Abstract
The surface energy balance, i.e. the partitioning of the energy exchange between the Earth’s surface and the atmosphere, plays an essential role in determining the characteristics and the evolution of the atmospheric boundary layer. An accurate assessment of its components is, therefore, crucial for a variety of applications. However, measurements of the surface energy balance terms are still affected by uncertainties. In particular, turbulent heat fluxes measured with the eddy-covariance technique generally do not balance the available energy. Several studies claim that the main reason for this gap is connected to the advection induced by secondary circulations, which can be present even over homogeneous surfaces in convective conditions but are more common over heterogeneous and complex terrain as a consequence of differential heating.
The INTERFACE project aims to evaluate the uncertainties connected to the measurement of the surface energy balance at different sites in the Alpine environment, where processes related to the lack of closure are expected to be particularly significant. Measurements at sites located in different contexts (e.g., valley floor, valley slope, mountain top) and climatic settings (North and South of the main Alpine crest) allow the investigation of the relationship between the non-closure of the surface energy balance, the surface heterogeneity, and the consequent development of different types of local and mesoscale thermally-driven circulations. These objectives are addressed by combining flux station and UAV measurements. The use of UAV allows spatially distributed measurements around the eddy-covariance sites, which are crucial for the estimation of advection. The INTERFACE project contributes to the TEAMx international research programme, which aims at improving our understanding of exchange processes in the atmosphere over mountains.
This contribution presents an overview of the project activities and results, focusing on the comparison of the characteristics of the surface energy balance and its closure at different Alpine sites. The analysis aims to point out similarities and differences between sites located in different topographic and land cover settings, providing a systematic quantification of the non-closure of the surface energy balance in several typical Alpine contexts. Moreover, particular attention is given to the evaluation of the role of thermally-driven circulations on the non-closure of the surface energy balance, selecting, by means of objective criteria, days with well-developed slope and valley circulations. Finally, different processing techniques, including different averaging and rotation approaches, are tested to quantify their effect on the final flux estimates and the surface energy balance closure.