GACP Projects
Form A: GACP ACCOMPLISHMENT REPORT
Name: Ralph Kahn
Institution: Jet Propulsion Laboratory / California Institute of Technology
Title: MISR Contributions to the GACP
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 Global Aerosol Climatology
Project (GACP) Science Team. As a member of the GACP, 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 GACP 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.
Goals: Our long-term research goals are (1) to provide the best possible aerosol products from MISR, (2) to coordinate with field observers and modelers in producing a next-generation global, monthly aerosol climatology, (3) to monitor the global budgets of different aerosol types using this climatology, and (4) with the help of other remote sensing and in situ observations, to do the best job we can at assessing the radiative implications of changes in global aerosol budgets.
Objectives: I have concentrated on characterizing the sensitivity of MISR to particle optical depth, shape, size distribution, and indices of refraction (composition), using theoretical simulations. In addition, I am working on characterizing MISR's sensitivity to natural mixes of particle types, and on analyzing AirMISR data as a means of testing the theoretical results. Once MISR flies, I will begin by working with the MISR validation team and other field observers to understand the actual performance of the instrument, then adjust the standard retrieval algorithms to make best use of the data. From there we will generate monthly climatologies, study the motions of aerosol air masses to develop an understanding of aerosol budgets on global scales, and work with MISR, MODIS, and CERES data, along with in situ measurements, to assess aerosol radiative effects.
Approach: For the GACP task in particular, I have participated in the GACP activities since its inception. I will continue participating in GACP, primarily building bridges with other members of the aerosol community: (1) contributing the MISR aerosol data to the GACP, in useful formats and with appropriate documentation, (2) enlisting the participation of in situ and field measurement scientists from the GACP community in characterizing the aerosol air masses we identify with MISR, so together we can provide a more complete picture of aerosol radiative effects globally, and (3) bringing back to the MISR team insights from the GACP community that may improve MISR aerosol products and validation.
My larger aerosol program, supported in part by the MISR Science Team, and in part by a separate grant in NASA's climate and Radiation program, includes modeling the MISR radiances under a variety of surface and atmospheric conditions, using the results of global aerosol transport models to obtain climatologically likely mixes of particles, and assessing the sensitivity of MISR to those mixes.
Tasks Completed: The incremental funding provided by GACP supports my participation in GACP meetings and planning activities. I participated actively in setting the scientific agenda for the first GACP Science Team meeting, Nov. 18-20, 1998, and co-chaired Discussion Session A: Strategy for the development and use of algorithms for satellite retrieval of aerosol parameters and development of an aerosol climatology. My notes on this session are part of the official meeting record.
In my larger aerosol program, supported by the MISR Science Team and a grant in the NASA Earth Science Climate and Radiaiton program, we completed our paper on MISR Sensitivity to pure particle properties (JGR 103, 32,195-32,213, 1998), and have 2 more papers near completion, one on MISR Sensitivity to climatological mixtures of pure particles, and a theoretical study of how MISR data, together with results from MODIS and CERES, can help constrain the cloud-free reflected solar radiation flux.
Future Plans: This grant consists of incremental funding to support my participation in the GACP effort. As I did last year, I plan to spend 2 weeks of my time working on GACP in the coming year, including travel to GACP meetings, preparing and giving presentations related to MISR aerosol products, communicating and coordinating with the GACP community, and helping out with the GACP program as needed.
In related tasks, supported in part by the MISR Science Team, and in part by a separate grant in NASA's climate and Radiation program, we will complete our papers on MISR Sensitivity to climatological mixtures of pure particles and on Constraining the cloud-free reflected solar radiation flux. We will then analyze aircraft data from AirMISR over dark water, using both the generic retrieval and the climatological retrieval approaches developed in our sensitivity studies, and will begin analysis of MISR data -- first by assessing instrument performance and the aerosol retrieval results over validation sites.
Results: Our primary results for this period are summarized in our recent publication, Sensitivity of Multiangle Imaging to aerosol optical depth and to pure-particle size distribution and composition over ocean (JGR 103, 32,195-32,213, 1998). Based on these simulations, we show that over calm ocean surfaces, and with commonly observed ranges of particle optical depth and size distribution, MISR can retrieve column optical depth for all but the darkest particles, to an accuracy better than 0.05 or 20%, whichever is larger, even if the microphysical properties of the particles are poorly known. It can distinguish spherical from non-spherical particles having Sahara dust-like composition. At most latitudes, MISR can also identify three to four distinct size groups between 0.1 and 2.0 microns characteristic radius, and two to three compositional groups over the natural range of indices of refraction. These results suggest that MISR will distinguish airmasses containing different particle types, a major improvement over current operational remote sensing aerosol retrievals.
In follow-on work, we identify about 10 climatologically likely mixtures of pure particles, using the published results of global transport models, that MISR should be able to distinguish. We expect these to correspond, at least roughly, to the "air mass types" that MISR will track as they move from source regions to places where the aerosols precipitate from the atmosphere.
Form C: FUTURE PLANS
Name: Ralph Kahn
Institution: Jet Propulsion Laboratory / California Institute of Technology
This grant consists of incremental funding to support my participation in the GACP effort. As I did last year, I plan to spend 2 weeks of my time working on GACP in the coming year, including travel to GACP meetings, preparing and giving presentations related to MISR aerosol products, communicating and coordinating with the GACP community, and helping out with the GACP program as needed.
In related tasks, supported in part by the MISR Science Team, and in part by a separate grant in NASA's climate and Radiation program, we will complete our papers on MISR Sensitivity to climatological mixtures of pure particles and on Constraining the cloud-free reflected solar radiation flux. We will then analyze aircraft data from AirMISR over dark water, using both the generic retrieval and the climatological retrieval approaches developed in our sensitivity studies, and will begin analysis of MISR data -- first by assessing instrument performance and the aerosol retrieval results over validation sites.
Form D: GACP BIBLIOGRAPHY
Name: Ralph Kahn
Institution: Jet Propulsion Laboratory / California Institute of Technology
Bibliography:
a. List of publications (including books, book chapters, and refereed papers).
- Kahn, R., R. West, E. Clothiaux, W. Lucht, N. A. Strahler, Strugnell,C. Schaff, and L. Thomason, Constraints on the Cloud-Free Reflected Solar Radiaiton Flux from combined multiangle, multispectral, and broadband flux remote sensing data, JGR, in preparation, 1999.
- Kahn, R., P. Banerjee, and D. McDonald, Sensitivity of multiangle imaging to natural mixes of aerosols over ocean, JGR, in preparation, 1999.
- R. Kahn, and A. Braverman, What shall we do with the data we are expecting from upcoming Earth observation satellites?, J. Comput. & Gaph. Statictics, in press (1999).
- Kahn, R., P. Banerjee, D. McDonald, and D. Diner, "Sensitivity of Multiangle imaging to Aerosol Optical Depth, and to Pure-Particle Size Distribution and Composition Over Ocean ", J. Geophys. Res., 103, 32,195-32,213, 1998.
- Martonchik, J.V., D.J. Diner, R. Kahn, M.M. Verstraete, B. Pinty, H.R. Gordon, and T.P. Ackerman, Techniques for the Retrieval of aerosol properties over land ocean using multiangle data, IEEE Trans. Geosci. Remt. Sensing 36, 1212-1227, 1998.
- Kahn, R., R. West, D. McDonald, B. Rheingans, and M.I. Mishchenko, Sensitivity of multiangle remote sensing observations to aerosol sphericity, J. Geophys. Res. 102, 16861-16870, 1997.
b. List of printed technical reports and non-refereed papers.
- Kahn, R., "Painting a New, Far More Detailed Portrait of Earth," The Los Angeles Times, February 26, 1998.
- Kahn, R., "The Box," Smithsonian Air and Space, 14-15, August/September, 1998.
c. List of oral presentations or posters at professional society meetings and conferences.
- What We Hope to Learn About Global Mineral Dust Aerosols from EOS Multi-angle Imaging SpectroRadiometer (MISR), Workshop on Mineral Dust, Boulder, CO, June 9-11, 1999.
- What We Can Learn about Aerosols from EOS-MISR multi-angle remote sensing observation, IGARSS'99, Hamburg, Germany, June 28-July 2, 1999.
- What We Can Learn about Aerosols from EOS-MISR multi-angle remote sensing observations over ocean, Second International Workshop on Multiangular Measurements and Models, Ispra, Italy, 15-17 September 1999
