Showing papers by "Kuo-Nan Liou published in 2004"
TL;DR: In this article, a multispectral scanning spectrometer was used to obtain measurements of the bidirectional reflectance and brightness temperature of clouds, sea ice, snow, and tundra surfaces at 50 discrete wavelengths between 0.47 and 14.0 mm.
Abstract: A multispectral scanning spectrometer was used to obtain measurements of the bidirectional reflectance and brightness temperature of clouds, sea ice, snow, and tundra surfaces at 50 discrete wavelengths between 0.47 and 14.0 mm. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) Arctic Clouds Experiment, conducted over a 1600 km 3 500 km region of the north slope of Alaska and surrounding Beaufort and Chukchi Seas between 18 May and 6 June 1998. Multispectral images in eight distinct bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) were used to derive a confidence in clear sky (or alternatively the probability of cloud) over five different ecosystems. Based on the results of individual tests run as part of this cloud mask, an algorithm was developed to estimate the phase of the clouds (liquid water, ice, or undetermined phase). Finally, the cloud optical thickness and effective radius were derived for both water and ice clouds that were detected during one flight line on 4 June. This analysis shows that the cloud mask developed for operational use on MODIS, and tested using MAS data in Alaska, is quite capable of distinguishing clouds from bright sea ice surfaces during daytime conditions in the high Arctic. Results of individual tests, however, make it difficult to distinguish ice clouds over snow and sea ice surfaces, so additional tests were added to enhance the confidence in the thermodynamic phase of clouds over the Chukchi Sea. The cloud optical thickness and effective radius retrievals used three distinct bands of the MAS, with a recently developed 1.62- and 2.13-mm-band algorithm being used quite successfully over snow and sea ice surfaces. These results are contrasted with a MODIS-based algorithm that relies on spectral reflectance at 0.87 and 2.13 mm.
186 citations
TL;DR: Two FDTD schemes for light scattering by dielectric particles to overcome the above-mentioned discontinuity on the basis of the electromagnetic boundary conditions for the two Cartesian grid configurations are presented.
Abstract: Two grid configurations can be employed to implement the finite-difference time-domain (FDTD) technique in a Cartesian system. One configuration defines the electric and magnetic field components at the cell edges and cell-face centers, respectively, whereas the other reverses these definitions. These two grid configurations differ in terms of implication on the electromagnetic boundary conditions if the scatterer in the FDTD computation is a dielectric particle. The permittivity has an abrupt transition at the cell interface if the dielectric properties of two adjacent cells are not identical. Similarly, the discontinuity of permittivity is also observed at the edges of neighboring cells that are different in terms of their dielectric constants. We present two FDTD schemes for light scattering by dielectric particles to overcome the above-mentioned discontinuity on the basis of the electromagnetic boundary conditions for the two Cartesian grid configurations. We also present an empirical approach to accelerate the convergence of the discrete Fourier transform to obtain the field values in the frequency domain. As a new application of the FDTD method, we investigate the scattering properties of multibranched bullet-rosette ice crystals at both visible and thermal infrared wavelengths.
23 citations
TL;DR: In this paper, a new methodology was developed to simultaneously retrieve the optical depths of aerosols and thin cirrus clouds over the oceans by using the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator data.
Abstract: [1] We have developed a new methodology to simultaneously retrieve the optical depths of aerosols and thin cirrus clouds over the oceans by using the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) data. This methodology takes advantage of the fact that the visible (or a near-infrared window) reflectance from cirrus can be characterized by its correlation to the reflectance from a near-infrared band located in a strong water vapor absorption region. Once the reflectance due to cirrus has been removed, the residual reflectance in these bands is used to retrieve aerosol optical depth. Cirrus cloud properties are subsequently obtained by minimizing the difference between observed and calculated reflectance values stored in look-up tables that are constructed a priori and correlated to the retrieved aerosol optical depth. Examination of the data gathered from the recent Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE) and Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) field campaigns demonstrates that our retrievals are consistent with both in-situ and ground based measurements.
22 citations
15 Sep 2004
TL;DR: In this article, 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.
Abstract: 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.
4 citations
14 Oct 2004
TL;DR: In this paper, a vector line-by-line equivalent radiative transfer program including the full Stokes parameters based on the adding method was developed, which employs the steepest descent method in the form of a series of numerical iteration procedures to search for the simulated polarization parameters that best match the measured polarization parameters.
Abstract: This paper presents a conceptual approach toward the remote sensing of cirrus cloud particle size and optical depth using the degree of polarization and polarized reflectance associated with the first three Stokes parameters I , Q , and U for the 0.865 and 2.25 μm wavelengths. A vector line-by-line equivalent radiative transfer program including the full Stokes parameters based on the adding method was developed. The retrieval algorithm employs the steepest descent method in the form of a series of numerical iteration procedures to search for the simulated polarization parameters that best match the measured polarization parameters. Sensitivity studies were performed to investigate the behavior of phase matrix elements as functions of scattering angles for three ice crystal size-shape combinations. Overall, each phase matrix element shows some sensitivity toward ice crystal shape, size, and suface roughness due to the various optical effects. Synthetic retrievals reveal that the retrieval algorithm itself is highly accurate, while polarimetric and radiometric error sources cause very small retrieval errors. Finally, an illustrative example of applying the retrieval algorithm to airborne POLDER data during EUCREX is presented.
2 citations
14 Oct 2004
TL;DR: In this article, a database of the optical properties of nonspherical ice crystals at the infrared wavelengths was developed and a fast infrared transfer model under cirrus cloudy condition, which is applied to the retrieval of ice clouds from satellite-based infrared measurements.
Abstract: Ice clouds have been identified as one of the most uncertain components in atmospheric research. In recent years, the atmospheric radiative transfer and remote sensing community has made a concerted effort to improve the characterization of cirrus clouds. A number of airborne and balloon-borne observations have demonstrated that cirrus clouds are essentially composed of nonspherical ice crystals with various habits (or shapes) and sizes. In this paper, we report on some recent progresses towards the computation of the single-scattering properties nonspherical ice crystals and the relevant applications to remote sensing and radiative transfer simulations. Specifically, we have developed a database of the optical properties of ice crystals at the infrared wavelengths. In conjunction with the application of the scattering database, we also developed a fast infrared transfer model under cirrus cloudy condition, which is applied to the retrieval of ice clouds from satellite-based infrared measurements.
1 citations