Filtering of pulsed lidar data using spatial information and a clustering algorithm

Abstract

Abstract. Wind lidars present advantages over meteorological masts, including simultaneous multipoint observations, flexibility in measuring geometry, and reduced installation cost. But wind lidars come with the “`cost” of increased complexity in terms of data quality and analysis. Carrier-to-noise ratio (CNR) has been the metric most commonly used to recover reliable observations from lidar measurements but with severely reduced data recovery. In this work we apply a clustering technique to identify unreliable measurements from pulsed lidars scanning a horizontal plane, taking advantage of all data available from the lidars – not only CNR but also line-of-sight wind speed (VLOS), spatial position, and VLOS smoothness. The performance of this data filtering technique is evaluated in terms of data recovery and data quality against both a median-like filter and a pure CNR-threshold filter. The results show that the clustering filter is capable of recovering more reliable data in noisy regions of the scans, increasing the data recovery up to 38 % and reducing by at least two-thirds the acceptance of unreliable measurements relative to the commonly used CNR threshold. Along with this, the need for user intervention in the setup of data filtering is reduced considerably, which is a step towards a more automated and robust filter.

The COTUR project: remote sensing of offshore turbulence for wind energy application

“…Nevertheless, this value is fairly close to the one used by e.g. the IEC standard (IEC 61400-3, 2005), documented offshore (Geernaert et al, 1987;Barthelmie et al, 1996) or near offshore (Andersen and Løvseth, 2006). Some studies report also average turbulence intensities lower than in the present case, e.g.…”

“…Although the majority of the performed LOS scan scenarios have a duration of 50 min, instrument acquisition errors led occasionally to loss of data and resulted in time series that were shortened. In the MABL, turbulence characteristics are typically studied using records equal to or longer than 30 min (Smith, 1980;Andersen and Løvseth, 2006;Cheynet et al, 2018). This aims to ensure that a sufficiently large number of eddies pass through the instrument measurement volume for precise estimation of the flow characteristics (Lumley and Panofsky, 1964;Kaimal and Finnigan, 1994).…”

“…One straightforward approach relies on a fixed value of the CNR, generally between −24 and −28 dB, below which data are discarded. Some recent studies (Beck and Kühn, 2017;Valldecabres et al, 2018;Alcayaga, 2020) argue that setting a fixed threshold value for the CNR can cause exaggerated data removal, which can be a critical issue when the overall data availability is low. While Beck and Kühn (2017) and Valldecabres et al (2018) used an iterative method based on a moving standard deviation window to increase the data availability, we used herein a two-stage method without iteration.…”

“…In the marine atmospheric boundary layer (MABL), much less information is available. In coastal sites, the lateral coherence has been studied using masts mounted on an islet (Mann, 1994) or an island (Andersen and Løvseth, 2006). However, many offshore sites are free of them, and the installation cost can become prohibitive if the mast structure must be anchored to the seabed.…”

The COTUR project: Remote sensing of offshore turbulence for wind energy application

“…One straightforward approach relies on a fixed value of the CNR, generally between −24 dB and −28 dB, below which data are discarded. Some recent studies (Beck and Kühn, 2017;Valldecabres et al, 2018;Alcayaga, 2020) argue that setting a fixed threshold value for the CNR can cause unnecessary exaggerated data removal, which can be a critical issue when the overall data availability is low. While Beck and Kühn (2017) and Valldecabres et al (2018) used an iterative method based on a moving standard deviation window to increase the data availability, we used herein a two-stage method without iteration.…”