KEYWORDS: Clouds, LIDAR, Signal detection, Diffusion, Mass attenuation coefficient, Signal attenuation, Geometrical optics, Multiple scattering, Optical filters, Data analysis
The Wide-Angle Imaging Lidar (WAIL), a new instrument that measures cloud optical and geometrical properties by means of off-beam lidar returns, was deployed as part of a multi-instrument campaign to probe a cloud field at ARM (Atmospheric Radiation Measurement) Southern Great Plain (SGP) site on March 25, 2002. WAIL is designed to determine physical and geometrical characteristics using the off-beam component of the lidar return that can be adequately modeled within the diffusion approximation. Using WAIL data, we estimate the extinction coefficient and geometrical thickness of a dense cloud layer; from there, we infer optical thickness. Results from the new methodology agree well with counterparts obtained from other instruments located permanently at the SGP ARM site and from the WAIL-like airborne instrument that flew over the site during our observation period.
In this paper the asymptotic law for the radial distribution of the radiance density from a point isotropic source placed in a slab of a homogeneous absorbing and scattering medium has been obtained. The final formulae have been derived for both the stationary and time-dependent problems. The obtained law has been verified by comparison wiht Monte Carlo simulations.
The method and computation algorithm have been developed to assess the effect of scattered light on a signal received by a sun photometer while a cloud optical thickness is measured. The approach provides estimations of impact of scattered light for any receiver field of view with allowance for angular dispersion of sunlight. The accuracy of measurements has been investigated as a function of zenith angle of the Sun, receiver field of view, wavelength, cloud optical thickness. The effect of aerosol scattering is an undercloud layer on the accuracy of cloud optical thickness measurement has also been considered.
A lot of different approaches to the problem of radiation propagation in scattering media is known, but a choice of an appropriate one to solve a specific problem is more art than science. There is no exact analytical solution to the stationary radiation transfer problem up to now. So, numerous approximate approaches should be used, each of them is reliable within its own range of characteristic parameters of the range. When the nonstationary problem is taken into consideration, the mathematical difficulties increase and, hence, a number of reliable approaches decreases.
A multicomponent approach to calculate a light field structure with allowance for multiple scattering in the media such as clouds, mists and ocean water is given. All characteristic properties of the real phase function are regarded and propagation in a scattering medium of any optical thickness with an arbitrary single scattering albedo can be considered. The phase function is represented as a sum of more simple functions. The radiance is given as a sum of appropriate components. The equations like the radiative transfer equation are obtained for each component. They can be solved using the known methods within domains where they work best of all. Comparison of various solutions (lidar returns, temporal structure of light pulse transmitted by cloud, an angular structure of the light reflected from and transmitted by cloud) with different numerical calculations shows a fairly satisfactory accuracy of our analytical formulas.
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