Studying Firn Properties with Radiometer, Radar, and the Community Firn Model

In studying the mass balance of polar ice sheets, the fluctuation of the firn density near the surface is a major uncertainty. Temporal knowledge of firn densities are used to interpret elevation change measured by altimeters in terms of ice mass change. In this paper, we use microwave radiometer from 0.5GHz to 2Ghz and radar datasets to infer firn density fluctuations. We examine data collected from four sites in the accumulation zone of Greenland (including those that experience significant melt and refreeze events), the FA-13 site on the south east coast of Greenland where firn aquifers are present, and DOME-C in Antarctica where the firn is believed to experience minimal melt effects. The Community Firn Model is used as a reference for the four locations in Greenland. UWBRAD, SMOS, and SMAP brightness temperature data and Snow radar echograms are used in the analysis. A radiative transfer model is used to account for the effects of firn density fluctuation and refrozen layers observed in the radar echograms in the accumulation zone in Greenland. The density fluctuation is treated as a 3D variation accounting for both vertical and horizontal density changes. Results over the Greenland accumulation zone show that the dynamic range of brightness temperature is correlated with the number of peaks in the radar echograms. Forward modelling results show a consistency of the input density parameters with the Community Firn Model when a good match between modeled and measured TB is achieved, and forward modeled multi-angle results also are in good agreement with SMOS observations. SMAP data at the FA- 13 site is also shown to be interpretable using the forward model. This work provides a combined active and passive method for the remote sensing of firn densities.