GACP Projects
Form A: GACP ACCOMPLISHMENT REPORT
Name: Larry L. StoweInstitution: NOAA/NESDIS Office of Research and Applications
TITLE: Remote Sensing of Aerosol over Land with AVHRR
ABSTRACT:
The objectives of this proposal are to:
1) develop an automated
algorithm for the
retrieval of aerosol optical thickness, AOT, over land where an aerosol signal
is present in
AVHRR cloud-free reflectance data;
2) validate this algorithm using regional
spectral AOT
datasets from NASA's AERONET surface sun-photometer/sky-radiometers;
3) explore
ways of
extending to other grid cells and times away from AERONET; and
4) evaluate the
performance
of the algorithm when implemented into a global AVHRR reprocessing system.
With the successful execution of reprocessing of AVHRR data back to 1981 by the AVHRR Pathfinder Atmosphere (PATMOS) project at NOAA/NESDIS, cloud-free radiance statistics exist for each day in a global 110 km grid for all five channels of AVHRR. Ocean grid cell data have been processed using the NOAA operational aerosol retrieval algorithm to create the most extensive record of AOT ever compiled. These data are currently in use by over 60 national and international scientific teams, and will be a baseline upon which future satellite measurements will be compared. However, as theoretical climate model studies indicate, the most significant concentrations and radiative effects of aerosols occur over land.
We propose to develop an AOT retrieval algorithm over land, comparable to the one over oceans, where and when an aerosol signal is present in the 0.63 micron reflectance channel of AVHRR. In a remote sensing approach analogous to the one successfully developed over oceans by NESDIS, observed radiances will be associated with an aerosol optical thickness which yields that radiance in a radiative transfer look-up table (LUT). Results will be validated against surface sun-photometer ground truth sites chosen from AERONET sites distributed over most continents to measure aerosol properties, including spectral AOT. Land surface reflectance characteristics will be derived empirically by forcing the regression line from AERONET comparisons through zero. The aerosol model will be adjusted to force the regression line from AERONET intercomparisons to have a slope of 1.0. Methods will be explored to extend this algorithm to other periods and locations without AERONET data, using climatologies of the surface and aerosol characteristics, and possibly using visibility measurements for validation. These modifications will be incorporated into the radiative transfer model used to produce LUTs, for each region and time period with measurable aerosol reflectance signals. A final set of LUTs and selection criteria will be developed for use by an AVHRR reprocessing center to help in the construction of a global aerosol climatology back to 1981.
GOALS:
To use AVHRR data to characterize changes in the Earth's radiation budget, cloud cover and atmospheric aerosol content since 1981, and determine how they are related to each other, and perhaps to global warming. Currently have the ability to do this over oceans, but not over land, which is why this particular GACP project is being done.
OBJECTIVES:
The objectives of the proposed research are to: 1) develop an automated algorithm for the retrieval of aerosol optical thickness, AOT, over land where an aerosol signal is present in AVHRR cloud-free reflectance data; 2) validate this algorithm using regional spectral AOT datasets from NASA's AERONET surface sun-photometer/sky-radiometers; 3) explore ways of extending to other grid cells and times away from AERONET; and 4) evaluate the performance of the algorithm when implemented into the GACP global AVHRR reprocessing system.
APPROACH:
Develop an AOT retrieval algorithm over land, comparable to the one we have developed over oceans, where and when an aerosol signal is present in the 0.63 micron reflectance channel of AVHRR. Using a remote sensing approach analogous to the one we have successfully used over oceans, observed radiances will be associated with an aerosol optical thickness which yields that radiance in a theoretically modeled radiative transfer look-up table (LUT). Results will be validated against surface sun-photometer ground truth sites chosen from AERONET sites distributed over most continents to measure aerosol properties, including spectral AOT. Land surface reflectance characteristics will be derived empirically by forcing the regression line from AERONET comparisons through zero, and then identified with similar land surface types using land classification databases. The aerosol model will be adjusted to force the regression line for each AERONET intercomparison site and season to have a slope of 1.0. Methods will be explored to extend these AERONET site specific algorithms to other periods and locations without AERONET data, using climatologies of the surface and aerosol characteristics, and possibly using visibility measurements for validation. These modifications will be incorporated into the radiative transfer model used to produce LUTs, for each region and time period with measurable aerosol reflectance signals. A final set of LUTs and rules for their use will be developed and delivered to the GACP reprocessing center to help in the construction of a global aerosol climatology back to 1981.
TASKS COMPLETED:
NASA funding was received in May, but the scientist we had been planning to hire with those funds, decided not to join us. We arranged with Colorado State University to advertise this post-doctoral position for us in June. So far ten candidates have applied by the August 1st deadline. It is likely that the selected applicant will not be able to start until around October 1st. As a result of this late start, this is the only progress to report at this time.
FUTURE PLANS:
Our first year of research will empirically determine regions with an aerosol signal and analyze the error of applying the NOAA/NESDIS ocean like algorithm at AERONET locations. The second year will adjust the surface and aerosol properties in a radiative transfer model to minimize the errors detected in the first year. In the third year, ways will be developed to extend the LUTs developed at AERONET sites, to non-AERONET locations and seasons, and the validated retrieval algorithm will be delivered to the NASA/GACP processing center.
RESULTS:
There are no scientific and technical results to report at this time because of the late start on this project.
Form C: FUTURE PLANS
Name: Larry L. Stowe
Institution: NOAA/NESDIS Office of Research and Applications
Brief description of second year's research: Due to the late start of this project, our future plans for the second year are to complete the work which was proposed for the first year. This includes empirically determining land regions with an aerosol signal and analyzing the error of applying the NOAA/NESDIS ocean like algorithm at AERONET locations.
