Dual-wavelength Lidar Research Articles

Ceilometer calibration for retrieval of aerosol optical properties

Ceilometers are durable compact backscatter lidars widely used to detect cloud base height. They are also useful for measuring aerosols. We introduced a ceilometer (CL51) for observing dust in a source region in Mongolia. For retrieving aerosol profiles with a backscatter lidar, the molecular backscatter signal in the aerosol free heights or system constant of the lidar is required. Although the system constant of the ceilometer is calibrated by the manufacturer, it is not necessarily accurate enough for the aerosol retrieval. We determined a correction factor, which is defined as the ratio of true attenuated backscattering coefficient to the measured attenuated backscattering coefficient, for the CL51 ceilometer using a dual-wavelength Mie-scattering lidar in Tsukuba, Japan before moving the ceilometer to Dalanzadgad, Mongolia. The correction factor determined by minimizing the difference between the ceilometer and lidar backscattering coefficients was approximately 1.2±0.1. Applying the correction to the CL51 signals, the aerosol optical depth (AOD) agreed well with the sky-radiometer AOD during the observation period (13–17 February 2013) in Tsukuba (9×10−3 of mean square error). After moving the ceilometer to Dalanzadgad, however, the AOD observed with the CL51 (calibrated by the correction factor determined in Tsukuba) was approximately 60% of the AErosol RObotic NETwork (AERONET) sun photometer AOD. The possible causes of the lower AOD results are as follows: (1) the limited height range of extinction integration (<3km); (2) change in the correction factor during the ceilometer transportation or with the window contamination in Mongolia. In both cases, on-site calibrations by dual-wavelength lidar are needed. As an alternative method, we showed that the backward inversion method was useful for retrieving extinction coefficients if the AOD was larger than 1.5. This retrieval method does not require the system constant and molecular backscatter signals, and can be applied to severe dust and air pollution aerosol cases in East Asia.

Measuring forests with dual wavelength lidar: A simulation study over topography

Accurate measurements of biophysical parameters are essential for understanding the distribution and dynamics of global vegetation, which exerts an influence on the carbon cycle and atmospheric circulation. Spaceborne, large footprint lidar has been shown to be a valuable tool. It is capable of measuring denser forests than other existing remote methods.However large-footprint lidar struggles to separate ground and canopy signals over topography and in the presence of short vegetation. This prevents the physically-based measurement of forest properties (such as canopy height and cover) at an acceptable accuracy (sub 10m root mean square error for height) without the use of external data. The necessary external datasets are not yet available at a global scale at high accuracy.In this paper the issues of measuring forests with large-footprint, monochromatic lidar are presented. A number of subtle effects, such as shadows beneath crowns, can hamper the reliable measurement of forests. It is proposed that a dual wavelength lidar will allow the separation of canopy from ground returns in these situations and so allow the physically-based measurement of forests.An initial algorithm is developed and tested with Monte-Carlo ray tracer simulations as a proof of concept. Some refinements are needed to make the method more robust, but the initial form was found to determine the start of the ground return over steep slopes and a range of forest densities, canopy heights and vertical structures with a root mean square error (RMSE) of 2.7m and mean bias of 67cm for canopies with covers below 99%. This resulted in canopy height RMSE of 2.88m with a bias of −23cm. Such a system will allow measurement of a much broader range of forests than is possible with monochromatic lidar and could form a second generation spaceborne lidar mission.

Analysis of Dust Aerosol by Using Dual-wavelength Lidar

Simulation of dust aerosol optical property is rather difficult, due to its extremely irregular shape, which often brings about difficulties in transforming its physical properties (such as size distribution) into optical properties (such as scattering phase function) in remote sensing retrieval and atmospheric radiation model. Some recent researches reveal that homogeneous spheroids seem to be an applicable optical model when dust particles are not much bigger than the wavelength, spheroids with reasonable shape distribution can simulate the scattering phase function of dust particles quite well. Based on the existed dual-wavelength lidar inversion algorithms, a modified method is proposed in the paper. Assuming the size distributions of dust aerosol can be modeled by bimodal lognormal distributions dominated by particles ranged in coarse mode, the size distributions and lidar ratios of dust aerosol at two wavelengths can be derived from dual-wavelength lidar measurement. By applying this algorithm to the data of dual-wavelength lidar at Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL), preliminary results show that for the case of pure dust the retrieved size distribution agree with that observed by Aerodynamic Particle Sizer Spectrometer, and the derived mean lidar ratios are 45.7 ± 5.3 sr at 532 nm and 33.9 ± 1.5 sr at 1064 nm.

24 h continuous observation of sodium layer over Wuhan by lidar

Based on the dual-wavelength lidar we have developed, the 24 h continuous observation has been realized in its sodium channel by using Faraday atomic filter technology and other relevant technologies. This will facilitate the continuous observation of the sodium layer and the relevant upper atmosphere over Wuhan. A result of about 50 h observation indicates that the daytime column density of sodium layer over Wuhan is slightly increased compared to that during the nighttime, and the characteristics of the sporadic sodium layer occurring during the daytime are compared with that during the nighttime.

In Situ Cloud Sensing with Multiple Scattering Lidar: Simulations and Demonstration

Due to the spatially inhomogeneous nature of clouds there are large uncertainties in validating remote sensing retrievals of cloud properties with traditional in situ cloud probes, which have sampling volumes measured in liters. This paper introduces a new technique called in situ cloud lidar, which can measure extinction in liquid clouds with sampling volumes of millions of cubic meters. In this technique a laser sends out pulses of light horizontally from an aircraft inside an optically thick cloud, and wide-field-of-view detectors viewing upward and downward measure the time series of the number of photons returned. Diffusion theory calculations indicate that the expected in situ lidar time series depends on the extinction and has a functional form of a power law times an exponential, with the exponential scale depending on the distance to the cloud boundary. Simulations of 532-nm wavelength in situ lidar time series are made with a Monte Carlo radiative transfer model in stochastically generated inhomogeneous stratocumulus clouds. Retrieval simulations are performed using a neural network trained on three parameters fit to the time series of each detector to predict 1) the extinction at four volume-averaging scales, 2) the cloud geometric thickness, and 3) the optical depth at four averaging scales. Even with an assumed 20% lidar calibration error the rms extinction and optical depth retrieval accuracy is only 12%. Simulations with a dual wavelength lidar (532 and 1550 nm) give accurate retrievals of liquid water content and effective radius. The results of a mountain-top demonstration of the in situ lidar technique show the expected power-law time series behavior.

Optical properties of the atmospheric boundary layer at 1.06 μm: Quantitative remote sensing of aerosols with a Nd: YAG lidar

Quantitative results on atmospheric extinction (α) and backscatter (β) coefficients are reported. From the observations, the lidar ratio ( α β ) was parametrized as a function of relative humidity. Using this parametrization and an integral-inversion technique, the calculated vertical extinction profiles from a lidar are in very good agreement with aircraft data. From a dual-wavelength lidar, a good estimate of the slope of the aerosol size distribution n( r) is obtained [ n( r)(:) r −4.3; r = particle radius] in accordance with the measured particle-size data. Some samples of routine boundary-layer profiles at 1.06 μm will be given. The profiles demonstrate the capability of lidar to monitor boundary-layer dynamics (convective motion, plume mixing). Preliminary experiments in conjunction with ground-level SO 2 measurements were performed, which indicate possible operational applications for IR lidars.