Characteristics and seasonal variations of cirrus clouds from ground-based lidar and satellite observations over Shouxian Area, China.

This study investigates the macroscopic and optical properties of cirrus clouds in the 32N region from July 2016 to May 2017, leveraging data from ground-based lidar observations and CALIOP to overcome the inconsistencies in detected cirrus cloud samples. Through extensive data analysis, statistical characteristics of cirrus clouds were discerned, revealing lidar ratio values of 28.5 ± 10.8 from ground-based lidar and 27.4 ± 11.2 from CALIOP. Validation with a decade of CALIOP data (2008-2018) confirmed these findings, presenting a consistent lidar ratio of 27.4 ± 12.0. A significant outcome of the analysis was the identification of a positive correlation between the lidar ratio and cloud centroid temperature, indicating a gradual decrease in the lidar ratio as temperatures dropped. The study established a fundamental consistency in their macroscopic properties, including cloud base height, cloud top height, cloud thickness, cloud centroid height, and cloud centroid temperature. The results for ground-based lidar (CALIOP) are: 10.0 ± 2.1 km (10.0 ± 2.2 km), 11.8 ± 2.1 km (11.5 ± 2.3 km), 1.87 ± 0.83 km (1.52 ± 0.71 km), and 10.5 ± 2.2 km, -46.9 ± 9.7°C (-47.1 ± 10.0°C).These properties exhibited seasonal variations, with cirrus clouds reaching higher altitudes in summer and lower in winter, influenced by the height of the tropopause. The optical properties of cirrus clouds were also analyzed, showing an annual average optical depth of 0.31 ± 0.35 for ground-based lidar and 0.32 ± 0.44 for CALIOP. The study highlighted the distribution of subvisible, thin, and thick cirrus clouds, with a notable prevalence of subvisible clouds during summer, suggesting their frequent formation above 14 km. Furthermore, the study observed linear growth in geometric thickness and optical depth up to 2.5 km from CALIOP and 2.9 km from ground-based lidar. Maximum optical depth was observed at cloud centroid temperatures of -35°C for CALIOP and -40°C for ground-based lidar, with optical depth decreasing as temperatures fell. This suggests that fully glaciated cirrus clouds exhibit the highest optical depth at warmer temperatures, within the complete glaciation temperature range of -35°C to -40°C.

Lidar signal processing method for atmospheric coherence length measurement based on the WD-ADMF.

A denoising method applied to atmospheric coherent length lidar is proposed. Wavelet decomposition (WD) and the adaptive median filter (ADMF) are combined in this method. In this research, the effectiveness of the WD-ADMF has been verified through simulation and measurement. The results show that this filter algorithm, when applied to lidar data, improves the average peak signal-to-noise ratio (PSNR) and centroid error while maintaining data integrity such that the measurement of coherence length or the inference of C n2 from coherence length more closely matches simulated truth and measured data.

Design of Lidar Data Acquisition and Control System in High Repetition Rate and Photon-Counting Mode: Providing Testing for Space-Borne Lidar.

For ground-based lidars in atmospheric observation, their data acquisition unit and control unit usually work independently. They usually require the cooperation of large-volume, high-power-consumption Industrial Personal Computer (IPC). However, the space-borne lidar has high requirements on the stability and integration of the acquisition control system. In this paper, a new data acquisition and lidar control system (DALCS) was developed based on System-on-Chip Field-Programmable Gate Array (SoC FPGA) technology. It can be used in lidar systems with high repetition rate and photon-counting mode and has functions such as data storage, laser control, automatic collimation, wireless communication, and fault self-test. DALCS has two working modes: in online mode, the echo data collected by DALCS are transmitted to the computer for display in real-time and then stored with the current time as the file name; in offline mode, the data are stored in local non-volatile memory, which can be read remotely and can work autonomously when there is no IPC. The test results showed that in the frequency range of 0-70 M, the counting linearity of DALCS reached 0.9999, and the maximum relative error between the DALCS card and the standard signal source was 0.211%. The comparison results showed that the correlation coefficient between DALCS and MCS-PCI was as high as 0.99768. The DALCS was placed in a self-developed lidar sensor system for continuous observation, and the system worked stably under different weather conditions. The range-squared-corrected signal profiles obtained from the observations reflect the spatial and temporal distribution characteristics of aerosols and clouds well. This provides scheme verification and experimental support for the development of space-borne lidar data acquisition and control system.

Radar-lidar ratio for ice crystals of cirrus clouds.

Simultaneous measurement of lidar and radar signals returned from the same cirrus clouds is a prospective method for retrieving the cloud microphysics, i.e. size and shape of the ice crystals constituting the clouds. In this study, the ratio of the backscattered signals of lidar and radar called the radar-lidar ratio has been calculated for the first time for typical shapes of ice crystals and wide distribution of the crystals over their sizes. It is shown that it is the lidar-radar ratio that is most sensitive to crystal sizes while the lidar depolarization ratio is most sensitive to crystal shapes.

High-spectral-resolution Mie Doppler lidar based on a two-stage Fabry-Perot etalon for tropospheric wind and aerosol accurate measurement.

A two-stage Fabry-Perot etalon (FPE)-based high-spectral-resolution (HSR) Mie Doppler lidar technology is proposed that is capable of simultaneously detecting tropospheric wind and aerosol optical properties with high precision. The lidar structure is designed, and the measurement principle is analyzed. A two-channel integrated FPE forming a two-stage FPE ensures the relative stability of the spectra. The HSR first-stage etalon can effectively suppress the contamination of Rayleigh signal. The transmission and reflection spectra of the second-stage etalon can form a double edge (DE) to measure wind speed. Two multimode polarization-insensitive optical circulators are used to achieve high-efficiency utilization for backscattering signals. The parameters of the two-stage FPE are optimized. According to the selected system parameters, the detection performance of the proposed lidar is simulated. Simulation results show that with 150 m range resolution and 1 min total accumulation time for the paired line-of-sight (LOS) measurement, within ±25 m/s LOS wind speed range, the nighttime and daytime LOS wind speed errors are below 0.48 m/s and 2.5 m/s, respectively, for a clear day, and below 0.58 m/s and 5.5 m/s, respectively, for a hazy day from 0.1 km to 8 km altitude; the backscatter ratio relative errors are below 2.7% up to 8 km for a clear day, and below 4.6% up to 5 km for a hazy day. Compared with the traditional dual-FPE-based DE Mie Doppler lidar, the wind speed accuracies are improved by 2.02-3.58 times for a clear day and 2.14-4.31 times for a hazy day.

Extinction matrix for cirrus clouds in the visible and infrared regions.

The extinction matrix for cirrus clouds has been calculated for the visible and infrared regions using the physical optics approximation. The cirrus clouds are modeled as a statistical ensemble of the hexagonal ice plates, distributed over their size and orientations by the gamma and Gaussian laws, respectively. Then, the extinction matrixes as the functions of the incident wavelength, incident direction, crystal size, and crystal orientation are numerically calculated for the first time. It is shown that the off-diagonal elements of the matrix are negligible. Therefore, the extinction in cirrus clouds is described with good accuracy by the scalar exponential law.

Fabry-Perot etalon-based ultraviolet trifrequency high-spectral-resolution lidar for wind, temperature, and aerosol measurements from 0.2 to 35 km altitude.

A novel ultraviolet trifrequency high-spectral-resolution lidar (HSRL) based on a triple Fabry-Perot etalon (FPE) and polarization discrimination technique is proposed, to the best of our knowledge, for measuring atmospheric wind, temperature, and aerosol optical properties simultaneously from the troposphere to low stratosphere. The measurement principle of wind speed, temperature, and aerosol is analyzed, and the structure of the proposed HSRL is designed. The parameters of the triple FPE are optimized. The multiparameter inversion method based on the nonlinear iterative approach and cubic spline interpolation method is also discussed, and the specific iteration steps are given. Finally, the detection performance of the proposed HSRL is simulated. The simulation results show that for 0.3 WSr-1 m-2 nm-1 at 355 nm sky brightness, by using a 350 mJ pulse energy, a 50 Hz repetition frequency laser, and a 0.45 m aperture telescope, the measurement errors of temperature, aerosol backscattering ratio and vertical wind speed are below 2.1 K, 2.5×10-3, and 2.2 m/s in nighttime and below 3.2 K, 3.4×10-3, and 2.6 m/s in daytime from 0.2 to 35 km with a temporal resolution of 5 min for temperature and aerosol, 1 min for vertical wind, and a vertical resolution of 30 m at 0.2-10 km, 100 m at 10-20 km, 200 m at 20-35 km; the measurement error of two other orthogonal line-of-sight wind speeds with a fixed zenith angle of 30° is below 2.9 m/s in nighttime and 3.9 m/s in daytime in the range of ±50 m/s from 0.2 to 35 km with a temporal resolution of 1 min and a vertical resolution of 26 m at 0.2-8.6 km, 87 m at 8.6-17.3 km, and 173 m at 17.3-35 km. Compared with the traditional double-edge wind-detection technique with the same complete instrumental parameters including those of the FPEs and FPE-based high-spectral-resolution temperature-detection technique with the optimal parameter values of FPEs for the same laser power and telescope aperture, the wind accuracy of the proposed technique improved by 1.5 times at night and by 1.5-1.9 times during the day, and the temperature accuracy of the proposed technique improved by 2.2-2.6 times at night and by 1.7-2.6 times during the day.

Backscatter by azimuthally oriented ice crystals of cirrus clouds.

The backscattering Mueller matrix has been calculated for the first time for the hexagonal ice columns and plates with both zenith and azimuth preferential orientations. The possibility of a vertically pointing polarization lidar measuring the full Mueller matrix for retrieving the orientation distributions of the crystals is considered. It is shown that the element m44 or, equivalently, the circular depolarization ratio distinguishes between the low and high zenith tilts of the crystals. Then, at their low or high zenith tilts, either the element m22 or m34, respectively, should be measured to retrieve the azimuth tilts.

Retrieval and Analysis of Atmospheric Temperature Using a Rotational Raman Lidar Observation.

Due to the existence of the aerosol, the traditional method of measuring atmospheric temperature by using Rayleigh scattering technique has limitations in the low altitude. A pure rotational Raman lidar to get tropospheric temperature profiles is built. We carried out the atmospheric temperature observation in Beijing for two months. The atmospheric temperature profile was retrieved using the observed rotational Raman scattering signals. The effect of smooth window, calibration range and calibration constant on the retrieval precision of the atmospheric temperature was evaluated and analyzed. The results show that with the increase of smooth window, the mean absolute deviation between the lidar and radiosonde firstly decreases and then increases; in order to remove effectively the effect of random error in the return signals, while maintaining the fine vertical structure of temperature profile, it is better to choose the range between 600 and 1 200 m for smooth window. When calibration range is different, the mean absolute deviation between the lidar and radiosonde is varied, the relative variation of the deviation is about 0.07 K. When both calibration constant a and b increase or decrease, the mean deviation between the lidar and radiosonde increases; when one increases and another decreases, the mean deviation has a tendency to cancel each other out. The variance probability of a or b is not equal, and the variance of a and b is always contrary in the sign; the mean deviation is not sensitive to variance of a or b, and it is sensitive to the whole variance of a and b, about 91.7% of the mean deviation is in the range between -3 and 3 K. These results provide the theoretical basis for the selection of smooth window and calibration range in pure rotational Raman lidar data retrieval, and the reference for the error of actual temperature inversion result caused by lidar calibration constant.

New experimental method for lidar overlap factor using a CCD side-scatter technique.

In theory, lidar overlap factor can be derived from the difference between the particle backscatter coefficient retrieved from lidar elastic signal without overlap correction and the actual particle backscatter coefficient, which can be obtained by other measured techniques. The side-scatter technique using a CCD camera is testified to be a powerful tool to detect the particle backscatter coefficient in near ground layer during night time. A new experiment approach to determine the overlap factor for vertically pointing lidar is presented in this study, which can be applied to Mie lidars. The effect of overlap factor on Mie lidar is corrected by an iteration algorithm combining the retrieved particle backscatter coefficient using CCD side-scatter method and Fernald method. This method has been successfully applied to Mie lidar measurements during a routine campaign, and the comparison of experimental results in different atmosphere conditions demonstrated that this method is available in practice.

Measurements of aerosol phase function and vertical backscattering coefficient using a charge-coupled device side-scatter lidar.

By using a charge-coupled device (CCD) as the detector, side-scatter lidar has great potential applications in the near range atmospheric detection. A new inversion method is proposed for CCD side-scatter lidar (Clidar) to retrieve aerosol phase function and vertical backscattering coefficient. Case studies show the retrieved results from Clidar are in good agreements with those obtained from other instruments. It indicates that the new proposed inversion method is reliable and feasible and that the Clidar is practicable.

Multimode Fano Resonances Sensing Based on a Non-Through MIM Waveguide with a Square Split-Ring Resonance Cavity.

In this article, a non-through metal-insulator-metal (MIM) waveguide that can excite fivefold Fano resonances is reported. The Fano resonances are obtained by the interaction between the modes excited by the square split-ring resonator (SSRC) and the bus waveguide. After a detailed analysis of the transmission characteristics and magnetic field strength of the structure using the finite element method (FEM), it was found that the independent tuning of Fano resonance wavelength and transmittance can be achieved by adjusting the geometric parameters of SSRC. In addition, after optimizing the geometric parameters, the refractive index sensing sensitivity (S) and figure of merit (FOM) of the structure can be optimal, which are 1290.2 nm/RIU and 3.6 × 104, respectively. Additionally, the annular cavity of the MIM waveguide structure can also be filled with biomass solution to act as a biosensor. On this basis, the structure can be produced for optical refractive index sensing in the biological, micro and nano fields.

Characteristics of aerosol optical properties in pollution and Asian dust episodes over Beijing, China.

Aerosol optical properties were continuously measured with the National Institute for Environmental Studies (NIES) compact Raman lidar over Beijing, China, from 15 to 31 December 2007. The results indicated that in a moderate pollution episode, the averaged aerosol extinction below 1 km height was 0.39+/-0.15 km(-1) and the lidar ratio was 60.8+/-13.5 sr; in heavy pollution episode, they were 1.97+/-0.91 km(-1) and 43.7+/-8.3 sr; in an Asian dust episode, they were 0.33+/-0.11 km(-1) and 38.3+/-9.8 sr. The total depolarization ratio was mostly below 10% in the pollution episode, whereas it was larger than 20% in the Asian dust episode. The distinct characteristics of aerosol optical properties in moderate and heavy pollution episodes were attributed to the difference in air mass trajectory and the ambient atmospheric conditions such as relative humidity.

[New mobile lidar for the measurement of tropospheric aerosol new mobile lidar for the measurement of tropospheric aerosol].

A new mobile lidar system has been developed for measuring tropospheric aerosol. Its main structure and specification as well as lidar data processing method are described. The results of measurement over Hefei city show that this lidar has ability to gain profiles of tropospheric aerosol day and night, and to manifest aerosol temporal and spatial distributions with high precision.