HAPpy Hour- Effects of urban roughness and thermal heterogeneity on scalar transport and cloud formation

Abstract:  
Land-surface heterogeneity comes in many varieties and occurs over a wide range of spatial scales. In urban environments, urban canopy and thermal heterogeneity, together with the mean advection driven by the large scale pressure gradient not only modulate urban flow structures and thermal environment, but also play a key role in the transport of water vapor and alter cloud formation.  

In the first part of this talk, a simplified large-eddy simulation (LES) is performed to investigate how coastal cities interact with the synoptic-scale marine heatwave (MHW). We consider two sets of simulations: simple urban roughness setup (Set A) and explicit urban roughness representation (Set B). Comparison between Set A and B reveals a substantial increase of vertical velocity and urban heat island (UHI) intensity in set B. To further understand this phenomenon, a canopy layer UHI model is proposed to explain different mechanisms that urban canopy, thermal heterogeneity, and mean advection contribute to UHI intensity. The effect of urban canopy is considered as an additional vertical velocity scale that facilitates heat transport within the canopy layer and therefore increases UHI intensity. This implies that overlooking the effect of urban canopy may misrepresent the overall flow and temperature fields in urban coastal environments.  

In the second part of this talk, FastEddy®, a graphics processing unit (GPU)-resident LES model, is used to investigate the urban-rural circulation and the transport of water vapor under free convection conditions. Different local climate zones (LCZs) with different building heights and building plan area fractions are considered. Results suggest that the convective boundary layer height and vertical velocity within the boundary layer are strongly affected by urban morphology. This implies that urban canopy impedes the vertical transport of momentum and scalars from the canopy layer to the region above. In addition, urban morphology also affects the total cloud cover and the horizontally averaged integrated liquid water path above urban area.

These findings highlight that urban canopy strongly impacts the transport of momentum, heat, and water vapor, and provide the basis for the parameterization of the urban-rural circulation and urban cloud formation.