Yong X. Hu

Scattering and absorption property database for nonspherical ice particles in the near- through far-infrared spectral region

The single-scattering properties of ice particles in the near- through far-infrared spectral region are computed from a composite method that is based on a combination of the finite-difference time-domain technique, the T-matrix method, an improved geometrical-optics method, and Lorenz-Mie theory. Seven nonspherical ice crystal habits (aggregates, hexagonal solid and hollow columns, hexagonal plates, bullet rosettes, spheroids, and droxtals) are considered. A database of the single-scattering properties for each of these ice particles has been developed at 49 wavelengths between 3 and 100 microm and for particle sizes ranging from 2 to 10,000 microm specified in terms of the particle maximum dimension. The spectral variations of the single-scattering properties are discussed, as well as their dependence on the particle maximum dimension and effective particle size. The comparisons show that the assumption of spherical ice particles in the near-IR through far-IR region is generally not optimal for radiative transfer computation. Furthermore, a parameterization of the bulk optical properties is developed for mid-latitude cirrus clouds based on a set of 21 particle size distributions obtained from various field campaigns.

Single-scattering properties of droxtals

Abstract Small ice crystals have been found to occur in high concentrations in polar stratospheric clouds and the upper portion of cirrus clouds, where temperatures are extremely low (often less than −50°C). The scattering properties of these small crystals are important to space-borne remote sensing, especially for the retrieval of cirrus properties using visible and near-infrared channels. Previous research has shown that the commonly used spherical and “quasi-spherical” approximations for these ice crystals can lead to significant errors in light scattering and radiative transfer calculations. We suggest that droxtals more accurately represent the shape of these small ice crystals. The single-scattering properties of ice droxtals have been computed at visible and infrared wavelengths using the finite-difference time domain method for size parameters smaller than 20. Further study of the optical properties of larger droxtals (size parameter greater than 20) will be carried out using an improved geometric optics method.

Sensitivity of cirrus bidirectional reflectance to vertical inhomogeneity of ice crystal habits and size distributions for two Moderate‐Resolution Imaging Spectroradiometer (MODIS) bands

A common assumption in satellite imager-based cirrus retrieval algorithms is that the radiative properties of a cirrus cloud may be represented by those associated with a specific ice crystal shape (or habit) and a single particle size distribution. However, observations of cirrus clouds have shown that the shapes and sizes of ice crystals may vary substantially with height within the clouds. In this study we investigate the sensitivity of the top-of-atmosphere bidirectional reflectances for two Moderate-Resolution Imaging Spectroradiometer (MODIS) bands centered at 0.65 μm and 2.11 μm to cirrus models composed of either a single homogeneous layer or three distinct, but contiguous, layers. First, we define the single- and three-layer cirrus cloud models with respect to ice crystal habit and size distributions on the basis of in situ replicator data acquired during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE-II), held in Kansas during the fall of 1991. Subsequently, fundamental light-scattering and radiative transfer theory is employed to determine the single-scattering and the bulk radiative properties of the cirrus cloud. For radiative transfer computations we present a discrete form of the adding/doubling principle that is computationally straightforward and efficient. For the 0.65 μm band, at which absorption by ice is negligible, there is little difference between the bidirectional reflectances calculated for the one- and three-layer cirrus models. This result suggests that the vertical inhomogeneity effect is relatively unimportant at 0.65 μm. At 2.11 μm the bidirectional reflectances computed for both optically thin (τ = 1) and thick (τ = 10) cirrus clouds show significant differences between the results for the one- and three-layer models. The reflectances computed for the three-layer cirrus model are substantially larger than those computed for the single-layer cirrus. Furthermore, our analysis shows that the cirrus reflectances at both the 0.65 and 2.11 μm bands are very sensitive to the optical properties of the small crystals that predominate in the top layer of the three-layer cirrus model. It is critical to define the most realistic geometric shape for the small “quasi-spherical” ice crystals in the top layer for obtaining reliable single-scattering parameters and bulk radiative properties of cirrus.

Spectral signature of ice clouds in the far-infrared region: Single-scattering calculations and radiative sensitivity study

(extinction efficiency, absorption efficiency, and the asymmetry factor of the scattering phase function) are calculated for small particles using circular cylinders and for large crystals using hexagonal columns. The scattering properties are computed for particle sizes over a size range from 1 to 10,000 mm in maximum dimension from a combination of the T-matrix method, the Lorenz-Mie theory, and an improved geometric optics method. Bulk scattering properties are derived subsequently for 30 particle size distributions, with effective particle sizes ranging from 15 to 150 mm, obtained from various field campaigns for midlatitude and tropical cirrus clouds. Furthermore, a parameterization of the bulk scattering properties is developed. The radiative properties of ice clouds and the clear-sky optical thickness computed from the line-by-line method are input to a radiative transfer model to simulate the upwelling spectral radiance in the far-IR spectral region at the research aircraft height (20 km). On the basis of the simulations, we investigate the sensitivity of far-IR spectra to ice cloud optical thickness and effective particle size. The brightness temperature difference (BTD) between 250 and 559.5 cm � 1 is shown to be sensitive to optical thickness for optically thin clouds (visible optical thickness t 8), the BTD between 250 and 410.2 cm � 1 is shown to be sensitive to the effective particle size up to a limit of 100 mm. INDEX TERMS: 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 0649 Electromagnetics: Optics; KEYWORDS: far-infrared, cirrus cloud, ice crystal

High Cloud Properties from Three Years of MODIS Terra and Aqua Collection-4 Data over the Tropics

Abstract This study surveys the optical and microphysical properties of high (ice) clouds over the Tropics (30°S–30°N) over a 3-yr period from September 2002 through August 2005. The analyses are based on the gridded level-3 cloud products derived from the measurements acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard both the NASA Earth Observing System Terra and Aqua platforms. The present analysis is based on the MODIS collection-4 data products. The cloud products provide daily, weekly, and monthly mean cloud fraction, cloud optical thickness, cloud effective radius, cloud-top temperature, cloud-top pressure, and cloud effective emissivity, which is defined as the product of cloud emittance and cloud fraction. This study is focused on high-level ice clouds. The MODIS-derived high clouds are classified as cirriform and deep convective clouds using the International Satellite Cloud Climatology Project (ISCCP) classification scheme. Cirriform clouds make up more tha...

Sensitivity of the backscattering Mueller matrix to particle shape and thermodynamic phase

The Mueller matrix (M) corresponding to the phase matrix in the backscattering region (scattering angles ranging from 175 degrees to 180 degrees) is investigated for light scattering at a 0.532-microm wavelength by hexagonal ice crystals, ice spheres, and water droplets. For hexagonal ice crystals we assume three aspect ratios (plates, compact columns, and columns). It is shown that the contour patterns of the backscattering Mueller matrix elements other than M11, M44, M14, and M41 depend on particle geometry; M22 and M33 are particularly sensitive to the aspect ratio of ice crystals. The Mueller matrix for spherical ice particles is different from those for nonspherical ice particles. In addition to discriminating between spherical and nonspherical particles, the Mueller matrix may offer some insight as to cloud thermodynamic phase. The contour patterns for large ice spheres with an effective size of 100 microm are substantially different from those associated with small water droplets with an effective size of 4 microm.

Effect of Cavities on the Optical Properties of Bullet Rosettes: Implications for Active and Passive Remote Sensing of Ice Cloud Properties

Bullet rosette particles are common in ice clouds, and the bullets may often be hollow. Here the singlescattering properties of randomly oriented hollow bullet rosette ice particles are investigated. A bullet, which is an individual branch of a rosette, is defined as a hexagonal column attached to a hexagonal pyramidal tip. For this study, a hollow structure is included at the end of the columnar part of each bullet branch and the shape of the hollow structure is defined as a hexagonal pyramid. A hollow bullet rosette may have between 2 and 12 branches. An improved geometric optics method is used to solve for the scattering of light in the particle. The primary optical effect of incorporating a hollow end in each of the bullets is to decrease the magnitude of backscattering. In terms of the angular distribution of scattered energy, the hollow bullets increase the scattering phase function values within the forward scattering angle region from 1° to 20° but decrease the phase function values at side- and backscattering angles of 60°–180°. As a result, the presence of hollow bullets tends to increase the asymmetry factor. In addition to the scattering phase function, the other elements of the phase matrix are also discussed. The backscattering depolarization ratios for hollow and solid bullet rosettes are found to be very different. This may have an implication for active remote sensing of ice clouds, such as from polarimetric lidar measurements. In a comparison of solid and hollow bullet rosettes, the effect of the differences on the retrieval of both the ice cloud effective particle size and optical thickness is also discussed. It is found that the presence of hollow bullet rosettes acts to decrease the inferred effective particle size and to increase the optical thickness in comparison with the use of solid bullet rosettes.

Use of circular cylinders as surrogates for hexagonal pristine ice crystals in scattering calculations at infrared wavelengths

We investigate the errors associated with the use of circular cylinders as surrogates for hexagonal columns in computing the optical properties of pristine ice crystals at infrared (8-12-microm) wavelengths. The equivalent circular cylinders are specified in terms of volume (V), projected area (A), and volume-to-area ratio that are equal to those of the hexagonal columns. We use the T-matrix method to compute the optical properties of the equivalent circular cylinders. We apply the finite-difference time-domain method to compute the optical properties of hexagonal ice columns smaller than 40 microm. For hexagonal columns larger than 40 microm we employ an improved geometric optics method and a stretched scattering potential technique developed in previous studies to calculate the phase function and the extinction (or absorption) efficiency, respectively. The differences between the results for circular cylinders and hexagonal columns are of the order of a few percent. Thus it is quite reasonable to use a circular cylinder geometry as a surrogate for pristine hexagonal ice columns for scattering calculations at infrared (8-12-microm) wavelengths. Although the pristine ice crystals can be approximated as circular cylinders in scattering calculations at infrared wavelengths, it is shown that optical properties of individual aggregates cannot be well approximated by those of individual finite columns or cylinders.

The spectral signature of mixed-phase clouds composed of non-spherical ice crystals and spherical liquid droplets in the terrestrial window region

Abstract An outstanding problem facing the cloud modeling and remote sensing community is to improve satellite-derived cloud microphysical and macrophysical properties when a single cloud layer exists within a temperature range for which a combination of water and ice particles may be present. This is typically known as a “mixed-phase” cloud condition, and is prevalent when the cloud-top temperature lies between −40°C and 0°C. In this paper we report on a sensitivity study of the spectral signature of mixed-phase clouds in the infrared terrestrial window region (8– 13 μm ). Mixed clouds are assumed to be a vertically uniform cloud layer composed of a mixture of pristine hexagonal ice crystals and spherical water droplets. Unlike the conventional approach that derives the bulk scattering properties of mixed-phase clouds by a linear weighting of the contributions of ice and water components, the bulk single-scattering properties of mixed-phase clouds are formulated on the basis of fundamental physics. With the aid of a line-by-line radiative transfer model and a discrete ordinates radiative transfer (DISORT) computational program, we investigate the high-resolution spectral signature, expressed in terms of brightness temperature, of mixed-phase clouds with various effective sizes, ice fraction ratios, and optical thicknesses. Small particles are found to have a significant impact on the infrared spectral signature of mixed-phase clouds when the size discrepancy between the ice and water particles is large. Furthermore, the simulation results show that the infrared radiative spectrum associated with cirrus clouds can be quite different from their counterparts for cirrus clouds even if a small amount of water droplets exist in the mixed-phase cloud layer.

Enhanced lidar backscattering by quasi-horizontally oriented ice crystal plates in cirrus clouds

The backscattering of light by quasi-horizontally oriented hexagonal ice plates is investigated because of its pertinence to lidar measurements of cirrus clouds. For oriented ice crystals, the commonly used geometric optics ray-tracing method is not applicable to the computation of the scattered field in certain scattering directions, in particular, the backscattering direction, because of the singularity problem inherent to the ray-tracing technique. In this study, we solve for the electric field due to scattering by quasi-horizontally oriented ice plates using an approach based on the electromagnetic wave theory. We simplify the analysis by ignoring the effect of the plates side faces on the internal field inside the particle. This is a reasonable approximation when the ratio of the particle diameter to its thickness is large. This approximation is also valid if the tilt of the particles symmetric axis from zenith is small and the size parameter is large. The present numerical results indicate that very strong oscillations in the backscattering cross section occur with the variation of size parameter. Furthermore, the bulk backscattering intensity has been calculated by including the effect of a particle size distribution and the random tilt of particle symmetric axis within a small angular region, for example, 2°. A strong dependence is found between the lidar backscattering cross section and the degree of the random tilt of the particles. A combination of the present method with the T-matrix method, which works well for particles having small and moderate size parameters, may provide a more complete picture of the lidar backscattering by quasi-horizontally oriented ice plates.