A32B-01
Using the multiangle polarimetric measuring capabilities of the 2008 NASA/Glory mission to characterize non-spherical particles
The remote sensing capabilities of the Aerosol Polarimetry Sensor (APS), scheduled for launch into the A-train in 2008 onboard the NASA/Glory Mission, offers an opportunity to retrieve aerosol properties such as particle refractive index, size, shape, and number density with unprecedented accuracies from space. It accomplishes this by combining the multiangle remote sensing capabilities of the MISR instrument, the multispectral remote sensing capabilities of the MODIS instrument, and the polarimetric remote sensing capabilities of the POLDER instrument. This talk provides an introduction to the objectives and measurements of the NASA/Glory mission, discusses the sensitivity of such measurements to the properties of non-spherical particles, and provides actual cases of non-spherical salt-like particle retrievals from measurements obtained during the CLAMS and MILAGRO field campaigns by an airborne prototype of the APS instrument. http://glory.giss.nasa.gov
A32B-02
Effect of The Surface Texture Of Ice Particles On The Retrieval Of Cirrus Properties
Surface roughness of ice crystals is a morphological parameter important to the scattering characteristics of these particles. The intent of this study is to quantify the effect of surface roughness on the retrieval of the optical and microphysical properties of cirrus clouds from satellite observations. We present two ray-tracing algorithms, a rigorous algorithm and an approximate algorithm with a simplified treatment of surface roughness, for calculating the single-scattering properties of randomly oriented hexagonal ice crystals with size parameters in the geometric optics regime. It is shown that the simplified scheme can account for most the effects of surface roughness on particle single-scattering properties without incurring substantial demand on computational resources, and thus provides an efficient way to compute the single-scattering properties of roughened particles. The simplified ray-tracing technique is employed to compute the single-scattering properties of hexagonal columns at wavelengths 0.86 and 2.13 ?m. Look-up tables built for the correlation between the bi-directional reflectances at wavelengths 0.86 and 2.13 ?m with different roughness conditions are used to retrieve ice cloud optical thickness and effective particle size over oceans. In general, the dominant effect of surface roughness on cloud property retrievals is to decrease the retrieved optical thickness and to increase the retrieved effective particle size in comparison with their counterparts in the case of smooth ice particles.
A32B-03
Resolving discrepancies between observational and theoretical representations of the asymmetry parameter of atmospheric ice crystals
The asymmetry parameter of ice crystals is a scattering property of primary interest to studies of the role of clouds in the earth's climate. Substantial effort has been devoted to ascertaining its value using both airborne sensors and theoretical calculations, However, observations and theory do not agree, even while the latter are employed almost exclusively in climate modeling and remote sensing applications. In general, observations indicate smaller values of the asymmetry parameter than theory, and a much weaker dependence on cloud particle size. In this review, it is argued that the discrepancy can be reconciled if it is considered that the scattering properties of atmospheric ice crystals are dominated by sub-crystal scale morphological features. The most likely candidate is ice crystal surface roughness.
A32B-04
Snow Optical Properties for Different Particle Shapes with Application to Snow Grain Size Retrieval and Simulation of MODIS/CERES Radiances Over the Antarctic Plateau
We investigated the single scattering optical properties of snow for different particle shapes and roughness. These optical properties were implemented and tested in a coupled atmosphere-snow radiative transfer model. The modeled surface spectral albedo and radiance distribution were compared with surface measurements. The results show that the usual equivalent sphere assumption rather significantly overestimates the forward reflected radiances and underestimates the backscattering radiances around the principal plane. On average, the aggregate shape assumption has the best agreement with the measured radiances. Optical properties with the aggregate assumption were used to retrieve snow grain size over the Antarctic plateau. The retrieved grain sizes showed similar and large seasonal variations in all years, and smaller interannual variation; our results for 2000-2005 are consistent with in situ measurements reported in the literature. Using the retrieved snow grain sizes based on MODIS channels 1 and 6, the modeled radiances agreed well with measurements for other MODIS channels and for broadband CERES. Except for the MODIS 2.13um channel, the mean relative model-observation differences are only few percent. The modeled MODIS radiances using measured surface reflectance at Dome C (Hudson et al.) also showed good agreement in visible channels, where radiation is not sensitive to snow grain size and the measured surface bidirectional reflectance is generally representative of the Antarctic plateau. But modeled radiances using measured reflectance in near infrared presented large errors because of the high sensitivity to the snow grain size, which varies spatially and temporally. The broadband shortwave radiance is moderately sensitive to the snow grain size, comparable to the MODIS 0.86um channel.
A32B-05
Evaluating Snow Surface BRDF Models with MISR
Most surface-viewing satellites-to-date have only provided single, near-nadir views of the earth's surface. This has limited our ability to test our understanding of how the earth scatters radiation, because we are solving an underconstrained problem. For example, one can simply match a radiative transfer simulation to the observed radiance by adjusting the albedo of its surface. That type of match does not make the model correct, because a single-view measurement from a satellite is inadequate to describe how that surface scatters radiation in different directions. This is evident through polar imagery from the Multi-angle Imaging SpectroRadiometer (MISR), which offers nine, near-simultaneous views of the same scene with view zenith angles ranging from ±70°. In the nadir camera from MISR, clouds, smooth glaciers, and rough snow may all appear similar. However, the off-nadir cameras of MISR all show significant differences between those three features (Di Girolamo and Wilson, 2003); a testimony that each of these features have different angular scattering properties. Therefore, the MISR observations can provide additional constraints in evaluating radiative transfer calculations. We use MISR to provide constraints on radiative transfer calculations of TOA radiances under clear sky polar conditions. We use MODTRAN4 for our radiative transfer simulation, which we have modified to accept external surface BRDF values. A simple Lambertian model, the Hapke (1981) model, and the Mishchenko (1999) model are all evaluated. We will demonstrate how a Lambertian surface is inadequate to explain angular scattering in ice and snow. We will also show from the MISR observations that surface roughness causes the phase function to shift, as desribed by Hudson et. al, 2006. This shift causes the Hapke and Mishchenko surface models to be mostly appropriate for smooth ice and snow.
A32B-06
Community Radiative Transfer Model for Satellite Radiance Simulation
The Community Radiative Transfer Model (CRTM) [Weng et al., 2005], developed at U.S. Joint Center for Satellite Data Assimilation (JCSDA), has been used for the satellite radiance simulation and the radiance derivatives to the surface/atmospheric parameters in the physical retrieval [Boukabara et al., 2007], data assimilation [Le Marshall et al., 2006] and many others [Han et al., 2006; Liu and Weng, 2006]. CRTM has been become a key component in U.S. data assimilation at the National Center for Environmental Prediction (NCEP) [Okamoto and. Derber, 2006]. It is a core engine for NOAA/NESDIS Microwave Integrated Retrieval System (MIRS) [Boukabara et al., 2007]. The CRTM has also been implemented into Weather Research Forecasting (WRF) model. The CRTM is known as modular program development [van Delst et al., 2006], which breaks down the radiative transfer model into components, each of which is encapsulated in one or several program modules and can be developed independently of the others. The key components of the CRTM are the advanced surface emissivity and reflectivity models [van Delst and Wu, 2000; English 1999; Weng et al. 2001] including a polarimetric surface emissivity model [Liu and Weng, 2003], the fast Optical Path Transmittance (OPTRAN) model [Xiong et al., 2006], the cloud absorption/scattering look-up tables [Yang et al., 2000], and the advanced radiative solver [Liu and Weng, 2006]. The CRTM can also compute aerosol radiance. The CRTM can deal with Zeeman splitting effect, the energy received in the channels for the stratosphere and mesosphere depends strongly on the geomagnetic field and its orientation with respect to the direction of observation [Han et al., 2007]. We will also present the applications of the CRTM in hurricane detection and forecasting, in the determination of stratospheric temperature, a key contributing factor to photochemical ozone depletion, and in reanalysis and climate studies.
A32B-07
The sensitivity of 3D cloud-aerosol radiative interaction to spatial variability of clouds and aerosol
Conversion of satellite-observed reflectances to the aerosol optical depth (AOD) in the vicinity of clouds is highly conjectural due to large three-dimensional (3D) cloud-induced enhancement. A few recent studies based on satellite-derived cloud optical depth have provided the first important demonstration of the spectral dependence of this enhancement and its sensitivity to the surrounding clouds. However, these studies have assumed the vertical variability of clouds and uniform distribution of aerosol. Real aerosols exhibit large spatial variability, which is often strongly correlated with the 3D cloud variability. The present work further evaluates the spectral dependence of the cloud-induced enhancement on the vertical/horizontal gradients in aerosol and cloud properties and its effect on the accuracy of the AOD retrieval in cloud-free pixels. We perform such evaluation using more realistic 3D fields of cumulus clouds and aerosol produced by Large Eddy Simulation model and Monte Carlo radiative calculations.
A32B-08
Modeling Optical Properties of Mineral Dust over "The Great Indian Desert"
The Thar desert, sometimes also described as ‘The Great Indian Desert', lying in the Northwest part of India with an area of 0.32*106 km2, is known to be the source of natural mineral dust . The mineral dust particles are mostly non-spherical having sharp edges, which show different scattering signature compared to that of equivalent spheres while interacting with the radiation. Furthermore accurate mineralogical information, that governs their refractive indices, is essential for scattering calculations. The radiative impacts of dust particles therefore depend on their morphology and mineralogy. Most of the present satellites consider the particle to be spherical while retrieving their optical properties. Some newly launched spacecraft instruments such as MISR accounts for non spherical nature by including spheroid particles in its retrieval algorithm . Clearly there exits a need for improvement in dust model used in retrieval algorithm to account for their sharp edges together with their index of refraction based on the latest chemical composition at the sensing wavelengths. To the best of our knowledge no such attempt has been made to calculate the optical properties of dust particles over the Thar desert. In this study, the optical properties of mineral dust of the Thar desert has been modeled using T-matrix method with realistic dust shapes based on Scanning Electron Microscope images of the dust over the desert with particle size ranging from 0.1-1.0 ìm at wavelengths spanning from ultraviolet to near infrared (0.38-1.2ìm). Representative dust particles shapes considered are sphere, cylinder, spheroids and chebyshev together with realistic mineral dust composition. Mineralogical analysis of airborne dust over Northwest India has revealed the presence of only basic non-metallic minerals such as Quartz, Feldspar, Mica and Calcite, which posses negligible imaginary part of refractive index at considered wavelength domain, however, the subsequent dust sampling over Rajasthan desert has disclosed significant iron content , which causes sufficient absorption of solar radiation in the considered wavelength domain. This iron occurs in the form of Hematite (Fe2O3) as metallic mineral with varying contribution in the desert dust. The effective refractive index of composite mineral dust accounting for hematite (Fe2O3) has been calculated using Bruggman's effective medium mixing rule. Since no exact volume percentage of Hematite in Thar dust particles is known till date therefore optical properties of mineral dust over the Thar desert have been modeled by varying the metallic mineral. The results pertaining to optical properties such as scattering and absorption cross-sections, phase functions together with asymmetry parameter will be discussed. http://home.iitk.ac.in/~snt