The impacts of large bluff roughness elements on turbulent transport of momentum and scalar in the urban boundary layer

More than half of the world population lives in cities. It is imperative to improve our predictive understanding of the urban boundary layer. In particular, considerable knowledge gaps still exist in turbulent transport of scalars (temperature, moisture and air pollutants) over urban rough surfaces, especially in the urban roughness sublayers. Using obstacle-resolving large eddy simulations, we first compare and contrast momentum and passive scalar transport over large, three-dimensional roughness elements. Dispersive scalar fluxes are shown to be a significant fraction of the total fluxes within the roughness sublayers. Strong dissimilarity is also noted between the dispersive momentum and scalar fluxes. The results highlight the need for distinct parameterizations of the turbulent and dispersive fluxes, as well as the importance of considering the contrasts between momentum and scalar transport for flows over very rough surfaces. In addition, the links between momentum and scalar roughness lengths (z_0m and z_0s) are explored by developing a conceptual framework that considers z_0m and z_0s at two distinct scales, namely micro and macro scales. Using a surface renewal theory for macro-scale roughness lengths, a log(z_0m/z_os) scaling with Re_*^1/2 is predicted and is supported by LES results. Overall, these results underline the potential of using wall-modeled, large-obstacle resolving LES to improve our process-based understanding, as well as to identify and represent the missing first-order physical processes in the ABL.