Abstract
Abstract. Satellite observations of microwave brightness temperatures between 19 GHz and 85 GHz are the main data sources for operational sea-ice monitoring and retrieval of ice concentrations. However, microwave brightness temperatures depend on the emissivity of snow and ice, which is subject to pronounced seasonal variations and shows significant hemispheric contrasts. These mainly arise from differences in the rate and strength of snow metamorphism and melt. We here use the thermodynamic snow model SNTHERM forced by European Re-Analysis (ERA) interim data and the Microwave Emission Model of Layered Snowpacks (MEMLS), to calculate the sea-ice surface emissivity and to identify the contribution of regional patterns in atmospheric conditions to its variability in the Arctic and Antarctic. The computed emissivities reveal a pronounced seasonal cycle with large regional variability. The emissivity variability increases from winter to early summer and is more pronounced in the Antarctic. In the pre-melt period (January–May, July–November) the standard deviations in surface microwave emissivity due to diurnal, regional and inter-annual variability of atmospheric forcing reach up to Δε = 0.034, 0.043, and 0.097 for 19 GHz, 37 GHz and 85 GHz channels, respectively. Between 2000 and 2009, small but significant positive emissivity trends were observed in the Weddell Sea during November and December as well as in Fram Strait during February, potentially related to earlier melt onset in these regions. The obtained results contribute to a better understanding of the uncertainty and variability of sea-ice concentration and snow-depth retrievals in regions of high sea-ice concentrations.
Highlights
The temporal and spatial variability of sea-ice coverage and its physical properties have been operationally observed with satellite passive microwave radiometers for more than 30 years (e.g. Eisenman et al, 2014; Stroeve et al, 2012; Cavalieri and Parkinson, 2008; Parkinson and Cavalieri, 2008)
This is mostly due to the fact that the model results represent 100 % sea-ice concentration, whereas observed data have been extracted for sea-ice concentration > 90 %, and are affected by emissivity variations arising from different open water fractions, surface heterogeneity and seaice drift
The simulated values are within a realistic range of observed PR and GR, the simulations indicate on average higher PR (Arctic: +0.005; Antarctic: +0.002) and lower GR (Arctic: −0.005; Antarctic: −0.014)
Summary
The temporal and spatial variability of sea-ice coverage and its physical properties have been operationally observed with satellite passive microwave radiometers for more than 30 years (e.g. Eisenman et al, 2014; Stroeve et al, 2012; Cavalieri and Parkinson, 2008; Parkinson and Cavalieri, 2008). From a comparison with field data or other ground-truth references, tie points or transfer functions are deducted to allow for an inversion from microwave measurements to sea-ice concentration, or surface properties like snow thickness or ice type (Markus and Cavalieri, 1998). Critical to this inversion are, seasonal and regional variations in the surface microwave emissivity that are caused by differences in atmospheric forcing and associated snow processes (Meier and Notz, 2010; Markus et al, 2006; Cavalieri et al, 1995; Gloersen and Cavalieri, 1986). As shown by Andersen et al (2007) variations in sea-ice concentration retrievals over high-concentration Arctic sea ice are dominated
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