GACP Projects
Form 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:
- 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.
- 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:
Numerous in-situ observations have illustrated that aerosols in the atmosphere
exhibit a wide variety of shapes spanning from quasi-spherical to highly
irregular. Many aerosols, such as sands, dust-like particles, soots, particles
from biomass burning, etc.,can be extremely irregular with rough surfaces.
The scattering of light by spherical particles can be solved by the exact Lorenz-Mie theory. Exact numerical programs such as the T-matrix method can also be used to compute the scattering and absorption properties of nonspherical particles with rotational symmetry such as spheroids (Mishchenko et al.1996a,b).To what extent the shape irregularity of aerosols may have on the scattering phase function patterns and the consequence of bi- directional reflectances associated with satellite remote sensing is a fundamental question that must be addressed. It is in this context that we have proposed a numerical program based on the finite-difference time domain(FDTD)technique for the solution of light scattering by aerosol particles.
The FDTD technique has been shown to be an efficient computational method for solving the scattering of light by irregular and inhomogeneous particles (Yang and Liou, 1999). For application to aerosols, we have modified the FDTD method that we have developed for convex polyhedronic particles. We use the concept of random growth to define the particle shape. An ellipsoid on which four points are randomly chosen is first selected as the apexes of the first pyramid in which the four facets are triangles. Subsequently, additional points on the ellipsoid surface can be selected randomly. Ten and 40 facets particles are illustrated in Fig. 1, along with the phase function patterns.For comparison, results for spheres and spheroids are also shown. Results of two cases involving moderate and essentially no absorption are presented for a size parameter of 10. Differences in terms of the ratios with respect to spheres and spheroids are shown in Fig. 2. Substantial differences are noted in the backscattering directions for the nonabsorption case. Because of the relatively smoother surfaces involving the 40-facet particle, smaller differences are displayed in this case.
Form C. Future Plans:
- Using the FDTD program developed for convex polyhedronic particles, we plan to carry out light scattering calculations for a number of aerosol types, shapes, and sizes using two AVHRR wavelengths: 0.63 and 0.86 um.
- The single-scattering properties for spherical, spheroidal, and irregular aerosols will be incorporated into an adding/doubling radiative transfer program to study the effects of aerosol shape and size on the bidirectional reflectances for the aforementioned two AVHRR channels. We plan to use aerosol optical depths of 0.03 and 0.3 over both land and ocean in this investigation. A number of AVHRR viewing and solar zenith angles will be used in the sensitivity computations.
- Further, we plan to add a thin cirrus cloud layer with an optical depth less than 0.1 to the aerosol atmosphere to explore the information content of thin cirrus and aerosols from the simulated bidirectional reflectance patterns.
Form D. GACP Bibliography: Publications acknowledging the partial support of NAG5-7738
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. (Accepted and in press).
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. (Submitted).
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