Characterization of the surface energy balance in complex terrain

The surface energy balance (SEB) is a physical principle that represents how the energy is distributed between the lowest layer of the atmosphere and the Earth's surface. Its understanding is pivotal not only for determining the meteorological and climatological conditions in the atmospheric boundary layer, but also for applications such as weather and climate modeling, agricultural and forest management, air pollution modeling, ecosystem carbon budget, and biological modeling. Indeed, all the mentioned applications assume a balance between the energy fluxes. Yet, even over flat and horizontally homogeneous terrain, the SEB closure is rarely observed. For an ideal massless layer at the surface, the SEB formulation states that net radiation is balanced by the sum of the turbulent fluxes of sensible and latent heat, and the soil heat flux (R_n = H + LE + G). Previous research suggests that the persistent lack of closure either results from measurements limitations or from omitted processes. In fact, measurements are usually not taken at the interface between the atmosphere and the surface, but at a certain distance above (R_n, H and LE) or below the ground (G). Consequently, fluxes should be referred to a volume rather than a surface. SEB studies identified the storage of heat in the volume of air/soil above/below the measurement point as a contributing factor to the SEB residual. Nowadays, it is widely recognized that non-turbulent, advective fluxes derived from surface heterogeneities are the main reason behind the SEB non-closure. 
We systematically assessed this ideal formulation using data collected from the sites of the i-Box network, a long-term measurement setup in the Inn Valley (Austria). These stations represent different topographic categories (valley floor, mountain top, and slopes) and present different slope angle, orientation and land-use characteristics. Different to assessing SEB closure (or non-closure) over ideal surfaces, the goal of the present study is to characterize the SEB residual in complex, mountainous terrain. In other words it is investigated which processes contribute to what degree to the expected non-closure of the SEB in this type of environment. Specifically, we determined the relation between the SEB imbalance and different flow regimes (thermally-driven valley-slope wind circulations and foehn events), stability classes, seasons and topographic categories. Furthermore, the effects of specific post-processing procedures on the magnitude of the SEB residual were assessed for a valley-floor and a north-facing slope site. We compared the turbulent fluxes using double rotation, planar-fit and sectorial-planar-fit coordinate rotation methods. In general, these procedures lead to small differences among fluxes such that they do not impact the relative importance of different processes to explain the SEB non-closure. However, this comparison allowed us to evaluate the uncertainty of the magnitudes of the turbulent fluxes and therefore of the SEB residual.