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Description of Naval Postgraduate School’s Algorithm of Aerosol Optical Depth (NPS AOD algorithm) |
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General Description:
The NPS aerosol optical depth (AOD) algorithm processes AOD image products from satellite visible radiance data sensed by the Polar Orbiting Operational Environmental Satellite (POES) and the Geosynchronous Operational Environmental Satellite (GOES) data. Documentation and performance evaluations about the algorithm can be found in Kuciauskas, 2002, Durkee, et al., 1999, Brown, 1997, and Durkee et al., 1991. The AOD products are centered within the visible wavelength, approximately 0.65 microns. The AOD products allow the user to view general variations of AOD over maritime, cloud- and sun glint-free conditions within a time window of either several hours or several days. The algorithm initially processes AOD from POES AVHRR data. For GOES AOD calculations, information about aerosol size distributions is passed from NOAA-AVHRR processing. The AOD products only occur during daylight hours when the solar elevation angle is at least 10 degrees above the horizon. For POES processing, the satellite pass must also cover at least 50% of the region.
Theory: For the NPS AOD algorithm, Brown (1997) and Durkee et al. (1991) derived a simplified form of the radiative transfer equation as:
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Within the equation, the subscript “a” refers to aerosol related quantities. La represents the satellite-detected upwelling radiance only from contributions of atmospheric aerosol particles. To eliminate radiances from non-aerosol sources, the following procedures were conducted. Radiances due to sun glint contamination (Cox and Munk, 1954), Rayleigh scatter (Turner, 1973), ozone absorption and surface reflected radiance are eliminated. Cloud detection is also incorporated into the algorithm. For the other variables in the equation, the single scatter albedo (wo) is a measure of the ratio of radiance scattered versus radiance absorbed. For marine aerosols sensed by visible wavelengths, the particles (salt, sulfate) are weakly absorbing and therefore, the single scatter albedo (wo) is assumed to be one. The incoming solar radiance (Fo) is obtained from the satellite radiometer’s characteristics (Kidwell, 1995), and the satellite zenith angle (m) is obtained by the satellite-earth geometry. The single scatter phase function phase function p(Ys) determines which direction the radiation is scattered when it encounters an aerosol particle, and finally, the satellite-detected aerosol optical depth (da) is expressed as the sum of the atmospheric extinction integrated vertically from the surface through the atmosphere. All but the scattering phase function variables are readily available.
Obtaining the scattering phase function values requires knowledge of the aerosol characteristics and size distribution and is parameterized. Brown (1997) and Durkee (1991) developed the parameterization technique using visible channel data (channels 1 and 2) from the POES AVHRR. The technique consists of calculating the ratio of the POES channel 1 and 2 radiances, ‘S12’. S12 will be larger for smaller size particle distributions and smaller for larger size aerosol particle distributions. Since S12 varies from pixel to pixel, variations in the aerosol size distribution can be detected within the pixel resolutions of the satellite image data. The S12 parameter is then input into a two-term Henyey-Greenstein scattering phase function that is described in Durkee (1991). Finally, the phase function values are parameterized into 7 models of aerosol size distributions that were developed by Brown (1997) to typify the maritime aerosol conditions over open water.
AVHRR visible raw data is calibrated according to the results by Rao and Chen (1995). GOES visible raw data was calibrated by applying a vicarious calibration technique (Rao and Zhang, 1999 and Rao, et al., 1999) as well as a correction factor (Kuciauskas, 2002) to the calibrated data.
References:
Brown, B. B., 1997: Remote measurement of aerosol optical properties using the NOAA POES AVHRR and GOES Imager during TARFOX. M.S. Thesis, Naval Postgraduate School, Monterey, CA, 73 pp.
Cox, C. and W. Munk, 1954: Measurement of the roughness of the sea
surface from photographs of the sun’s glitter. Journal of the optical Society of America., 44, 838-850.
Durkee, P. A., F. Pfeil, E. Frost, and R. Shema, 1991: Global analysis of aerosol particle
characteristics. Atmos. Env., 25A, 2457-2471.
Durkee, P. A., K. E. Nielsen, P. J. Smith, P. B. Russell, B. Schmid, J. M. Livingston, B. N. Holben, C. Tomasi, V. Vitale, D. Collins, R. C. Flagan, J. H. Seinfeld, K. J. Noone, E. �str�m, S. Gasso, D. Hegg, L. M. Russell, T. S. Bates and P. K. Quinn, 2000: Regional aerosol optical depth characteristics from satellite observations: ACE-1, TARFOX and ACE-2 results. Tellus, 52B, 1-14.
Kidwell, K. B., 1995: NOAA Polar Orbiter Data Users Guide. National Environmental Satellite, Data, and Information Service (NESDIS), National Oceanic and Atmospheric Administration, 394 pp.
Kuciauskas, A.P., 2002: Aerosol optical depth analysis with NOAA GOES and POES in the western Atlantic. M.S. Thesis, Naval Postgraduate School, Monterey, CA, 88 pp.
Rao, C. R. N. and J. Chen, 1995: Inter-satellite calibration linkages for the visible channels of the Advanced Very High Resolution Radiometer on the NOAA-8, -9, and –11. Int. J. Rem. Sen., 16, 1931-1942.
Rao, C. R. N. and N. Zhang, 1999: Calibration of the visible channel of the GOES images using the Advanced Very High Resolution Radiometer. Pre-print volume, 10th Atmospheric Radiation Conference (Madison, Wisconsin), 560-563.
Rao, C. R. N., C. J. Sullivan, and N. Zhang, 1999: Post-launch calibration of meterorlogical satellite sensors. Adv Space Res., 23, 1357-1365.
Turner, R., 1973: Atmospheric effects in remote sensing. In Remote Sensing of the Earth Resources, II, 549-583, F. Shahrocki (ed), Unversity of Tennessee.
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Description of NAAPS AOD / TOMS AI
Composite Plots
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Plots of combined the NPS AOD algorithm results are presented as individual images
for a day, or as 8-day loops containing 9 images (1-daily plots).
There are 3 major regions of coverage: (note: the following underlined phrases would contain hypertext pointing to maps of each of the regions).
1. Persian Gulf: These images are centered on 27N and 53E, with an areal coverage of approximately 1000 X 1000 km about the center point. The focus of the coverage is the entire Persian Gulf and most of the Gulf of Oman regions.
2. Eastern Pacific Ocean (US west coast divided into 2 regions)
a. north: These images are centered on 42N, and 128W, with an areal coverage of approximately 1100 km along the north-south dimension and approximately 1000 km along the east-west dimension about the center point. The focus of the coverage is along the Washington, Oregon, and northern California coast.
b. south: These images are centered on 35N and 122W, with an areal coverage of approximately 900 km along the north-south dimension and approximately 800 km along the east-west dimension about the center point. The focus of the coverage is along the southern California coast.
3. Puerto Rico – Caribbean Sea: These images are centered on 19N and 63W, with an areal coverage of 850 X 850 km about the center point. The focus of the coverage is the Caribbean Sea area including the islands of Puerto Rico and Guadaloupe.
Assumptions to the AOD
products:
� Aerosol compositions and size distributions are not expected to vary much throughout the time period. This assumption is greatly compromised during the advections of significant dust or aerosol plume events.
� Cloud and sub-cloud fields are still undergoing refinements. Users should be especially careful in assessing aerosol optical depth conditions within regions that are near clouds.
� POES-derived AOD products are considered to be more accurate than GOES-derived AOD products. Users should be aware that the GOES calibration and post-calibration radiances contain a higher degree of uncertainty, that is passed onto the AOD calculations.
Naval Research Laboratory
Marine Meteorology Division
7 Grace Hopper Avenue, Stop 2
Monterey, CA 93943-5502
(831) 656-4613