Peng-Wang Zhai

A vector radiative transfer model for coupled atmosphere and ocean systems based on successive order of scattering method

A vector radiative transfer model has been developed for coupled atmosphere and ocean systems based on the Successive Order of Scattering (SOS) Method. The emphasis of this study is to make the model easy-to-use and computationally efficient. This model provides the full Stokes vector at arbitrary locations which can be conveniently specified by users. The model is capable of tracking and labeling different sources of the photons that are measured, e.g. water leaving radiances and reflected sky lights. This model also has the capability to separate florescence from multi-scattered sunlight. The delta - fit technique has been adopted to reduce computational time associated with the strongly forward-peaked scattering phase matrices. The exponential - linear approximation has been used to reduce the number of discretized vertical layers while maintaining the accuracy. This model is developed to serve the remote sensing community in harvesting physical parameters from multi-platform, multi-sensor measurements that target different components of the atmosphere-oceanic system.

Invisibility cloaks for irregular particles using coordinate transformations

Invisibility cloaks for ellipsoids, rounded cuboids and rounded cylinders have been studied on the basis of the coordinate transformation approach. The resultant material property tensors for irregular cloaks are more complicated in comparison with those for the spherical invisibility cloak. A generalized Discrete Dipole Approximation (DDA) formalism has been used to simulate the scattered field distribution in the vicinity of the aforementioned irregular cloaks illuminated by an incident plane wave. Simulated scattering efficiencies are on the order of 10(-5), and the simulated electric-field distribution outside of a cloak is the same as that of the incident radiation.

Electric and magnetic energy density distributions inside and outside dielectric particles illuminated by a plane electromagnetic wave

We have studied the distribution of the electric and magnetic energy densities within and in the vicinity outside a dielectric particle illuminated by a plane electromagnetic wave. Numerical simulations were performed by using the Lorenz-Mie theory and the finite-difference time-domain method for spheres and spheroids, respectively. We found that the electric and magnetic energy densities are locally different within the scatterers. The knowledge of the two components of the electromagnetic energy density is essential to the study of the dipole (electric or magnetic) transitions that have potential applications to Raman and fluorescence spectroscopy.

Cirrus optical depth and lidar ratio retrieval from combined CALIPSO‐CloudSat observations using ocean surface echo

Ocean surface observations from the CloudSat radar and the spaceborne lidar aboard the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) platform are combined in the Synergized Optical Depth of Aerosol (SODA) algorithm and used to retrieve the optical depth of semitransparent single-layered cirrus clouds. In the operational CALIPSO data analysis, lidar-derived optical depths are typically estimated using a correction factor for multiple scattering effects and a single global mean lidar ratio. By combining the SODA approach with observations from the CALIPSO Imaging Infrared Radiometer, accurate values for both of these parameters can be derived directly from the measurements. Application of this approach yields a multiple scattering factor of 0.61 ± 0.15 sr, which is essentially identical to the value used operationally. However, the standard lidar ratio used in the CALIPSO daytime operational analysis is found to be biased low by around 25%. As a consequence, the lidar-derived optical depths retrieved from the daytime operational analyses are more than 30% smaller than those derived using SODA. The lidar ratio for semitransparent cirrus is found to be rather stable over ocean (33 ± 5 sr) with slight variations as a function of temperature and latitude. The geographic distribution shows a moderate decrease of average lidar ratio values over Indonesia during daytime, which may be attributed to a larger occurrence of high-altitude cirrus layers in this convectively active area.

Impulse response solution to the three-dimensional vector radiative transfer equation in atmosphere-ocean systems. I. Monte Carlo method

We have developed a powerful 3D Monte Carlo code, as part of the Radiance in a Dynamic Ocean (RaDyO) project, which can compute the complete effective Mueller matrix at any detector position in a completely inhomogeneous turbid medium, in particular, a coupled atmosphere-ocean system. The light source can be either passive or active. If the light source is a beam of light, the effective Mueller matrix can be viewed as the complete impulse response Green matrix for the turbid medium. The impulse response Green matrix gives us an insightful way to see how each region of a turbid medium affects every other region. The present code is validated with the multicomponent approach for a plane-parallel system and the spherical harmonic discrete ordinate method for the 3D scalar radiative transfer system. Furthermore, the impulse response relation for a box-type cloud model is studied. This 3D Monte Carlo code will be used to generate impulse response Green matrices for the atmosphere and ocean, which act as inputs to a hybrid matrix operator-Monte Carlo method. The hybrid matrix operator-Monte Carlo method will be presented in part II of this paper.

Integrating cavities: temporal response

The temporal response of an integrating cavity is examined and compared with the results of a Monte Carlo analysis. An important parameter in the temporal response is the average distance d between successive reflections at the cavity wall; d was calculated for several specific cavity designs--spherical shell, cube, right circular cylinder, irregular tetrahedron, and prism; however, only the calculation for the spherical shell and the right circular cylinder will be presented. A completely general formulation of d for arbitrary cavity shapes is then derived, d =4V/S where V is the volume of the cavity, and S is the surface area of the cavity. Finally, we consider an arbitrary cavity shape for which each flat face is tangent to a single inscribed sphere of diameter D (a curved surface is considered to be an infinite number of flat surfaces). We will prove that for such a cavity d =2D/3, exactly the same as d for the inscribed sphere.

Zero-backscatter cloak for aspherical particles using a generalized DDA formalism.

The Discrete Dipole Approximation (DDA) formalism has been generalized to materials with permeabilities mu; not equal 1 to study the scattering properties of impedance-matched aspherical particles and cloaked spheres. We have shown analytically that any impedance-matched particle with a four-fold rotational symmetry with respect to the direction of the incident radiation has the feature of zero backscatter. Moreover, an impedance-matched coat with the aforementioned symmetry property acting on an irregular dielectric particle with the same symmetry property can substantially reduce the backscatter. This leads to a substantial reduction of the signals from an object being detected by a monostatic radar/lidar system. The DDA simulation also provides accurate information about electric field distributions in the vicinity of a cloaked sphere.

Implementing the near- to far-field transformation in the finite-difference time-domain method

When the finite-difference time-domain (FDTD) method is applied to light-scattering computations, the far fields can be obtained by means of integrating the near fields either over the volume bounded by the particles surface or on a regular surface encompassing the scatterer. For light scattering by a sphere, the accurate near-field components on the FDTD-staggered meshes can be computed from the rigorous Lorenz-Mie theory. We investigate the errors associated with these near- to far-field transform methods for a canonical scattering problem associated with spheres. For a scatterer with a small refractive index, the surface-integral approach is more accurate than its volume counterpart for computation of the phase functions and extinction efficiencies; however, the volume-integral approach is more accurate for computation of other scattering matrix elements, such as P12, P32, and P43, especially for backscattering. If a large refractive index is involved, the results computed from the volume-integration method become less accurate, whereas the surface method still retains the same order of accuracy as in the situation for the small refractive index.

Equivalent path lengths in an integrating cavity: comment

The equivalent absorption path length in an integrating cavity is examined. In an otherwise excellent paper, Tranchart et al. [Appl. Opt. 35, 7070 (1996)] made an important error in obtaining the expressions for the equivalent path length in an integrating cavity. This error has been propagated through several other publications in the literature. Since the equivalent path length is the sine qua non for obtaining an accurate absorption coefficient when using an integrating cavity, it is our intent here to give the correct formulas to prevent further errors when extracting absorption coefficients.

Application of the symplectic finite-difference time-domain method to light scattering by small particles

A three-dimensional fourth-order finite-difference time-domain (FDTD) program with a symplectic integrator scheme has been developed to solve the problem of light scattering by small particles. The symplectic scheme is nondissipative and requires no more storage than the conventional second-order FDTD scheme. The total-field and scattered-field technique is generalized to provide the incident wave source conditions in the symplectic FDTD (SFDTD) scheme. The perfectly matched layer absorbing boundary condition is employed to truncate the computational domain. Numerical examples demonstrate that the fourth-order SFDTD scheme substantially improves the precision of the near-field calculation. The major shortcoming of the fourth-order SFDTD scheme is that it requires more computer CPU time than a conventional second-order FDTD scheme if the same grid size is used. Thus, to make the SFDTD method efficient for practical applications, one needs to parallelize the corresponding computational code.