Alexander A. Kokhanovsky

Satellite Aerosol Remote Sensing over Land

Introduction, 1. The dark-land MODIS collection 5 aerosol retrieval: algorithm development and product evaluation, 2. The time series technique for aerosol retrievals over land from MODIS, 3. Iterative procedure for retrieval of spectral aerosol optical thickness and surface reflectance from satellite data using fast radiative transfer code and its application to MERIS onboard ENVISAT measurements, 4. Aerosol retrieval over land using the ATSR dual-view algorithm, 5. Aerosol optical depth from dual-view (A)ATSR satellite observations, 6. Oxford-RAL Aerosol and Cloud (ORAC): aerosol retrievals from satellite radiometers, 7. Benefits and limitations of the synergistic aerosol retrieval SYNAER, 8. Retrieval of aerosol properties over land using MISR observations, 9. Polarimetric remote sensing of aerosols over land surfaces, 10. Optimal estimation applied to the joint retrieval of aerosol optical depth and surface BRF using MSG/SEVIRI observations, 11. Remote sensing data combinations: superior global maps for aerosol optical depth

Optical properties of terrestrial clouds

The aim of this review is to consider optical characteristics of terrestrial clouds. Both single and multiple light-scattering properties of water clouds are studied. The numerous results discussed can be used for solutions to both inverse and direct problems of cloud optics. An introduction to main microphysical characteristics of water and ice clouds, including the effective size, the liquid water content and path, the concentration of particles, is given. The refractive index of water and ice, which is of importance for optical waves propagation in clouds, is also considered. We consider extinction and absorption coefficients, phase functions, and asymmetry parameters of water and ice clouds and their relationships with the size, shape, and concentration of water droplets and ice crystals. Simple analytical results for absorption, reflection, and transmission characteristics of cloudy media are given. They are valid for optically thick clouds with optical thickness larger than 5. We have reviewed studies of polarization of solar light reflected from clouds. Image transfer problems have been considered as well. Modern methods of passive remote sensing of water and ice clouds are discussed. The review is finished by the discussion of optical properties of inhomogeneous clouds, which is a main topic of modern cloud optics research. D 2003 Elsevier B.V. All rights reserved.

Reflective properties of natural snow: approximate asymptotic theory versus in situ measurements

Results of measurements of the bidirectional reflection function of snow for the solar zenith angle close to 54/spl deg/ are compared with a recently developed snow optical model based on the representation of snow grains as fractal particles. The model has a high accuracy out of the principal plane for the observation zenith angles smaller than 60/spl deg/. However, the accuracy is reduced in the principal plane. Specular light reflection by partially oriented snow plates on the snow surface not accounted for by the model can play a role for measurements in the principal plane. The model discussed can be used for the grain size retrieval using both ground and spaceborne measurements of the snow reflectance. This is supported by a high accuracy of the model in a broad spectral range 545-2120 nm as demonstrated in this work.

Atmospheric Aerosol Monitoring from Satellite Observations: A History of Three Decades

More than three decades have passed since the launch of the first satellite instrument used for atmospheric aerosol detection. Since then, various powerful satellite remote sensing technologies have been developed for monitoring atmospheric aerosols. The application of these new technologies to different satellite data have led to the generation of multiple aerosol products, such as aerosol spatial distribution, temporal variation, fraction of fine and coarse modes, vertical distribution, light absorption, and some spectral characteristics. These can be used to infer sources of major aerosol emissions, the transportation of aerosols, interactions between aerosols and energy and water cycles, and the involvement of aerosols with the dynamic system. The synergetic use of data from different satellite sensors provides more comprehensive information to better quantify the direct and indirect effects of aerosols on the Earth’s climate. This paper reviews how satellite remote sensing has been used in aerosol monitoring from its earliest beginnings and highlights future satellite missions.

The physical parameterization of the top-of-atmosphere reflection function for a cloudy atmosphere—underlying surface system: the oxygen A-band case study

Abstract The paper is devoted to the physical parameterization of the top-of-atmosphere reflection function. The accuracy of the parameterization is checked against exact radiative transfer calculations in a cloudy atmosphere for various cloud-top-heights, cloud optical and geometrical thicknesses, solar illumination and surface reflection conditions. It was found that the error of approximation is smaller than 5% for most cases studied at the wavelength interval 758 – 768 nm , which corresponds to the oxygen A-band. This band is routinely used in cloud-top-height retrievals. The model proposed can be used for cloud-top-height and cloud geometrical thickness retrievals. This allows to avoid a standard look-up-table retrieval scheme, involving complex numerical procedures.

Optical properties of irregularly shaped particles

The phase matrix of irregularly shaped randomly oriented large fractal particles is studied in the framework of the ray tracing approach for various refractive indices. The fractal particle model can be used for modelling local optical characteristics of various natural light scattering media with irregular particles, including oceanic water, crystalline clouds and solid aerosols. For nonabsorbing large particles the theory does not contain any fitting parameters except the refractive index of particles, which is either known (ice clouds) or has generally unknown distribution in a local volume of a scattering medium (dust aerosols and oceanic suspensions).

The determination of cloud altitudes using GOME reflectance spectra: multilayered cloud systems

This paper is devoted to the application of the Semi-Analytical Cloud Retrieval Algorithm (SACURA) to the cloud-top height determination using data from the Global Ozone Measurement Experiment (GOME) instrument onboard Earth Remote Sensing satellite (ERS-2). In particular, measurements of the top-of-atmosphere reflectance in the oxygen absorption A-band are used. The technique is based on the asymptotic radiative transfer theory as applied to studies of the oxygen absorption bands in reflected light. Our approach is valid for optically thick clouds with cloud optical thickness larger than approximately 5. The accuracy of the algorithm is checked against independent retrieval techniques for completely cloudy pixels. In particular, the close coincidence with data obtained from the Along Track Scanning Radiometer (ATSR-2) onboard ERS-2 is found. Some results of retrievals using these different instruments disagree (up to 2 km). This is explained by the strong horizontal inhomogeneity of clouds under investigation, which is not accounted by the SACURA or, possibly, by well-known problems of infrared techniques as applied to low-level clouds. The effective cloud geometrical thickness l is also retrieved. This parameter is defined as the geometrical thickness of a vertically homogeneous cloud, which allows for the minimization of differences between observed and calculated top-of-atmosphere reflectance spectra. For inhomogeneous clouds, the value of l differs from a real cloud geometrical thickness, but it gives us an indication of the possible existence of the multilayered cloud system in the field of view of the optical instrument.

Sizing snow grains using backscattered solar light

In this article, we describe a technique to determine dry snow grain size from optical observations. The method is based on analysis of the snow reflectance in the near-infrared region, in particular, the Medium Resolution Imaging Spectrometer (MERIS) band at 865 nm, which is common to many spaceborne optical sensors, is used. In addition, the algorithm is applied to the Moderate Resolution Imaging Spectroradiometer (MODIS) 1240 nm band. It is found that bands located at 1020 and 1240 nm are the most suitable for snow grain size remote-sensing applications. The developed method is validated using MODIS observations over flat snow deposited on a lake ice in Hokkaido, Japan.

The reflection function of optically thick weakly absorbing turbid layers: a simple approximation

The paper is devoted to the derivation of correction terms for the asymptotic formulae of the radiative transfer theory, which allow to increase their accuracy for smaller optical thicknesses (typically, down to the optical thickness equal to 5). Most of the examples are limited to the case of water clouds. However, the main results are easily generalized on other types of disperse media.

Physical interpretation and accuracy of the Kubelka–Munk theory

Kubelka–Munk theory is used extensively in a number of industrial applications including colour matching. The main aim of this paper is the derivation of alternative analytical equations, which are more accurate than correspondent Kubelka–Munk formulae for light scattering materials of a finite optical depth. The derived equations are based on asymptotic radiative transfer theory valid for optically thick turbid media with arbitrary phase functions and single scattering albedos.