S. V. Samoilova

Investigations of the vertical distribution of troposphere aerosol layers based on the data of multifrequency Raman lidar sensing: Part 1. Methods of optical parameter retrieval

A technique intended for interpreting the data of multifrequency Raman lidar sensing is developed. An algorithm for separating aerosol layers with different scattering properties and the subsequent estimation of the average value of the lidar ratio and Angström parameter within the individual layers is proposed. The algorithm allows the error of retrieving the backscattering coefficient from daytime observations to be at least halved. To interpret the data of nighttime observations, a well-posed algorithm of numerical differentiation intended for determining the extinction coefficient based on the transformation of the range of allowable values which requires a solution of nonlinear equations is developed. An iterative procedure yielding an improved spatial resolution as compared with the conventional methods is envisaged for linearization. The methods can be successfully used for processing routine lidar measurements under conditions of a priori uncertainty.

Results of joint observations of aerosol perturbations of the stratosphere at the CIS-LiNet network in 2008

The results of lidar observations of stratospheric aerosol perturbations for the period of July–November 2008 at three lidar stations of the CIS-LiNet network in Tomsk, Minsk, and Vladivostok are presented along with the results obtained in the Gobi Desert during a research expedition. The behavior of stratospheric profiles of the scattering ratio R(H) (ratio of the total aerosol and molecular backscattering coefficient to the molecular backscattering coefficient) is analyzed at different wavelengths characterizing the aerosol stratification in the stratosphere. The transport of air masses in the stratosphere is studied by the method of direct and backward trajectories using the NOAA HYSPLIT model. It is shown that stratospheric aerosol perturbations are connected with explosive eruptions of volcanoes of the Aleutian islands Okmok (53.4° N, 168.1° W; July 12, 2008) and Kasatochi (52.2° N, 175.5° W; August 6–8, 2008).

Reconstruction of the aerosol optical parameters from the data of sensing with a multifrequency Raman lidar

A method of interpreting data of multifrequency Raman lidar sensing is developed. An algorithm for separating aerosol layers with different scattering properties and subsequently estimating the average value of the lidar ratio and Angström parameter within individual layers is suggested. The algorithm allows the error of reconstructing the backscattering coefficient from daytime observations to be at least halved. A well-posed numerical differentiation algorithm for determining the extinction coefficient is suggested for the interpretation of nighttime measurements based on the transformation of the range of allowable values that requires a solution of nonlinear equations. An iterative procedure envisaged for linearization improves the spatial resolution compared with the conventional methods. The methods can be successfully used to process routine lidar measurements under conditions of a priori uncertainty.

Investigation of the vertical distribution of tropospheric aerosol layers from multifrequency laser sensing data. Part 2: The vertical distribution of optical aerosol characteristics in the visible region

Regular lidar measurements of the vertical aerosol distribution were conducted in Tomsk (56° N, 85° E) from March 2006 to October 2007 as part of the CISLINET (CIS Lidar Network) project. The statistical analysis of the profiles of the aerosol backscattering coefficients βa (532 nm), extinction coefficients σa (532 nm), and lidar ratio Sa (532 nm) from the data of nocturnal measurements by Raman lidar (532 and 607 nm) in the altitude range from 0.45 to 7 km is presented. According to these measurements, the mean height of the top boundary of the boundary layer (BL) is 1.22 km for the cold period of observations (from October to March) and 2.3 km for the warm period (from April to September). The mean value of σa (532 nm) for the cold period of observations in the BL is 0.025 km−1, which is more than two times lower than the mean value of 0.061 km−1 for the warm observation period. The mean value of Sa (532 nm) in the BL is independent of the observation season and is equal to 52 sr. Above the BL, in the free troposphere (FT), the coefficients βa (532 nm) and βa (532 nm) are proportional to the molecular scattering coefficient. The mean value of σa (532 nm) is 0.0083 km−1 for the cold period and 0.011 km−1 for the warm period. The lidar ratio in the FT is 43.5 sr in the cold period. This value is nearly 10 sr lower than the mean lidar ratio for the warm period (52.8 sr).

Spectral behavior of optical coefficients and microphysical characteristics of aerosol particles

The paper considers the regularities of the spectral variations in the extinction and backscattering coefficients and in a lidar ratio, on the one hand, and the microphysical characteristics of aerosol particles, described by a complex refractive index and particle size distribution function, on the other hand. Based on the Mie calculations for mono- and bimodal distributions, it is shown that the Ångström parameters for the extinction coefficient are informative with respect to the contribution of small particles to the volume concentration Vf/Vt and weakly depend on the geometric mean radius of small particles Rf. On the contrary, the Ångström parameters for the backscattering coefficient are informative with respect to Rf and are almost independent of Vf/Vt. The lidar ratio and Ångström parameters for the backscattering coefficient depend strongly on the real mR and imaginary mI parts of the refractive index. Restricting the variability range of lidar ratio limits the variability range of mI (for a fixed mR), which shifts toward larger mI values with an increase in mR. The lidar ratio may increase with growth of the wavelength only for large particles when Vf/Vt < 0.2 and the real part of the refractive index is ∼1.40.

Method for reconstructing atmospheric optical parameters from the data of polarization lidar sensing.

Inversion of polarization lidar sensing data based on the form of the lidar sensing equation with allowance for contributions from multiple-scattering calls for a priori information on the scattering phase matrix. In the present study the parameters of the Stokes vectors for various propagation media, including those with the scattering phase matrices that vary along the measuring range, are investigated. It is demonstrated that, in spaceborne lidar sensing, a simple parameterization of the multiple-scattering contribution is applicable and the polarization signals characteristics depend mainly on the lidar and depolarization ratios, whereas differences in the angular dependences of the matrix components are no longer determining factors. An algorithm for simultaneous reconstruction of the profiles of the backscattering coefficient and depolarization and lidar ratios in an inhomogeneous medium is suggested. Specific features of the methods are analyzed for the examples of interpretation of lidar signal profiles calculated by the Monte Carlo method and are measured experimentally.

Structure of aerosol fields of the atmospheric boundary layer according to aerosol and Doppler lidar data during passage of atmospheric fronts

The paper presents the results of complex observations of the atmospheric boundary layer dynamics performed at the Fonovaya Observatory of the Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences, in September 2013, with the use of remote sensing facilities, i.e., aerosol and Doppler lidars. The structure of aerosol and wind fields in the period of occurrence of internal buoyancy waves and low-level jet streams in the boundary layer is considered.

Investigation of the Vertical Distribution of Tropospheric Aerosol Layers Using the Data of Multiwavelength Lidar Sensing. Part 3. Spectral Peculiarities of the Vertical Distribution of the Aerosol Optical Characteristics

The spectral peculiarities of the distributions of the backscattering βa(λi, z) and extinction σa(λi, z) coefficients, as well as lidar ratio Sa(λi, z) estimated from the data of multi-wavelength sensing in Tomsk (56° N, 85° E) in the height range from 0.5 to 7.5 km are presented here. Based on observations since April till October 2007 it is shown that in the boundary layer (except of the internal mixing layer) ηβ(532/1064) > ηβ(355/532), and, simultaneously, ησ(532/1064) > ησ(355/532), where ηi are the values of the Ågström parameter for the respective coefficients. Such a distribution of the Ågström parameters is caused by prevalence of small particles with mean geometric radius Rf < 0.15 μm in the volume distribution. On the contrary, in the free troposphere ηβ(532/1064) < ηβ(355/532) and ησ(532/1064) < ησ(355/532). Hence, Rf > 0.15 μm, and the contribution of large particles is governing. In the boundary layer, the lidar ratio decreases with increasing wavelength; the average values are 59.7 (15) sr at 355 nm, 51.1 (8.3) sr at 532 nm, and 47.3 (13.5) sr at 1064 nm. In the free troposphere, the wavelength behavior of the lidar ratio can be different; the average values are 50.4 (8.5) sr at 355 nm, 49.5 (5.7) sr at 532 nm, and 55.3 (10) sr at 1064 nm. The aerosol contribution of the free troposphere to the total aerosol optical depth grows with decreasing boundary layer height; on average, it is 22 (17)% at 355 nm, 27 (19)% at 532 nm, and 34 (22)% at 1064 nm.

“LOSA-S” – basic lidar of the CSF “ATMOSPHERE” IAO SB RAS for tropospheric studies

Stationary lidar “LOSA-S” of the center of shared facilities (CSF) “ATMOSPHERE” IAO SB RAS is intended for the study of aerosol fields in the boundary layer of the troposphere in the height range 0.5 up to 15 km, as well as for the study of crystal clouds using the polarization unit with linear and circular polarization of radiation. The scheme of simultaneous observation of the elastic and Raman scattering signals when irradiating the medium at the wavelengths of 1064, 532 and 355 nm is realized in the lidar. The lidar is based on the LOTIS-2135 Nd:YAG laser and the receiving specular telescope of the Cassegrain system with the diameter of 300 mm. In addition to the return signals of elastic scattering recorded in analog mode, the lidar records the Raman scattering signals on molecular nitrogen (387 and 607 nm) and water vapor (407 nm) in the photon counting mode. To realize the aforementioned height range, two receiving telescopes are used in the lidar for near and far zones, the signals are recorded by the same photodetectors.

Retrieval of the vertical distribution of aerosol microphysical characteristics from lidar measurements in Tomsk

Regular lidar measurements of the vertical distribution of aerosol optical parameters are carried out in Tomsk (560N, 850E) since April, 2011. We present the results of retrieval of microphysical characteristics from the data of measurements by means of Raman lidar in 2013. Section 2 is devoted to the theoretical aspects of retrieving the particle size distribution function U(r) (SDF) assuming a known complex refractive index m (CRI). It is shown that the coarse fraction cannot be retrieved unambiguously. When estimating U(r) and m together (section 3), the retrieved refractive index is non-linearly related to the optical coefficients and the distribution function, which leads to appearance of different, including false values of m . The corresponding U(r) differs only slightly, so the inaccuracy in m does not essentially affect the retrieval of the distribution function.