Retrieval of snow grain size and albedo using EnMAP spaceborne observations

Abstract

Cryosphere is an integral part of the terrestrial ecosystem with important linkages and feedbacks generated through its influence on moisture fluxes, hydrology, and climate change due to temporal changes in snow/ice extent, impurity load and albedo. Therefore, snow and ice properties including ice and snow albedo and extent are monitored using ground and satellite instrumentation. The measurements are performed at various temporal and spatial scales using passive and active remote sensing instrumentation in a broad spectral range. The high spatial resolution is highly relevant for studies of cryosphere due to the rapid horizontal variability of snow properties and impurity load (dust outbreaks, algae blooms, structures on the snow surface/sastrugi). The instruments with low spatial resolution are not capable to resolve fine scales of snow variability. The accurate information on specific snow features (e.g., spatial distribution of algae blooms) can be hardly detected using measurements performed on the scale 0.3-1.0km.  The Environmental Mapping and Analysis Program (EnMAP) is a German hyperspectral satellite mission (Guanter et al., 2015), which provides information on evolution of aquatic and terrestrial ecosystems including cryosphere on the spatial scale of 30m, is capable to resolve the fine scale variability of snow properties. The measurements of EnMAP can be also used to assess the sub-pixel snow variability for coarse spatial resolution satellite missions and assess the accuracy of satellite products derived on the coarse spatial grid (e.g., snow fraction). This paper is aimed at the adaptation of the previously proposed snow remote sensing technique (Kokhanovsky et al., 2023) to EnMAP measurements. The retrievals are based on the asymptotic radiative transfer theory valid for weakly absorbing multiply light scattering turbid media (Kokhanovsky, 2021). The local optical properties of snow are calculated using the geometrical optics approximation, which is a valid technique for snow due to large size of ice grains as compared to the wavelength of the incident solar light in the spectral range under study. In particular, the spectral snow albedo and snow grain size are retrieved using EnMAP measurements performed by the SWIR EnMAP detector in the spectral range 900 - 1283nm. The snow specific surface area (SSA) and broadband albedo (BBA) are also derived using EnMAP measurements. The example of retrievals over Concordia station in Antarctica is given. It has been found that the effective ice grain diameter is around 0.23mm, SSA=28m*m/kg, and BBA=0.81, which is similar to the values of snow parameters measured at this location at the same season using both ground and satellite instrumentation.ReferencesGuanter, L., H. Kaufmann, K. Segl, et al., 2015: The EnMAP Spaceborne Imaging Spectroscopy Mission for Earth Observation. Remote Sensing 7: 8830-8857.Kokhanovsky, A., 2021: Snow Optics, Cham: Springer.Kokhanovsky, A., B. Vandecrux, A. Wehrlé, O. Danne, C. Brockmann, and J. E. Box, 2023: Improved Retrieval of Snow and Ice Properties Using Spaceborne OLCI/S-3 Spectral Reflectance Measurements: Updated Atmospheric Correction and Snow Impurity Load Estimation. Remote Sensing 15: 1-25.