Deriving wind gusts from Doppler lidar measurements

Doppler lidar systems allow for a reliable determination of the profiles of wind speed and wind direction in the Atmospheric Boundary Layer (ABL) based on classical measurement strategies such as a VAD scan (Velocity Azimuth Display, e.g. Päschke et al., 2015, Atmos. Meas. Tech. 8, 2251–2266). For many practical applications, however, short-term fluctuations of the wind, such as those that occur in connection with wind gusts, are of great interest in addition to the mean wind profile.

A study by Suomi et al. (2017, Q.J.R. Meteorol. Soc. 143, 2061-2072) has shown that it is, in principle, possible to derive wind gusts from Doppler lidar measurements. However, the high temporal resolution in the determination of the wind vector required for this is not achieved with usual measurement strategies. The authors therefore introduced a correction of the gust values derived from the lidar data based on a scaling approach using in-situ wind measurements.

In our study, an alternative measurement strategy for Doppler lidar systems of the type "Streamline" (Halo Photonics) was developed and tested over several months in 2020/21 at the boundary layer measurement field site (GM) Falkenberg of the German Meteorological Service (DWD). The gust derivation is based on a so-called continuous scan mode (CSM) where the radial velocity measurements taken continuously during a complete rotation of the lidar scan head are assigned to 10-11 beam directions and the wind vector for each rotation is determined using the VAD method. The duration of a scan cycle is about 3.4s, thus a time resolution can be achieved that corresponds to the widely-accepted definition of a wind gust (3s moving average; WMO (2018)).

This new configuration brings challenges to the data processing. In the fast CSM, comparatively few lidar pulses per measurement beam have to be used, so that classical approaches for data filtering (signal-to-noise thresholding, consensus filtering) cannot be used. An alternative method for processing the raw lidar data is proposed. The results of deriving both the mean wind vector and the respective maximum wind gust for a 10-minute averaging interval are compared with sonic measurements at 90m height for a two-month period during the FESSTVaL experiment (Field Experiment on Sub-Mesoscale Spatio-Temporal Variability in Lindenberg, www.fesstval.de). Further measurements were realised in Hamburg in spring 2022 in order to make comparisons with sonic measurements on a tower at higher levels (up to 250m). First results from this 4-week experiment will also be presented.

World Meteorological Organization (WMO) (2018): Measurement of surface wind. In Guide to Meteorological Instruments and Methods of Observation, Volume I -Measurement of Meteorological Variables, No.8: 196–213, URL: https://library.wmo.int/doc_num.php?explnum_id=10616 (accessed April 2022)