Objective Wind is a critical variable for applications such as numerical weather prediction and climate study. The direct-detect Rayleigh-Mie Doppler lidar is currently one of the most effective tools for tropospheric to stratospheric wind field detection with high accuracy and spatiotemporal resolution. However, owing to the strong aerosol backscattering and Brillouin backscattering signals in the lower atmosphere, the traditional wind retrieval method for Rayleigh-Mie Doppler lidar yields large retrieval errors of low-altitude atmospheric wind speeds. In this study, we quantitatively analyze the wind speed retrieval error of the traditional wind retrieval method in the low-altitude wind field inversion. Further, we propose a new retrieval algorithm and present its specific steps for Rayleigh-Mie Doppler lidar. The proposed new retrieval method can accurately and simultaneously retrieve wind speeds and aerosol backscattering ratios. We expect that the proposed method can effectively expand the capabilities of the Rayleigh-Mie Doppler lidar for detecting low-altitude wind field and aerosol backscattering ratios.
Methods First, factors causing the large retrieval errors in the low-altitude wind field inversion using the traditional retrieval method are analyzed, starting from the principle of wind field detection. Second, considering the influence of Brillouin scattering and Mie scattering, the S6-model-based Rayleigh-Brillouin scattering spectrum model, which is closer to the reality, is used instead of the Gaussian approximate spectrum model. Furthermore, a nonlinear iterative algorithm is used to separate the Rayleigh-Brillouin backscattering signal from the Mie backscattering signal using the signals of energy channel and two edge channels. After combining the above two points, a new retrieval algorithm based on the S6 model and nonlinear iterative method is established, which can simultaneously retrieve the low-altitude wind field and aerosol backscattering ratio. Third, the effectiveness of the proposed retrieval method is verified using the inversion simulation test of the wind field and aerosol backscattering ratio. Finally, based on the measured data of the Rayleigh-Mie Doppler lidar verification system of Anhui institute of optics and fine mechanics (AIOFM) in a comparison experiment with a sounding balloon, the traditional and proposed methods are used to retrieve the horizontal wind speed. By comparing the retrieval results of the horizontal wind speed obtained by employing the two methods using the measured data of the sounding balloon, the advantages of the proposed method in the actual low-altitude wind field inversion are further analyzed and confirmed.
Results and Discussions Based on the U.S. standard atmosphere model and design parameters of the Rayleigh-Mie Doppler lidar of AIOFM, the wind speed retrieval error below the 3-km altitude will reach 4--5 m/s and the relative error will exceed 10% within a wind speed range of ±50 m/s using the traditional wind field retrieval method, confirming the necessity of the study on low-altitude wind field retrieval method for Rayleigh-Mie Doppler lidar (Fig. 7). The parameter inversion simulation tests show that the retrieval value of the radial wind speed using the proposed method is obviously closer to the true value than that using the traditional method; additionally, the lower the altitude, the more obvious the advantages of the proposed method (Fig. 9). Moreover, the proposed method can simultaneously retrieve the aerosol backscattering ratio (Fig. 10). Using the measured raw data of the AIOFM Rayleigh-Mie Doppler lidar verification system in a comparison experiment with a sounding balloon, the horizontal wind speed profile retrieved using the proposed method is more consistent with that measured using the sounding balloon on the whole and this observation is particularly obvious below 6 km (Fig. 11). The statistical results of the difference in the horizontal wind speed data pairs measured using the two detection devices at the same altitude further verify that the proposed method has obvious advantages in retrieving the low-altitude wind field compared with the traditional method (Fig. 12).
Conclusions To address the problem of large retrieval errors yielded by the traditional wind field retrieval method of Rayleigh-Mie Doppler lidar in the low-altitude wind field inversion, a new retrieval algorithm based on the S6 model of Rayleigh-Brillouin scattering spectrum and nonlinear iterative method is proposed and the specific inversion steps are presents. The results of parameter inversion simulation tests show that the proposed method can simultaneously retrieve the wind speed and aerosol backscattering ratio with high accuracy. The inversion results of actual wind fields using the measured data of the AIOFM Rayleigh-Mie Doppler lidar verification system also show that the horizontal wind speed retrieved using the proposed method is obviously more consistent with the measurement results of the sounding balloon in the comparison experiment in the lower altitude. These findings fully verify that the proposed method can more accurately retrieve the low-altitude wind speed than the traditional method. The proposed low-altitude wind retrieval method can effectively expand the detection capability of Rayleigh-Mie Doppler lidar and has a high practical application value.
