Challenges in Representing the Vertical Control Volume of Urban Canyons in Earth System Models

Representing the city-climate-energy nexus across scales is critical for capturing thermal, dynamic, and emission effects of cities in Earth System Models (ESMs). The common practice nowadays is to embed a single-layer urban canopy model (SLUCM) in the lowest model level of ESMs, and to provide surface fluxes of momentum, sensible heat, and latent heat as the lower boundary conditions to the planetary boundary layer (PBL) models in ESMs. However, this type of flux coupling strategy assumes that the lowest ESM model layer (~50-100 m above the ground) comprises the physical processes in both the urban canopy layer and the roughness sublayer. While this type of traditional flux coupling approach works in general for modeling rural surfaces (i.e., with relatively low surface roughness elements), it may not be valid for modeling urban surfaces where tall buildings may protrude into hundreds of meters above the ground. Therefore, it is imperative to understand the vertical control volume of UCMs and how to factor that into the coupling approach with ESMs. This study takes advantage of two different methods in coupling UCMs in WRF-Urban to investigate the vertical control volume of urban canopy models. The quasi-idealized modeling approach is adapted here in that the WRF-Urban simulations used real weather conditions, but an idealized city. We conducted a number of WRF V4.1 numerical experiments coupled with the SLUCM and multi-layer BEP models, respectively, under various degrees of urban land-use heterogeneity, and analyzed the effects of different UCM coupling strategies on simulating the temporal and vertical evolution of PBL thermodynamic structures.