Abstract
Abstract. Sea ice thickness is an essential climate variable. Current L-Band sea ice thickness retrieval methods do not account for sea ice surface roughness that is hypothesised to be not relevant to the process. This study attempts to validate this hypothesis that has not been tested yet. To test this hypothesis, we created a physical model of sea ice roughness based on geometrical optics and merged it into the L-band emissivity model of sea ice that is similar to the one used in the operational sea ice thickness retrieval algorithm. The facet description of sea ice surface used in geometrical optics is derived from 2-D surface elevation measurements. Subsequently the new model was tested with TB measurements performed during the SMOSice 2014 field campaign. Our simulation results corroborate the hypothesis that sea ice surface roughness has a marginal impact on near-nadir TB (used in the current operational retrieval). We demonstrate that the probability distribution function of surface slopes can be approximated with a parametric function whose single parameter can be used to characterise the degree of roughness. Facet azimuth orientation is isotropic at scales greater than 4.3 km. The simulation results indicate that surface roughness is a minor factor in modelling the sea ice brightness temperature. The change in TB is most pronounced at incidence angles greater than 40∘ and can reach up to 8 K for vertical polarisation at 60∘. Therefore current and future L-band missions (SMOS, SMAP, CIMR, SMOS-HR) measuring at such angles can be affected. Comparison of the brightness temperature simulations with the SMOSice 2014 radiometer data does not yield definite results.
Highlights
The L-band brightness temperature (TB) is sensitive to sea ice thickness
We demonstrate that the probability distribution function of surface slopes can be approximated with a parametric function whose single parameter can be used to characterise the degree of roughness
In this paper we address the knowledge gap concerning the influence of the decimetre-scale surface roughness on the Lband brightness temperature of sea ice
Summary
The L-band brightness temperature (TB) is sensitive to sea ice thickness. This feature is used for sea ice thickness retrieval from L-band TB (over thin ice, < 1.5 m) (Tian-Kunze et al, 2014; Huntemann et al, 2014; Kaleschke et al, 2016). Several factors influence TB measurements over ice-covered regions, among them ice concentration, ice temperature, snow cover, sea ice surface roughness, and the shape of the interfaces between the snow and ice layers We investigate the effects of surface roughness on the L-band TB, the large-scale roughness. This factor is not included in the modelling of sea ice emissions in operational sea ice thickness retrieval. These roughness statistical parameters are derived from measurements of surface elevation (z), conducted with altimeters that are characterised by their accuracy (δ) and sampling distance ( x). Sea ice elevation measurements obtained from airborne altimeters (Ketchum, 1971; Dierking, 1995) and supplemented with terrestrial laser scanners (Landy et al, 2015) draw a picture of sea ice roughness as Published by Copernicus Publications on behalf of the European Geosciences Union
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