Understanding the physical processes modulating the transport of scalars, such as heat, over very rough surfaces is essential for understanding the thermal environment of
cities, how wind and solar farms modify heat and water exchanges between the
atmosphere and Earth surface, and parameterizing surface physics in coarser Earth
systems models. This talk examines this problem in the urban context. Since passive
scalars such as water vapor are advected with the flow, broad similarity is expected
between the widely-studied momentum transfer problem and its scalar counterpart.
However, unlike momentum that is dominated by form drag over very rough walls,
scalar transport must occur through the viscous exchanges at the solid-fluid interface.
This results in transport dissimilarity, as well as in a continuous dependence on the
Reynolds number. In addition, the spatial variability of the geometry gives rise to the so-
called dispersive fluxes that can be larger than conventional turbulent fluxes in the
roughness layer. In this talk we will use large eddy simulations to examine the role of
surface topography, the resulting transport dissimilarity, and novel approaches for
parametrizing scalar exchanges using the surface renewal theory.