Aleksey V. Malinka

Dual‐FOV Raman and Doppler lidar studies of aerosol‐cloud interactions: Simultaneous profiling of aerosols, warm‐cloud properties, and vertical wind

Date of Acceptance: 24/04/2014 This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made

Fraunhofer diffraction by arbitrary-shaped obstacles

We consider Fraunhofer diffraction by an ensemble of large arbitrary-shaped screens that are randomly oriented in the plane of a wavefront and have edges of arbitrary shape. It is shown that far outside the main diffraction peak the differential scattering cross section behaves asymptotically as theta(-3), where theta is the diffraction angle. Moreover, the differential scattering cross section depends only on the length of the contours bordering the screens and does not depend on the shape of the obstacles. As both strictly forward and total diffraction cross sections are specified by obstacle area only, the differential cross section of size-distributed obstacles is expected to be nearly independent of obstacle shape over the entire region of the diffraction angles.

Approximation of the Fraunhofer diffraction peak, produced by particles of arbitrary shape

A simple analytical formula is developed to describe light diffraction by chaotically oriented particles of arbitrary shape. The formula expresses the angular pattern through three well-defined microphysical characteristics of an ensemble: the average cross-sectional area, the average area squared, and the average length of the contour bordering the particle projection.

Light scattering by optically soft large particles of arbitrary shape

Light scattering by chaotically oriented optically soft large particles of arbitrary shape is considered within the framework of the Rayleigh-Gans approximation. It has been shown that outside the forward direction, the scattering pattern has the dependence of Δk⁻⁴(1+cos²θ), where is an average particle surface area, Δk is the difference between scattered and initial wave vectors, θ is the scattering angle, and this pattern is independent of particle shape. A simple approximating formula is suggested, which correctly describes the scattering pattern in the entire range of scattering angles. This formula is compared to the particular case of size-distributed spherical particles and is shown to have high accuracy. Also, it is shown that the inherent optical properties, as total, transport, and backward scattering coefficients, are determined by the specific particle surface area and the effective particle size.

Raman lidar remote sensing of geophysical media

Lidars are equipment, consisting of a laser and a photo-receiver, that measures the backward scattering of light. They appeared in the 1960s (Fiocco and Smullin, 1963), i.e., immediately after the invention of the laser, and since then they have been actively used in the problems of natural media monitoring. Lidars are of great use in providing atmosphere and ocean pollution control, in control of atmospheric gases, and in measuring meteorological and climate characteristics. Generation of a beam of high power and small angular divergence makes the great advantage of lidars over projector sounding, having existed before. The possibility of accurate wavelength tuning, as well as spectral return measuring, allows the determination of the chemical composition of the atmosphere and the biochemical composition of the ocean. Thanks to the measurement of scattered light polarization degree one can learn about the shape of scatterers. Furthermore, as lasers are able to generate powerful pulses of short duration, there appears the possibility of measuring time-dependent returns, i.e., measuring not only the integral optical characteristics of a medium, but also their spatial distribution. These features made lidars a powerful tool in the investigation of geophysical media.

Analytical expressions for characteristics of light scattering by arbitrarily shaped particles in the WKB approximation.

Light scattering in the Wentzel-Kramers-Brillouin (WKB) approximation is considered from the point of view of stereology. The extinction and absorption cross sections for an ensemble of chaotically oriented particles of arbitrary shape are expressed analytically through the chord length distribution. The analytical approximation for the scattering phase function is proposed. The derived analytical expressions are compared to the calculations with the discrete-dipole-approximation method.

Fraunhofer diffraction by a random screen.

The stochastic approach is applied to the problem of Fraunhofer diffraction by a random screen. The diffraction pattern is expressed through the random chord distribution. Two cases are considered: the sparse ensemble, where the interference between different obstacles can be neglected, and the densely packed ensemble, where this interference is to be taken into account. The solution is found for the general case and the analytical formulas are obtained for the Switzer model of a random screen, i.e., for the case of Markov statistics.

Retrieving seawater-backscattering profiles from coupling Raman and elastic lidar data

We propose a technique for retrieving seawater-backscattering profiles that is based on the joint use of elastic and Raman lidar returns. We suggest using two lidar channels: the Raman channel and the elastic channel with a light frequency equal to a half-sum of initial and Raman-shifted frequencies of the Raman channel. These specific wavelengths provide the same attenuation laws for elastic and Raman signals if absorption and scattering spectra can be approximated by a power law. In particular, seawater supplies such a possibility in the region of 400-500 nm if extremely bioproductive waters are not considered and the chlorophyll absorption peak at 440 nm does not come out of the background of dissolved organic matter absorption. With these specific initial wavelengths, the elastic and Raman lidar returns differ only in the backscattering coefficients. Because the Raman-backscattering coefficient is constant along the profile, the (elastic-to-Raman) ratio of these lidar returns directly produces the profile of the elastic-backscattering coefficient. This technique stays valid even under multiple-scattering conditions, which is of great importance for seawater sounding.

Analytical modeling of Raman lidar return from clouds and ocean including multiple scattering

An analytical approach for modeling Raman lidar return with multiple scattering is presented. The approach is based on a small-angle quasi-single scatteirng approximation developed earlier for elastic lidar sounding. Spatial-angular structure of Raman lidar return is investigated. For particular case of warm clouds it is shown that multiple-field-of-view lidar technique allows one to retrieve the effective size of scattering particles.

Retrieval of microphysical properties of liquid water clouds from atmospheric lidar measurements: comparison of the Raman dual field of view and the depolarization techniques

Since 2010, the Lidar MARTHA at TROPOS permits the retrieval of microphysical properties of liquid-water clouds during nighttime by estimations of the multiple scattering effects with the so called dual-FOV Raman technique. A recent lidar single-FOV depolarization approach which permits the retrieval of these properties as well, was implemented in the MARTHA system. Additionally, a new simple dual-FOV depolarization approach was developed and tested in the lidar measurement for several cloud periods. The first preliminary retrieval results and a comparison between the three methods are presented.