2014-07-10: Contiguous polarisation spectra of the Earth from 300 to 850 nm measured by GOME-2 onboard MetOp-A

In this paper we present the first contiguous high-resolution spectra of the Earth's polarisation observed by a satellite instrument. The measurements of the Stokes fraction Q/I are performed by the spectrometer GOME-2 onboard the MetOp-A satellite.

Polarisation measurements by GOME-2 are performed by onboard polarisation measurement devices (PMDs) and the high-resolution measurements discussed in this paper are taken in the special "PMD RAW" mode of operation. The spectral resolution of these PMD RAW polarisation measurements varies from 3 nm in the ultraviolet (UV) to 35 nm in the near-infrared wavelength range. We first compare measurements of the polarisation from cloud-free scenes with radiative transfer calculations for a number of cases. We find good agreement but also a spectral discrepancy at 800 nm, which we attribute to remaining imperfections in the calibration key data. Secondly, we study the polarisation of scenes with special scattering geometries that normally lead to near-zero Q/I. The GOME-2 polarisation spectra indeed show this behaviour and confirm the existence of the small discrepancy found earlier. Thirdly, we study the Earth polarisation for a variety of scenes. This provides a blueprint of Q/I over land and sea surfaces for various degrees of cloud cover. Fourthly, we compare the spectral dependence of measurements of Q/I in the UV with the generalised distribution function proposed by Schutgens and Stammes (2002) to describe the shape of the UV polarisation spectrum. The GOME-2 data confirm that these functions match the spectral behaviour captured by the GOME-2 PMD RAW mode.

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Earth reflectance (left panel) and polarisation Stokes fraction Q/I (right panel) derived from GOME-2 PMD RAW mode measurements for case 1 in Fig. 2 of the paper (in red). The green curves represent the high-resolution simulated reflectance and polarisation spectra, based on surface albedo input from the GOME-1 surface LER database. The dashed blue curve represent the same, but convolved with the PMD slit function. The FWHM of this slit function typically ranges between 3 nm in the UV up to 35 nm in the NIR. The solid and dotted blue curves also represent convolved simulation results but for these simulations the surface albedo input was taken from the GOME-2 and MERIS surface reflectivity databases, respectively. The two horizontal green lines in the right panel indicate the unpolarised situation, for which Q/I = 0, and the single scattering situation, for which Q/I = (Q/I)ss.
Spectra of Q/I for scenes over (vegetated) land. Cloud presence varies from one scene to the other. For the rest the scene properties are comparable. The numbers indicate effective cloud fractions. For low effective cloud fractions the spectra show the chlorophyll ``bump'' at ~500 nm and the vegetation red edge at ~700 nm.