Rain measurement using nacelle-mounted Doppler lidar

Rain measurements based on the remote sensing technique play an important role in many fields. In this paper, the DTU-developed continuous-wave SpinnerLidar, deployed on top of a wind turbine, is used to investigate the feasibility of rain measurement. Considering the wind-driven effect on raindrops, a retrieval method is proposed to obtain the rain vector velocity, rain drop size distribution (DSD), and rain intensity from the combined Doppler spectrum of precipitation and wind based on a nacelle-mounted Doppler lidar. To evaluate the performance of the rain identification method, the rain vector velocities and wind speeds identified from the SpinnerLidar are compared with those from the disdrometer and the sonic anemometers, respectively. The results indicate that a nacelle-mounted wind lidar has the ability to observe the rainfall velocity even with low elevation angles of about 20° - 30° of the laser beams. The multi-peak structure produced by raindrops in the Doppler lidar spectrum was reported for the first time, which may be attributed to the wide range of sizes of raindrops and snowflakes, and their different corresponding falling velocities. Additionally, the rain and wind spectra retrieved from the SpinnerLidar are in good agreement with those calculated from the rain drop size and speed distributions from the disdrometer combined with backscattering models, and the probability density function of wind speeds measured by the sonic anemometer, respectively, on both clear and rainy days. Finally, the errors of the DSD and rain intensity estimation related to the rain attenuation effect, rain echo signals beyond the Rayleigh length of continuous-wave lidar, and the two assumptions underlying the proposed method were analyzed and discussed. The results indicated that under typical atmospheric conditions, the absolute value of the error induced by the two assumptions is mainly less than 0.5 m/s, fluctuating between 0.1 to 0.3 m/s.