Investigating Spatial Inhomogeneity of Surface Layer Turbulence in Complex Terrain

In this project it is proposed to investigate Surface Layer (SL) turbulence characteristics in complex terrain. Its inherently inhomogeneous forcing (thermal and dynamical) raises the question whether or to what degree current theoretical treatment of SL turbulence, which is based on the assumption of horizontally homogeneous and flat (FHH) terrain, can be used. The theoretical understanding, known as Monin-Obukhov Similarity Theory (MOST), has widely been verified over HHF surfaces, but evidence from complex terrain is extremely sparse. Still, due to the lack of better knowledge it is generally used in a wide range of applications, such as numerical weather prediction (surface exchange parameterization), hydrological modeling, air pollution modeling, etc., whether they are intended for HHF or for complex terrain. In the present project we will use use an existing long-term data set for essentially two tasks: i) to characterize SL turbulence (surface fluxes, integral statistics) and their relative importance at various characteristic sites (`valley floor`, `slope`, `mountain top`) in an area of highly complex topography and ii) to explore the degree of applicability of MOST and, if necessary, potential pathways of its extension in complex terrain. To successfully perform these tasks the post-processing of turbulence data must first be adapted to the conditions of complex terrain. As a synthesizing view, finally, the findings of the two main tasks will be used in analyzing the spectral characteristics of SL turbulence in complex terrain. The data set available for this project is part of a recent larger initiative at the applicant`s institution (which also includes high-resolution numerical modeling) that aims at investigating the entire (not only turbulent) earth- atmosphere exchange in complex terrain. The individual measurement sites being concentrated in a confined `box` of some 15 x 25km extension yields the possibility to assess spatial characteristics of SL turbulence (task i) for different conditions (boundary layer stability, surface cover and state, specific local weather conditions like Föhn). The observations` long-term character (several years of measurements) allows for investigating a broad range of atmospheric states with a large enough data basis to obtain statistically significant results (task ii). Additional data sets from earlier, more episodic experimental campaigns are available to the project to generalize findings.

At the interface between the atmosphere and the Earths surface (Atmospheric Boundary Layer) there is a constant exchange of energy, mass and momentum. His leads to the important fact that this layer is turbulent. Hence The understanding of turbulence in the atmosphere is an important pre-requisite to build physically consistent weather and climate models. Our theoretical understanding of these turbulent exchange processes is restricted to flat and horizontally homogeneous surfaces. The project Investigating Spatial Inhomogeneity of Surface Layer Turbulence in Complex Terrain therefore aims at assessing the characteristics of turbulence and exchange processes over inhomogeneous and mountainous surfaces. For this, a unique data set could be employed that has been made available by the Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck (ACINN) in the frameworks of the Innsbruck Box (short i-Box) program. It consists (among others) of 6 so-called flux sites in very complex topography with partly long data sets. Results can be summarized as follows. The first focus of the project, i.e. the characterization of turbulence in complex terrain (here, questions like Are there consistent patterns for example in the daily cycles of turbulence variables?; Where is inhomogeneity the strongest?; Have similar structures been previously observed [typically from short campaign data sets]? should be answered) has yielded a broad range of results. In a valley as the Inn Valley, it is primarily the local topography that determines the turbulence structure, and not so much the exposition as this is the case in a north-south oriented valley. Quite generally, turbulence is stronger over the slopes than over the valley floor thus leading to some degree of homogeneity along the valley axis, but not across. Finally, there is a strong interaction between local flow systems (like the thermally driven valley wind system) and the turbulence structure. Second focus: the theory that has been developed under the assumption of flat and horizontally homogeneous conditions (Monin-Obukhov Similarity Theory) cannot be adopted one-to-one in mountainous areas. The results suggest, however, that there is potential for a generalization. A so-called local scaling approach proved successful for a number of variables and locations. These results from the present project are only first indication, which must theoretically be substantiated and verified based on data from other sites. This last point has important implications for the numerical simulation of weather and climate in the mountains (and similarly for applied simulations e.g., hydrology that are based on atmospheric input) since the corresponding models employ - due to a lack of better knowledge the flat and horizontally homogeneous theory also at grid points in mountainous terrain. This aspect is being investigated in other projects, parallel to INHOM- TCT.