The Performance of a Time-Varying Filter Time Under Stable Conditions over Mountainous Terrain

Area of Science:

• Atmospheric Science
• Turbulence Research
• Boundary Layer Meteorology

Background:

• Eddy-covariance technique is crucial for measuring turbulent fluxes.
• Distinguishing between turbulent and non-turbulent motions is essential for accurate flux calculations.
• Previous studies have used various filter times, with inconsistent results.

Purpose of the Study:

• Determine the optimal gap scale for separating turbulent and non-turbulent motions.
• Evaluate the performance of time-varying versus constant filter times.
• Investigate the influence of atmospheric stability and wind speed on filter time selection.

Main Methods:

• Analysis of eddy-covariance data from five Austrian stations under stable conditions.
• Application of Fourier analysis and multi-resolution flux decomposition to calculate (co)spectra.
• Correlation analysis between gap scale, mean wind speed, and stability parameter (z/L).

Main Results:

• A correlation was found between gap scale, mean wind speed, and stability parameter (z/L).
• Both a time-varying filter time and a constant filter time of 2-3 minutes effectively separated turbulent from non-turbulent motions.
• The impact of filter time on turbulence statistics varied with parameters and location, affecting horizontal components and fluxes more than vertical wind variance.

Conclusions:

• A constant filter time of 2-3 minutes is recommended for separating turbulent and non-turbulent motions in the Inn Valley.
• Time-varying filter times, while theoretically sound, did not offer significant advantages over well-chosen constant filter times in this study.
• The findings provide practical guidance for processing eddy-covariance data to improve turbulence measurements.