Retrieving the availability of light in the ocean utilising spectral signatures of vibrational Raman scattering in hyper-spectral satellite measurements
The availability of light in the ocean is an important parameter for the determination of phytoplankton photosynthesis processes and primary production from satellite data. It is also a useful parameter for other applications, e.g. the determination of heat fluxes. In this study, a method was develo...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-05-01
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| Series: | Ocean Science |
| Online Access: | http://www.ocean-sci.net/11/373/2015/os-11-373-2015.pdf |
| Summary: | The availability of light in the ocean is an important parameter for the
determination of phytoplankton photosynthesis processes and primary
production from satellite data. It is also a useful parameter for other
applications, e.g. the determination of heat fluxes. In this study, a method
was developed utilising the vibrational Raman scattering (VRS) effect of
water molecules to determine the number of photons available in the ocean
water, which is expressed by the depth integrated scalar irradiance
<span style="border-top: 1px solid #000; color: #000;">E</span><sub>0</sub>. Radiative transfer simulations with the SCIATRAN fully
coupled ocean–atmosphere radiative transfer model (RTM) show clearly the
relationship of <span style="border-top: 1px solid #000; color: #000;">E</span><sub>0</sub> with the strength of the VRS signal measured
at the top of the atmosphere (TOA).
<br><br>
Taking advantage of VRS structures in hyper-spectral satellite measurements,
a retrieval technique to derive <span style="border-top: 1px solid #000; color: #000;">E</span><sub>0</sub> in the wavelength region
from 390 to 444.5 nm was developed. This approach uses the weighting
function differential optical absorption spectroscopy (WF-DOAS) technique,
applied to TOA radiances, measured by the Scanning Imaging Absorption
Spectrometer for Atmospheric Chartography (SCIAMACHY). Based on the approach
of Vountas et al. (2007), where the DOAS method was used to fit modelled spectra of
VRS, the method was improved by using the weighting function of VRS (VRS-WF)
in the DOAS fit. This was combined with a look-up table (LUT) technique,
where the <span style="border-top: 1px solid #000; color: #000;">E</span><sub>0</sub> value was obtained for each VRS satellite fit
directly. The VRS-WF and the LUT were derived from calculations with the
SCIATRAN RTM (Rozanov et al., 2014). RTM simulations for different
chlorophyll <i>a</i> concentrations and illumination conditions clearly
show that low fit factors of VRS retrieval results correspond to low amounts
of light in the water column and vice versa.
<br><br>
Exemplarily, 1 month of SCIAMACHY data were processed and a global map of the
depth integrated scalar irradiance <span style="border-top: 1px solid #000; color: #000;">E</span><sub>0</sub> was retrieved. Spectral
structures of VRS were clearly identified in the radiance measurements of
SCIAMACHY. The fitting approach led to consistent results and the WF-DOAS
algorithm results of VRS correlated clearly with the chlorophyll
concentration in case-I water. Comparisons of the diffuse attenuation
coefficient, extracted by VRS fit results, with the established GlobColour
<i>K</i><sub>d</sub>(490) product show consistent results. |
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| ISSN: | 1812-0784 1812-0792 |