Variability of Turbulence Dissipation Rate in Complex Terrain: the Perdigão 2017 Field Campaign

Accurate forecasting of turbulence in the atmospheric boundary layer is required for a variety of socio-economic activities, such as pollutant dispersion and air quality forecasting, forest fires prediction, and wind energy production. However, improved turbulence parametrizations are currently crucially needed to refine the accuracy of model representations at fine horizontal scales in complex terrain. To improve turbulence parameterizations, we need improved understanding of how turbulence is produced and dissipated.

We calculate turbulence dissipation rate (ε) from measurements taken during the Perdigão (Portugal) field campaign in summer 2017. Several different instruments were deployed in the valley and along the two ridges, which stay parallel for about 2km, with an elevation difference of about 200m from the ridges to the bottom of the valley. We use traditional approaches to derive ε from the 18-Hz measurements of sonic anemometers and the 1-kHz measurements from a hotwire anemometer suspended by a tethered lifted system. Further, we apply a novel approach to calculate ε from profiling wind Doppler lidar measurements.

We use the unprecedentedly extensive network of instruments at Perdigão to understand the variability of turbulence dissipation in complex terrain, with comparison between measurements taken at the bottom of the valley, on the ridges, and outside the valley. Figure 1 compares ε calculated from sonic anemometers mounted on 100m meteorological towers on the two ridges and at the bottom of the valley, on 3 May 2017. As case studies, we focus on the impact that mountain wave activity, recirculation and wind turbine wakes have on the variability of turbulence dissipation.