Analysis of the filter time scale under stable conditions in mountainous terrain

To calculate turbulent fluxes from eddy-covariance measurements, an appropriate filter time needs to be selected to remove non-turbulent larger-scale motions from the raw time series, while retaining all of the turbulent contributions. Common choices include 30 min for convective conditions and 1-5 min for stable conditions. Eddy-covariance data from five i-Box stations in the Austrian Inn Valley are analyzed to determine the appropriate filter time scale under stable conditions using spectral analysis. The i-Box (Innsbruck Box) is a long-term measurement platform, which was designed to study boundary-layer processes in highly complex terrain and has been operational since 2012. The five stations are located in an approximately 6.5-km long section of the 2-3-km wide valley, with one station at the almost flat valley floor, two stations on relatively low-angle slopes of the south-facing sidewall, and two stations on steep slopes of the north-facing sidewall. Different methods, including Fourier analysis and multi-resolution flux decomposition, are tested to determine the filter time scale. As submeso motions affect temperature and the horizontal and vertical wind components differently, not all variances and covariances are equally well suited to identify the time scale. Using the correlation between the identified filter time and the mean near-surface wind speed and stability, the impact of different filter times, including a flexible, time-varying filter time, on near-surface turbulent fluxes is further discussed.