The influence of topography on meteorological variables and surface-atmosphere interactions

Topography perturbs practically every process and variable relevant to land-atmosphere interactions, including radiation, air temperature and saturation deficit, wind and turbulence, cloudiness and precipitation, and properties of the soil and vegetation. We review progress in understanding some of these influences, concentrating on the wind field and the exchanges of scalars (especially sensible and latent heat) over low to moderate hills, both without and with flow separation. The treatment of the wind field is based on a physical division of the flow into inner, outer and wake regions. This framework is used to discuss modelling approaches, measurements of both the mean flow and the turbulence, special processes in the wake region, and the effects of stratification. Scalar transfer and the surface energy balance are discussed first in terms of a linear theory which accommodates radiative, aerodynamic, and elevation perturbations on the surface energy balance over hills. The linear theory provides general scales for determining the relative magnitudes of these perturbations, but its quantitative predictions are limited to low slopes without separation. Next, wind tunnel experiments are used to study the aerodynamic perturbations on scalar transfer caused by hills in situations outside the scope of the linear theory, such as flows with separation. Finally, the wind tunnel results are extrapolated to the atmosphere, using the framework of a convective boundary layer model. It is found that, for relatively gentle topography, the regional surface energy balance (spatially averaged over several hills) is insensitive to the presence of the topography.