Menghua Wang NOAA/NESDIS/ORA E/RA3, Room 102, 5200 Auth Rd.
Published byModified over 4 years ago
Presentation on theme: "Menghua Wang NOAA/NESDIS/ORA E/RA3, Room 102, 5200 Auth Rd."— Presentation transcript:
1 Atmospheric Correction for Turbid Waters in Coastal Regions: Need Bands > 1000 nm Menghua WangNOAA/NESDIS/ORAE/RA3, Room 102, 5200 Auth Rd.Camp Springs, MD 20746, USAThe Coastal Ocean Applications and Science Team MeetingSeptember 7-8, 2005, Corvallis, Oregon
2 Solar IrradiancePassive Remote Sensing: Sensor-measured signals are all originated from the sun!
3 Ocean Color Remote Sensing Sensor-Measured“Green” oceanBlue oceanAtmospheric Correction (removing >90% signals)Calibration (0.5% error in TOA >>>> 5% in surface)From H. Gordon
4 Atmospheric WindowsUV bands can be used for detecting the absorbing aerosol casesTwo long NIR bands (1000 & 1240 nm) are useful for of the Case-2 waters
5 The Ocean Radiance Spectrum TOA ReflectanceOpen Ocean WatersCoastal Waters
6 Atmospheric Correction MODIS and SeaWiFS algorithm (Gordon and Wang 1994)w is the desired quantity in ocean color remote sensing.Tg is the sun glint contribution—avoided/masked/corrected.Twc is the whitecap reflectance—computed from wind speed.r is the scattering from molecules—computed using the Rayleigh lookup tables (atmospheric pressure dependence).A = a + ra is the aerosol and Rayleigh-aerosol contributions —estimated using aerosol models.For Case-1 waters at the open ocean, w is usually negligible at 750 & 865 nm. A can be estimated using these two NIR bands. Ocean is usually not black at NIR for the coastal regions.Gordon, H. R. and M. Wang, “Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: A preliminary algorithm,” Appl. Opt., 33, , 1994.
12 SeaWiFS Global Deep Ocean Results Wang, M., K. Knobelspiesse, and C. R. McClain, “Study of the SeaWiFS aerosol optical property data over ocean in combination with the ocean color products,” J. Geophys. Res., 110, D10S06, doi: /2004JD
17 Turbid Waters: Examples from MODIS Data (Short Wave Infrared Bands for Coastal Regions)Wang, M. and W. Shi, “Estimation of ocean contribution at the MODIS near-infrared wavelengths along the east coast of the U.S.: Two case studies,” Geophys. Res. Letters, 32, L13606, doi: /2005GL (2005).
18 Case-2 Water Complications For productive ocean waters at coastal regions, the ocean is usually not black at the NIR wavelengths at 765 and 865 nm. In these cases, the ocean contributions at the NIR are often mistakenly accounted as radiance scattering from atmosphere, thereby leading to over-correction of atmospheric radiance and underestimation of water-leaving radiance at the visible.Examples of atmospheric correction for the non-black ocean at the NIR bands.Negative nLw
19 Atmospheric Correction: Short Wave Infrared (SWIR) In general, to effect the atmospheric correction operationally using the NIR bands at 765 and 865 nm, or using the spectral optimization with measurements from nm, Case-2 bio-optical model that has strongly regional & temporal dependences is needed.At the short wave infrared (SWIR) wavelengths (>~1000 nm), ocean water is much strongly absorbing and ocean contributions are significant less. Thus, atmospheric correction may be carried out at the coastal regions without using the bio-optical model.Examples using the MODIS 1240 and 1640 nm data to derive the ocean contributions at the NIR bands.We use the SWIR (1640 nm) for the cloud masking. This is necessary for the coastal region waters.
27 Aerosol Single-Scattering Epsilon (0 = 1640 nm) Spectral aerosol contribution relative to wavelength 1640 nm for 12 models
28 Data Processing Using SWIR Bands Lookup Tables Generation and Implementation:Rayleigh lookup tables for the SWIR bands.Aerosol optical property data (scattering phase function, single scattering albedo, extinction coefficients) for the SWIR bands (12 models same as for SeaWiFS/MODIS).Aerosol lookup tables (12 aerosol models--same as for SeaWiFS/MODIS) for the SWIR bands.Data Processing:Developed cloud masking using the 1640 nm band. This is necessary for the high-productive waters (e.g., coastal regions).Implemented the sun glint mask.
29 MODIS-Measured TOA vs. Theoretical TOA Open Ocean, MODIS Terra GranuleNeed vicarious calibration for 1240 and 1640 nm bands: Adjusting the gains so that the intercept=0 and slope=1.
30 Procedures For the MODIS 748 and 869 nm Bands Ocean-Contributed Reflectance EstimationMODIS Terra L1bVicarious CalibrationB1240 and B1640 at open oceanAerosol Type from B1240 and B1640Reflectance w/o OceanContribution (B748, B865)Ocean-ContributedReflectance (B748, B865)
31 Histogram of Ocean Contributed Reflectance (Open Ocean)
32 The Rayleigh-Corrected TOA Reflectance 748 nm869 nm1240 nm1640 nmRayleigh-RemovedMODIS Terra Granule: (March 11, 2004)
33 The NIR Ocean Contributions Very large ocean contribution at the NIR bands in coastal regions with significant spatial & temporal variations.
34 Real values VS Theoretical Values ( )MODIS Aqua 2130 nmMODIS Aqua 1240 nm
37 Ocean-Contributed Reflectance at NIR Very significant ocean contributions!
38 ConclusionsBoth SeaWiFS/MODIS provide high quality ocean color products in the open oceans.For the turbid waters in coastal regions, ocean is not black at the NIR bands. This leads to underestimation of the sensor-measured water-leaving radiances with current SeaWiFS/MODIS atmospheric correction algorithm.Ocean is black for turbid waters at wavelengths >~1000 nm. Thus, the longer NIR bands can be used for atmospheric correction over the turbid waters.In addition, we need longer NIR bands for cloud masking in the coastal regions.We need longer NIR bands (> ~1000 nm) for GOES-R HES-CW for atmospheric correction for turbid waters in coastal regions.