Profiles of turbulence kinetic energy derived from Doppler lidar measurements

Turbulence kinetic energy (TKE) is an important process variable to characterize the atmospheric boundary layer. State-of-the-art numerical weather prediction (NWP) models solve a prognostic TKE equation, this generates the interest in measurement data of this variable for verification of the NWP model output. Operational TKE measurements are typically performed using 3D ultrasonic anemometers; this limits their availability to near-surface levels (up to about 200 m above ground at a few tower sites). Alternatively, TKE may be derived from measurements with ground-based remote sensing instruments, such as Doppler lidars.

At the Meteorological Observatory Lindenberg – Richard-Aßmann-Observatory (MOL-RAO) of the German Meteorological Service (DWD) we have implemented an algorithm suggested by Smalikho und Banakh (2017, Atmos. Meas. Tech. 10, 4191–4208) to derive TKE profiles from Doppler lidar measurements. In addition, this algorithm allows to derive profiles of momentum flux, eddy dissipation rate and the integral length scale of turbulence. Thus, a consistent data set to characterize turbulent processes in the atmospheric boundary layer can be obtained. The method includes a correction for an underestimation of TKE due to pulse volume averaging effects. It is based on a special Doppler lidar scan regime - a continuous scan mode (CSM) with very high azimuthal resolution (< 2 deg). This implies using a small number of lidar pulses per ray such that classical data filtering approaches cannot be applied.

We briefly introduce the scan configuration and the methodology for TKE derivation and discuss an alternative data filtering approach which has been realized and tested at MOL-RAO. The methodology has been applied to a data set covering one year of quasi-operational measurements with a Halo Photonics Streamline Doppler lidar at MOL-RAOs boundary-layer field site (GM) Falkenberg. Here, the intercomparison of the derived TKE values versus sonic measurements at a height of 90m at the GM Falkenberg tower shows good agreement. Case studies illustrate the potential to characterize enhanced turbulence associated with cold pools affiliated to thunderstorms or in the shear zone below the axis of a nocturnal low-level jet. Finally, the Doppler lidar TKE values have been compared to the output of DWD’s NWP models.