimes identified in the bibliography. We
recognize the characteristics of the well-known arch vortex forming on the
leeward side of the first building and document possible generation and
destruction mechanisms of this vortex based on the resulting temporal modes.
These so-called vortex-generating and vortex-breaking modes are further
analyzed via proper-orthogonal decomposition. We show that the arch vortex
plays a prominent role in the dispersion of pollutants in urban environments,
where its generation leads to an increase in their concentration; therefore,
the reported mechanisms are of extreme importance in the context of urban
sustainability.
