GACP Projects
A. GACP Accomplishment Report
Name: Kuo-Nan Liou
Institution: University of California, Los Angeles
Title: Radiative Transfer and Remote Sensing in Aerosol Atmosphere
Goals:
To support the GACP team by providing the scientific information of the aerosol nonsphericity and thin cirrus effects on the retrieval of aerosol optical depth (and particle size) from AVHRR channels.
Objectives:
(1) To investigate the potential effect of aerosol nonsphericity on the radiance distribution for AVHRR 0.63 and 0.86 um channels and its impact on the retrieval of aerosol optical depth.
(2) To develop an efficient light scattering program based on the finite- difference time domain technique for nonspherical and inhomogeneous aerosols.
(3) To explore the sensitivity of thin cirrus (optical depth less than 0.1) on the retrieval of aerosol optical depth and particle size.
Abstract:
We propose to undertake research on light scattering and absorption by aerosols with nonspherical shapes and internal inclusions, commonly occurring in the atmosphere, to obtain the fundamental data essential for a reliable understanding of aerosol radiative forcings and for correct applications to satellite remote sensing of aerosol properties. We plan to use the finite-difference time domain method, an exact method in numerical sense, developed by our research group to perform this task. Spectral aerosol radiative forcing studies will also be carried out using a line-by-line equivalent solar radiative model that includes absorption, multiple scattering, polarization, and potential emission by all pertinent gases and particulates including aerosols, water droplets, and ice crystals in realistic atmospheric conditions. In view of the fact that cirrus clouds are ubiquitous, particularly in the tropics, reliable aerosol retrievals must account for cirrus effects. This will also enhance the retrieval domain for aerosols in time and space using past and present (AVHRR) and future (MODIS) satellite data. From this perspective and in the spirit of collaborating with pertinent researchers working in aerosols retrieval areas, we wish to contribute to a scientific team in the specific area of reconstruction of clear (aerosol) radiances in cirrus cloudy atmospheres. Our proposed programs begin with the detection of cirrus using AVHRR and MODIS channels and then present the IR technique that has been developed by our group for retrieval of the cirrus optical depth, ice crystal size, and temperature (height) for the purpose of removing the cirrus effect employing AVHRR channels. We further discuss a proposed solar technique for applications to MODIS channels for the removal of the cirrus effect in aerosol atmospheres including validations.
Approach, Tasks Completed, and Results (2nd year):(1) We have improved the finite-difference time-domain (FDTD) program for the solution of light scattering by small nonspherical particles by properly evaluating the dielectric constant for the grid cells and by employing the perfectly matched layer absorbing boundary condition and coding in FORTRAN90. We applied the improved FDTD code to the study of light scattering by nonspherical aerosols, assuming convex and concave shapes to resemble dust and some irregular aerosols. We carried out calculations of phase function and linear polarization and compared the results with those computed from the T matrix for spheroids and the Lorenz-Mie theory for spheres. Substantial differences in the phase functions are shown in the backscattering directions.
(2) We have performed calculations for the AVHRR 0.63 and 0.86 um bidirectional reflectances based on the phase functions for three aerosol types: dust (10-face convex), spheroid, and sphere, using the adding/doubling radiative transfer program over the ocean. The mean size of the aerosols was assumed to be about 1 um and the aerosol optical depths range from 0.03 to 0.5. Five different scattering angles from 120 to 180 degrees associated with the sun-sensor geometry were used in the analysis. Uncertainties in the retrieved aerosol optical depth resulting from the assumed spherical and spheroidal shapes, as compared to convex dust, are on the order of 100% and 40%, respectively. These sensitivity analyses demonstrate that it is critical to incorporate the effect of aerosol nonsphericity in radiative transfer calculations and remote sensing algorithm developments, particularly for the retrieval of optical depths and particle sizes involving Asian and Saharan dust events.
(3) We have constructed look-up tables from light scattering and radiative transfer programs involving coupled aerosol-cirrus layers for a thin cirrus case observed by MAS over the Pacific during SUCCESS. The oceanic aerosol model was assumed for these calculations. Employing adjacent cloud-free and cirrus scenes, we were able to differentiate the cirrus signal from the upwelling cloud-based reflectance that includes the aerosol contribution. The cirrus optical depth and mean effective ice crystal size can be retrieved based on the scheme developed by Rolland et al.(2000). These two parameters can then be used to remove the cirrus signal on a pixel-by-pixel basis. Consequently, the aerosol optical depth can be inferred from the reconstructed radiances.
B. Future Plans:
Our future plan includes the continuation of the works described in items (2) and (3) above. We plan to complete a paper in these areas for submission to the Special Issue for the Aerosol Climatology Project in the Journal of the Atmospheric Sciences.
C. GACP Bibliography:
Publications acknowledging the partial support of NAG5-7738
(1) Ou., S.C., K.N. Liou, M.D. King, and S.C. Tsay, 1999: Remote sensing of cirrus cloud parameters based on a 0.63-3.7 um radiance correlation technique applied to AVHRR data. Geophys. Res. Lett., 26, 2437-2440.
(2) Rolland, P., K.N. Liou, M.D. King, S.C. Tsay, and G.M. McFarquhar, 1999: Remote sensing of optical and microphysical properties of cirrus clouds using MODIS channels: methodology and sensitivity to assumptions. J. Geophys. Res., 105, 11,721-11,738.
(3) Yang, P., K.N. Liou, M.I. Mishchenko, and B.C. Gao, 2000: An efficient finite difference time domain scheme for light scattering by dielectric particles: Application to aerosols. App. Opt., 39, 3727-3737.
(4) Liou, K.N., P. Rolland, Q.Cai, P. Yang, and M. Mishchenko, 1999:Satellite remote sensing of aerosol optical depth: nonspherical effect. Paper presented at the AGU Fall meeting in San Francisco, CA, December 13, 1999.
