数据说明
MODIS/Aqua Particulate Inorganic Carbon (PIC)
上传时间：2019-05-22 14:24:18 浏览次数：作者与来源：admin
Particulate inorganic carbon is defined as the inorganic matter that is not able to pass through a filter (filters generally range in size between 0.7 and 0.22 um).
Unit: mol/m3
Resolution: 9km pixel, monthly
Duration: Jul. 2002 to on-going
Source: Moderate Resolution Imaging Spectroradiometer(MODIS), Aqua, NASA
Version: 2018
Processing: Original daily and monthly mean products are shown.
PIC here calculated using observed in situ relationships between water-leaving radiances, spectral backscattering coefficients (PIC = bbc(546)/1.628(calcite-specific backscattering coefficient)). The PIC algorithm is a hybrid of two independent approaches, defined here as the 2-band approach (Balch et al. 2005) and the 3-band approach (Gordon et al. 2001). The 3-band approach is used when the 2-band approach fails.
The 2-band approach：
The algorithm makes use of a precomputed look-up table, derived from in situ measurements, that contains the total backscattering coefficient for calcite at 546 nm, bbc(546) in m-1, as a function of nLw(443) and nLw (555). The concentration of calcite (PIC) is computed by dividing bbc(546) by a calcite-specific backscattering coefficient (1.628 m2/mol), as also derived from in situ measurements.
The 2-band algorithm may fail for two primary reasons: 1) the normalized water-leaving radiances could not be retrieved due to atmospheric correction failures or other masking conditions (e.g., clouds or land), and 2) the retrieved water-leaving radiances may be outside the range of values in the precomputed LUT. A common reason for either of these conditions is that the PIC concentration is very high.
The 3-band approach:
Observed TOA radiances, Lt(λ), at three spectral bands near 670, 765, and 865 nm are converted to reflectance and then elated to the components of the radiant path reflectance through:
ρt(λ)=(ρr(λ)+ts(λ)×ρf(λ)×ts(λ)×ρw(λ)+ρa(λ))tg(λ)
where: ρt(λ) is top-of-atmosphere reflectance (measured), ρr(λ) is reflectance due to Rayleigh scattering in the absence of aerosols (calculated), ρf(λ) is reflectance due to whitecaps and foam (calculated), ts(λ) is diffuse transmittance of the atmosphere from surface to sensor (calculated), tg(λ) is atmospheric gas transmittance Sun to surface to sensor (calculated), ρw(λ) is water-leaving reflectance (unknown), and ρa(λ) is aerosol reflectance (unknown).
Aerosol and water-leaving reflectances can be expressed roughly as:
ρa(λ)≈ρa(λ0)×exp(a×(λ0-λ))ρa(λ)≈ρa(λ0)×exp(a×(λ0-λ))
and
ρw(λ)≈bb(λ)/(6.179×(aw(λ)+bb(λ)))
where: aw is the absorption coefficient of seawater, bb is the total backscattering coefficient, and λ0 = 865 nm.Backscattering by calcite and seawater can be roughly expressed as:
bb(λ)≈bbc(546)×(546/λ)1.35+bbw(λ)
Known issues
1. A limited set of satellite-to-in-situ match-up validation results are available for MODIS from the validation tool of the SeaWiFS Bio-Optical Archive and Storage System (SeaBASS). And the result shows a mean bias of 0.30065 and a mean absolute error(MAE) of 3.63329.
2. It is worth noting that the products here are mainly used to estimate the PIC of the global ocean. When estimating the PIC of a regional area, it is necessary to use the measured PIC data to verify the applicability of the products presented here.
Reference
Balch, W. M., Gordon, H. R., Bowler, B. C., et al. Calcium carbonate measurements in the surface global ocean based on Moderate-Resolution Imaging Spectroradiometer data. Journal of Geophysical Research,2005,110(C07001). DOI:10.1029/2004jc002560
Gordon, H. R., Boynton, G. C., Balch, W. M., et al. Retrieval of coccolithophore calcite concentration from SeaWiFS imagery. Geophysical Research Letters, 2001,28(8):1587-1590. DOI:10.1029/2000gl012025
All the contents of this page are quoted from NASA. All project documentation and related publications can be found at the website: https://oceancolor.gsfc.nasa.gov/
Unit: mol/m3
Resolution: 9km pixel, monthly
Duration: Jul. 2002 to on-going
Source: Moderate Resolution Imaging Spectroradiometer(MODIS), Aqua, NASA
Version: 2018
Processing: Original daily and monthly mean products are shown.
PIC here calculated using observed in situ relationships between water-leaving radiances, spectral backscattering coefficients (PIC = bbc(546)/1.628(calcite-specific backscattering coefficient)). The PIC algorithm is a hybrid of two independent approaches, defined here as the 2-band approach (Balch et al. 2005) and the 3-band approach (Gordon et al. 2001). The 3-band approach is used when the 2-band approach fails.
The 2-band approach：
The algorithm makes use of a precomputed look-up table, derived from in situ measurements, that contains the total backscattering coefficient for calcite at 546 nm, bbc(546) in m-1, as a function of nLw(443) and nLw (555). The concentration of calcite (PIC) is computed by dividing bbc(546) by a calcite-specific backscattering coefficient (1.628 m2/mol), as also derived from in situ measurements.
The 2-band algorithm may fail for two primary reasons: 1) the normalized water-leaving radiances could not be retrieved due to atmospheric correction failures or other masking conditions (e.g., clouds or land), and 2) the retrieved water-leaving radiances may be outside the range of values in the precomputed LUT. A common reason for either of these conditions is that the PIC concentration is very high.
The 3-band approach:
Observed TOA radiances, Lt(λ), at three spectral bands near 670, 765, and 865 nm are converted to reflectance and then elated to the components of the radiant path reflectance through:
ρt(λ)=(ρr(λ)+ts(λ)×ρf(λ)×ts(λ)×ρw(λ)+ρa(λ))tg(λ)
where: ρt(λ) is top-of-atmosphere reflectance (measured), ρr(λ) is reflectance due to Rayleigh scattering in the absence of aerosols (calculated), ρf(λ) is reflectance due to whitecaps and foam (calculated), ts(λ) is diffuse transmittance of the atmosphere from surface to sensor (calculated), tg(λ) is atmospheric gas transmittance Sun to surface to sensor (calculated), ρw(λ) is water-leaving reflectance (unknown), and ρa(λ) is aerosol reflectance (unknown).
Aerosol and water-leaving reflectances can be expressed roughly as:
ρa(λ)≈ρa(λ0)×exp(a×(λ0-λ))ρa(λ)≈ρa(λ0)×exp(a×(λ0-λ))
and
ρw(λ)≈bb(λ)/(6.179×(aw(λ)+bb(λ)))
where: aw is the absorption coefficient of seawater, bb is the total backscattering coefficient, and λ0 = 865 nm.Backscattering by calcite and seawater can be roughly expressed as:
bb(λ)≈bbc(546)×(546/λ)1.35+bbw(λ)
Known issues
1. A limited set of satellite-to-in-situ match-up validation results are available for MODIS from the validation tool of the SeaWiFS Bio-Optical Archive and Storage System (SeaBASS). And the result shows a mean bias of 0.30065 and a mean absolute error(MAE) of 3.63329.
2. It is worth noting that the products here are mainly used to estimate the PIC of the global ocean. When estimating the PIC of a regional area, it is necessary to use the measured PIC data to verify the applicability of the products presented here.
Reference
Balch, W. M., Gordon, H. R., Bowler, B. C., et al. Calcium carbonate measurements in the surface global ocean based on Moderate-Resolution Imaging Spectroradiometer data. Journal of Geophysical Research,2005,110(C07001). DOI:10.1029/2004jc002560
Gordon, H. R., Boynton, G. C., Balch, W. M., et al. Retrieval of coccolithophore calcite concentration from SeaWiFS imagery. Geophysical Research Letters, 2001,28(8):1587-1590. DOI:10.1029/2000gl012025
All the contents of this page are quoted from NASA. All project documentation and related publications can be found at the website: https://oceancolor.gsfc.nasa.gov/
