Understanding the influence of surface composition and properties on surface fluxes in urban areas

Our study combines eddy flux data from four different sites with distinct land cover characteristics and an urban canopy model to determine the surface characteristics that control the surface energy budget over urbanized terrain. Turbulent fluxes of heat and water vapor and other micrometeorological variables were continuously sampled from four different sites (UMBC, Cubhill, Princeton and Broadmead) located in the North Eastern United States. Of the four sites, two (UMBC and Princeton) were located in dense urban areas, while the other two sites, Cubhill and Broadmead were located in a suburban and a rural environment, respectively. Our preliminary analysis has shown that while the latent heat flux dominates at the rural and suburban sites, the more urbanized UMBC and Princeton sites were dominated by sensible heat flux. This is expected and could directly be attributed to the vegetative cover fraction surrounding the respective sites. A more interesting finding is that, over the more urbanized surfaces, a considerable shift in peak flux times can be noted due to the heat retaining capacity of urban materials, especially concrete. To quantitatively assess these effects, the Princeton Urban Canyon Model (PUCM) was used. PUCM combines basic meteorological data with aerodynamic and geometric properties of built environment and thermal properties of built and vegetated surfaces to estimate the surface energy budget for urban canopies. The model was able to reproduce the shift in peaks observed in the eddy flux data, at the densely urban site (UMBC). The sector-averaged fluxes produced by the model show relatively high sensible heat fluxes from sectors filled with asphalt and concrete.