Aerosol Workshop — June 2-3, 1997
Session 6: Potential of Satellites for Future Aerosol Data
(Facilitator: Bill Rossow; Recorder: Kuo-Nan Liou)
Quantitative Aerosol Data from MODIS
Yoram Kaufman, Goddard Space Flight Center
MODIS will retrieve optical thickness and aerosol size information at 10 km resolution daily globally. The size information will be detailed over oceans. Data will be gridded on one degree by one degree resolution. The mass concentrations and fluxes can be retrieved even better.
We anticipate that in the future it also will be possible to derive the single scatter albedo over land and to detect dust in the MODIS infrared channels.
Validation against aircraft data over land show errors in τ of ±0.05 ±25%. A higher accuracy is expected over ocean: ±0.03 ±15%.
Quantitative Aerosol Data from MISR
Ralph Kahn, Jet Propulsion Laboratory
Climate models now require information about the distribution of aerosol amount and type, globally, on weekly to monthly time scales — coverage that can only be achieved with satellite instruments. However, the current operational satellite remote sensing product relies on single-angle, monospectral data, and provides only column aerosol optical depth over ocean for an assumed aerosol type.
The Multi-angle Imaging SpectroRadiometer (MISR) is scheduled for launch in June 1998 aboard the EOS AM-1 platform. MISR will simultaneously measure the upwelling radiance in 4 spectral bands (443, 550, 670, and 865 nm), at each of 9 emission angles spread out in the forward and aft directions along the flight path between ±70.5 degrees. Global coverage will be acquired about once in 9 days at the equator; the nominal mission lifetime is 6 years.
With these data, we plan to retrieve aerosol optical depth and aerosol "type," which represents a combination of index of refraction, size distribution, and shape constraints, globally, at 17.6 km spatial resolution. According to theoretical simulations, we will retrieve column aerosol optical depth over calm ocean surfaces to an accuracy of at least 0.05 or 10%, whichever is larger, for natural ranges of aerosol type and amount, even if the particle type is not well known. In addition, three to four distinct aerosol size groups between 0.1 and 2.0 microns effective radius can be identified at most latitudes. And we can distinguish spherical from non-spherical particles under similar conditions, according to these studies (Kahn et al., J. Geophys. Res., in press 1997). Our sensitivity to particle composition is currently under investigation. The "at-launch" MISR algorithm will also retrieve aerosol optical depth over heterogeneous land, and surfaces covered with dense dark vegetation.
Quantitative Aerosol Data from EOSP
Michael Mishchenko, NASA Goddard Institute for Space Studies
Most current and proposed satellite remote sensing of tropospheric aerosols relies upon radiance measurements that are interpreted using algorithms that determine best fits to precalculated scattered sunlight for one or more "standard" aerosol models. However, the number of different aerosol types and their typical space and time variations can pose a severe uniqueness problem even for the multiple constraints provided by multispectral radiances of a scene at a number of observation zenith angles. We show that, in contrast, algorithms utilizing high-accuracy polarization measurements are much more sensitive to aerosol microphysics, are less dependent on the availability and use of a priori information, and can provide a physically based retrieval of aerosol characteristics (optical thickness, refractive index, and size) with accuracy needed for long-term monitoring of global climate forcings and feedbacks.
The confirmation and quantification of the Twomey effect on a global basis requires accurate satellite retrievals of CCN concentrations. Our sensitivity study shows that the AVHRR retrieval algorithm based on single-channel single-viewing-angle radiance measurements is incapable of accurately determining CCN concentrations and that an algorithm based on multiangle radiance measurements provides much better retrievals. However, even for the latter algorithm the errors in the retrieved CCN concentration can be as large as a factor of several. The poor performance of single-channel radiance-only algorithms is explained by the strong dependence of the extinction cross section and weak dependence of the phase function on aerosol effective radius. In contrast, high-precision multiangle polarization measurements are capable of constraining CCN concentrations to within a few tens of percent.
Quantitative Aerosol Data from Satellite Lidar
M. Patrick McCormick, Hampton University
A spaceborne lidar is capable of yielding tropospheric (and stratospheric) aerosol information on a global scale, with high vertical resolution (tens of meters) and a small footprint (tens to hundreds of meters). These vertical profile data greatly complement retrievals by many passive sensors (e.g., MODIS and MISR) and can be obtained even for low aerosol concentrations, thin cloud layers, and over bright surfaces. Using lidar data alone, retrievals of aerosol optical depth (AOD) are accurate to no better than 30%. For example, at an optical depth of 0.01, the lidar retrieval error is 0.003. Lidars meet the requirements set forth by a recent NRC panel convened to examine the issue of aerosol forcing for AODs less than 0.05, but not for greater values where the required uncertainty is 0.03. In order to improve the retrieval errors at the higher optical depths (values representative of many boundary layer depths), complementary information is required. Currently, lidar and a high spectral resolution oxygen A/band spectrometer seem to be the best combination for providing the required AOD information with significantly higher accuracy, over both land and water, than any other technique. For example, optical depths down to 0.02 can be retrieved with an uncertainty of about 0.005. In addition, it is recommended that future spaceborne lidars fly in a tailored orbit to optimize coincident sampling with instruments on other spacecraft such as MODIS, MISR and CERES.
Quantitative Aerosol Data from Polder
Didier Tanre, Laboratoire d'Optique Atmospherique
The POLDER (POLarization and Directionality of the Earth Reflectance) instrument is a camera composed of a two-dimensional CCD (charge coupled device) detector array with a wide FOV (field of view), telecentric optics, and a rotation wheel carrying spectral and polarized filters. POLDER was launched in August 1996 aboard the Japanese ADEOS platform and operated until the end of June 1997.
POLDER allows spectral, directional and polarization measurements with daily global coverage. One of its scientific objectives is retrieval of aerosol properties, specifically the aerosol spectal optical thickness, size distribution and refractive index.
Over ocean, the algorithm uses both the spectral dependence derived from the two near infrared channels (670 and 865 nm) and the angular information. The polarization measurements are then used for getting an estimate of the refractive index. Over land, the retrieval scheme uses the polarized channels in the three bands (443, 670 and 865 nm) over the 14 observation angles. Preliminary results show good comparisons with ground-based measurements. The expected accuracy of the optical thickness is about 0.05; accumulation and coarse model particles can be separated; and three classes of refractive indices, 1.33, 1.40 and 1.50, can be discriminated.