Retrieval of light availability in ocean waters utilizing 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 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 E_0. Radiative transfer simulations with the SCIATRAN fully coupled ocean–atmosphere radiative transfer model (RTM) show clearly the relationship of E_0 with the strength of the VRS signal measured at the top of the atmosphere (TOA). Taking advantage of VRS structures in hyper-spectral satellite measurements, a retrieval technique to derive E_0 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 E_0 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 a 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. Exemplarily, one month of SCIAMACHY data were processed and a global map of the depth integrated scalar irradiance E_0 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 Kd490 product show consistent results