GACP Projects
Contributing MISR Global Multiangle Aerosol Results to the Aerosol Radiative Forcing Initiative
Ralph Kahn
Abstract: I am the aerosol scientist on the EOS-MISR (Earth Observing System - Multiangle Imaging SpectroRadiometer) instrument, and have requested incremental funding to participate in the Aerosol Radiative Forcing Science Team (ARFST). As a member of the ARFST, I offer both to contribute what we learn about aerosols from MISR to the overall effort, and to coordinate the continuing MISR aerosol studies with those of the larger aerosol community.
MISR will measure the upwelling visible radiance from Earth in 4 spectral bands centered at 446, 558, 672, and 866 nm, at each of 9 emission angles spread out in the forward and aft directions along the flight path at ±70.5 deg, ±60.0 deg, ±45.6 deg, ±26.1 deg, and nadir. The spatial sampling rate is 275 meters in the cross-track direction at all angles. Over a period of 7 minutes, a 360 km wide swath of Earth comes into the view of the cameras at each of the 9 emission angles, providing a wide range of scattering angle coverage for each surface location. The data will be used to characterize aerosol properties, surface albedo and bi-directional reflectance, and cloud properties. Global coverage will be acquired about once in 9 days at the equator; the nominal mission lifetime is 6 years. MISR is scheduled for launch into a 10:30 AM, sun-synchronous polar orbit in 1999.
Multiangle, multi-spectral remote sensing observations, such as those anticipated from MISR, provide a type of information about the characteristics of aerosols rarely obtained from satellites. The MISR team has developed algorithms to produce global aerosol products at 17.6 km spatial resolution. Over ocean, we will retrieve optical depth and aerosol mixture "type," which represents a combination of index of refraction, size distribution, and shape constraints. We also anticipate retrieving optical depth, and possibly some information about particle properties, over dense, dark vegetation and over heterogeneous land with the multiangle data.
Using theoretical simulations, we have assessed the sensitivity of the algorithm to characteristics of pure particles having a wide range of sizes, shapes, and compositions, over ocean (Kahn et al., 1997; 1998). As best we can tell prior to launch, we can retrieve column optical depth from measurements over calm ocean, for all but the darkest particles, with typical size distributions and compositions, to an accuracy better than 0.05 or 20%, whichever is larger, even if the particle properties are poorly known. MISR should be able to distinguish spherical from non-spherical particles, to separate 2 to 4 compositional groups according to indices of refraction, and to separate 2 to 4 compositional groups according to indices of refraction, and to identify 3 to 4 distinct size groups between 0.1 and 2.0 microns characteristic radius at most latitudes.
Based on these results, we expect to distinguish air masses containing different aerosol mixes, routinely and globally over ocean, with MISR data. We anticipate contributing information about the global distribution of air masses by aerosol type to global climatologies, and plan to collaborate with field measurement teams that can provide the detailed microphysical properties of aerosols in the air masses we identify and track.
Work continues on the MISR aerosol algorithm, the sensitivity studies, and an assessment of the conditions under which MISR aerosol data can make the biggest contribution to our knowledge of the cloud-free, reflected solar radiation flux. This work is funded in part by the MISR science budget, and in part by a research grant in the NASA Code Y Climate and Radiation Program. The ARFST offers an opportunity to place the MISR results into a larger context, and to participate in a synthesis of complementary results from satellite, field, and modeling efforts from an active community of researchers.
