Yoshihide Takano

Remote sensing of cirrus cloud optical thickness and effective particle size for the National Polar-orbiting Operational Environmental Satellite System Visible/infrared imager Radiometer Suite: sensitivity to instrument noise and uncertainties in environmental parameters

We describe sensitivity studies on the remote sensing of cirrus cloud optical thickness and effective particle size using the National Polar-orbiting Operational Environmental Satellite System Visible/Infrared Imager Radiometer Suite 0.67-, 1.24-, 1.61-, and 2.25-microm reflectances and thermal IR 3.70- and 10.76-microm radiances. To investigate the accuracy and precision of the solar and IR retrieval methods subject to instrument noise and uncertainties in environmental parameters, we carried out signal-to-noise ratio tests as well as the error budget study, where we used the University of California at Los Angeles line-by-line equivalent radiative transfer model to generate radiance tables for synthetic retrievals. The methodology and results of these error analyses are discussed.

Parry arc: a polarization lidar, ray-tracing, and aircraft case study

Using simple ray-tracing simulations, the cause of the rare Parry arc has been linked historically to horizontally oriented columns that display the peculiar ability to fall with a pair of prism faces closely parallel to the ground. Although we understand the aerodynamic forces that orient the long-column axis in the horizontal plane, which gives rise to the relatively common tangent arcs of the 22 degrees halo, the mechanism leading to the Parry crystal orientation has never been resolved adequately. On 16 November 1998, at the University of Utah Facility for Atmospheric Remote Sensing, we studied a cirrus cloud producing a classic upper Parry arc using polarization lidar and an aircraft with a new high-resolution ice crystal imaging probe. Scanning lidar data, which reveal extremely high linear depolarization ratios delta a few degrees off the zenith direction, are simulated with ray-tracing theory to determine the ice crystal properties that reproduce this previously unknown behavior. It is found that a limited range of thick-plate crystal axis (length-to-diameter) ratios from approximately 0.75 to 0.93 generates a maximum delta approximately 2.0-5.0 for vertically polarized 0.532-microm light when the lidar is tilted 1 degrees -2 degrees off the zenith. Halo simulations based on these crystal properties also generate a Parry arc. However, although such particles are abundant in the in situ data in the height interval indicated by the lidar, one still has to invoke an aerodynamic stabilization force to produce properly oriented particles. Although we speculate on a possible mechanism, further research is needed into this new explanation for the Parry arc.

A Polarized Delta-Four-Stream Approximation for Infrared and Microwave Radiative Transfer: Part I

Abstract The delta-four-stream polarized (vector) thermal radiative transfer has been formulated and numerically tested specifically for application to satellite data assimilation in cloudy atmospheres. It is shown that for thermal emission in the earth’s atmosphere, the [I, Q] component of the Stokes vector can be decoupled from the [U, V] component and that the solution of the vector equation set involving the four-stream approximation can be expressed in an analytic form similar to the scalar case. Thus, the computer time requirement can be optimized for the simulation of forward radiances and their derivatives. Computations have been carried out to illustrate the accuracy and efficiency of this method by comparing radiance and polarization results to those computed from the exact doubling method for radiative transfer for a number of thermal infrared and microwave frequencies. Excellent agreement within 1% is shown for the radiance results for all satellite viewing angles and cloud optical depths. For...

Phase matrix for light scattering by concentrically stratified spheres: comparison of geometric optics and the "exact" theory.

We have developed a hit-and-miss Monte Carlo geometric ray-tracing program to compute the scattering phase matrix for concentrically stratified spheres. Using typical refractive indices for water and aerosols in the calculations, numerous rainbow features appear in the phase matrix that deviate from the results calculated from homogeneous spheres. In the context of geometric ray tracing, rainbows and glory are identified by means of their ray paths, which provide physical explanation for the features produced by the exact Lorenz-Mie theory. The computed results for the phase matrix, the single-scattering albedo, and the asymmetry factor for a size parameter of approximately 600 compared closely with those evaluated from the exact theory.

Laser transmission-backscattering through inhomogeneous cirrus clouds

We have developed a two-dimensional (2D) model for inhomogeneous cirrus clouds in plane-parallel and spherical geometries for the analysis of the transmission and backscattering of high-energy laser beams. The 2D extinction-coefficient and mean effective ice-crystal size fields for cirrus clouds can be determined from a combination of the remote sensing of cirrus clouds by use of the Advanced Very High Resolution Radiometer on board National Oceanic and Atmospheric Administration satellites and the vertical profiling of ice-crystal size distributions available from limited measurements. We demonstrate that satellite remote sensing of the position and the composition of high cirrus can be incorporated directly in the computer model developed for the transmission and backscattering of high-energy laser beams in realistic atmospheres. The results of laser direct transmission, forward scattering, and backscattering are analyzed carefully with respect to aircraft height, cirrus cloud optical depth, and ice-crystal size and orientation. Uncertainty in laser transmission that is due to errors in the retrieved ice-crystal size is negligible. But uncertainty of the order of 2% can be produced if the retrieved optical depth has errors of +/-0.05. With both the aircraft and the target near the cloud top, the direct transmission decreases, owing to the propagation of the laser beam through the curved portion of the cloud top. This effect becomes more pronounced as the horizontal distance between the aircraft and the target increases.

Diurnal effects in the composition of cirrus clouds

[1]xa0Extended (7-y) polarization lidar data, which are sensitive to ice crystal shape and orientation, are used to search for diurnal changes in cirrus clouds caused by solar heating. Average linear depolarization ratios are ∼0.02 lower during the day than night, minimizing at a 45° sun angle at ∼0.05 lower than the 0.350 nighttime average. Analysis of the diurnal pattern using scattering simulations suggests that plate crystals are more common during daylight, and that a feedback occurs between solar heating and the maintenance of horizontal plate orientations. This anisotropy also affects the cirrus solar albedo, representing an additional complication confronting climate modelers.

Retrieval of Cirrus Cloud Properties From the Atmospheric Infrared Sounder: The K-Coefficient Approach Using Cloud-Cleared Radiances as Input

We have developed a k-coefficient retrieval approach for Atmospheric Infrared Sounder (AIRS) observations, using AIRS cloud-cleared radiances (ACCRs) as input. This new approach takes advantage of the available ACCR, reduces computational expense, offers an efficient and accurate cirrus cloud retrieval alternative for hyperspectral infrared (IR) observations, and is potentially applicable to the compilation of a long-term cirrus cloud climatology from hyperspectral IR observations. The retrieval combines a lookup-table method coupled to a residual minimization scheme using observed cloudy and cloud-cleared AIRS radiances as input. Six AIRS channels between 766 and 832 cm-1 with minimal water vapor absorption/emission have been selected, and their spectral radiances have been demonstrated to be sensitive to both cirrus cloud optical depth (τc) and ice crystal effective particle size (De). The capability of the k-coefficient approach is demonstrated by comparison with a more accurate retrieval program, which combines the delta-four stream (D4S) approximation with the currently operational Stand-alone AIRS Radiative Transfer Algorithm (SARTA). The distribution patterns and the range of retrieved cloud parameters from the k-coefficient approach are nearly identical to those from SARTA+D4S retrievals, with minor differences traced to uncertainties in parameterized cloudy radiances in the k-coefficient approach and in the ACCR. The k -coefficient approach has also been applied to four AIRS granules over North Central China, Mongolia, and Siberia containing a significant presence of cirrus clouds, and its results are quantitatively compared to simultaneous Moderate Resolution Imaging Spectroradiometer/Aqua cirrus cloud retrievals. Finally, AIRS retrieved τc and De are consistent with the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat derived values for semitransparent cirrus clouds, with more significant differences in thicker cirrus and multilayer clouds.

An analog light scattering experiment of hexagonal icelike particles. Part II: Experimental and theoretical results

Abstract The scattering properties of hexagonal icelike crystals as measured in the analog manner by the experimental apparatus described in Part I are presented. The crystals are made out of sodium fluoride (NaF), which has an index of refraction similar to that of water ice. The experimentally determined light intensities scattered from fixed and integrated random orientations of a NaF hexagonal crystal, oriented to produce a two-dimensional scattering profile, compares favorably to the expectations derived from geometric ray tracing methods. Also, the three-dimensional scattering properties of a simulated NaF Parry column, a NaF crystal aggregate, and a NaF plate with a rough surface are compared to results computed from the geometric ray tracing approach. From these comparisons the authors conclude that within the experimental measurement uncertainties and to the degree in which the NaF crystal models approach the geometric and optical ideal, the geometric ray tracing approach is an excellent method t...

Experimental and theoretical spectral reflection properties of ice clouds generated in a laboratory chamber

In a preliminary experimental program, the measured bidirectional reflection properties between 1.0 and 3.5 mum from a grating spectrometer with a resolution of approximately 0.1 mum for ice crystal clouds generated in a cold chamber are compared with theoretical results computed from a line-by-line equivalent solar radiative transfer model. The theoretical calculations are based on the measured habits, concentrations, and sizes of the ice particles from replicas of the ice crystals that show a mean maximum size of approximately 7 mum. The experimental design was first tested with transmission measurements in a pure water-vapor environment that compare closely with theoretical expectations. Within the uncertainties and in consideration of the assumptions necessitated by the preliminary nature of this program, there is a close comparison between the experimental and theoretical results.

Comparison of the UCLA-LBLE radiative transfer model and MODTRAN for accuracy assessment of the NPOESS-VIIRS cloud optical property algorithms

To support the verification and implementation of the Visible/Infrared Imaging/Radiometric Suites algorithms used for inferring cloud environmental data records, an inter-comparison effor has been carried out to assess the consistency between the simulated cloudy radiances/relectances from the University of California at Los Angeles line-by-line equivalent radiative transfer model (UCLA-LBLE RTM) and those from the Moderate-Resolution Transmission Model (MODTRAN) with the 16-stream Discrete Ordinate Radiative Transfer Model (DISORT) incorporated. For typical ice and water cloud optical depths and particle sizes, we find discrepancies in the visible and near-infrared reflectances from the two models, presumably due to the difference in phase function (non-spherical vs. Henyey-Greenstein), different numbers of phase function expansion terms (16-term vs. 200-term), and different treatment of forward peak truncation in each model. Using MODTRAN4, we also find substantial differences in the infrared radiances for optically thick clouds. These differences led to the discovery by MODTRAN4 developers of an inconsistency in the MODTRAN4/DISORT interface. MODTRAN4 developers corrected the inconsistency, which provided dramatic reductions in the differences between the two radiative transfer models. The comparison not only impacts the prospective test plan for the VIIRS cloud algorithms, but also leads to improvements in future MODTRAN releases.