The budgets of turbulence kinetic energy and heat in the urban roughness sublayer

Quantification of the terms in the transport equations of turbulence kinetic energy k and heat in the real urban roughness sublayer is inherently difficult due to variation of terms with thermal stability, wind speed, wind direction, and horizontal heterogeneity of most urban sites concerning morphometric parameters and land use. There is also a paucity of observations in the real urban environment, specifically pertaining to the heat budget, which motivate such budget analyses to understand the physical processes, inform urban development, and parametrize microclimate models. The budget terms of k and heat in the urban roughness sublayer were quantified using field observations conducted in the Reek Walk, Guelph, Canada, inside a quasi two-dimensional urban canyon located at the University of Guelph, from 15 July 2018 to 5 September 2018. The budget terms were analyzed under four stability classes, from thermally unstable to stable conditions, and under different wind speeds, from very low to high wind speed conditions. The budget terms were further analyzed under varying wind directions in eight sectors with respect to the canyon axis. For k, the budget terms quantified were storage, advection, buoyant production/consumption, shear production/consumption, turbulent transport, and dissipation. It was found that the main transport mechanism for k was driven by the turbulent transport that relocated k from the shear layer above roof (speculated but not measured) to the urban canyon, where it was balanced by shear production/consumption and dissipation terms. The advection term had lower magnitude to other terms but was greater in magnitude than the buoyant production/consumption term. For heat, the budget terms quantified were storage, advection, and flux divergence. It was found that the main transport mechanism for heat was driven by advection, where either warm or cold air masses were transported to the urban canyon depending on wind direction. The advection was found to be balanced by flux divergence. For both k and heat the storage terms were at least one order of magnitude smaller than other budget terms. For k it was found that with decreasing wind speeds, the residual (unexplained) portion of the budget increased, suggesting more difficulties in the Reynolds decomposition approach and budget apportionment in such cases. The findings for the k budget were in agreement with previous studies, while the findings for the heat budget could inspire further investigations. It was noted that the advective transfer mechanisms for heat could be overlooked in simplified urban microclimate models, but such transfer mechanisms need to be accounted for.