15 Aug 2022
 | 15 Aug 2022

Polar Firn Properties in Greenland and Antarctica and Related Effects on Microwave Brightness Temperatures

Haokui Xu, Brooke Medley, Leung Tsang, Joel T. Johnson, Kenneth Jezek, Macro Brogioni, and Lars Kaleschke

Abstract. In studying the mass balance of polar ice sheets, the fluctuation of the firn density near the surface is a major uncertainty. In this paper, we explore these variations at locations in the Greenland Ice Sheet and at the Dome C location in Antarctica. Borehole in situ measurements, snow radar echoes, microwave brightness temperatures, and modelling results from the Community firn model (CFM) are used. It is shown that firn density profiles can be represented using 3 processes: “long” and “short” length scale density variations and “refrozen layers”. Consistency with this description is observed in the dynamic range of airborne 0.5–2 GHz brightness temperatures and snow radar echo peaks in measurements performed in Greenland in 2017. Based on these insights, a new analytical partially coherent model is implemented to explain the microwave brightness temperatures using the three scale description of the firn. Short and long scale firn processes are modelled as a 3D continuous random medium with finite vertical and horizontal correlation lengths as opposed to past 1D random layered medium descriptions. Refrozen layers are described as deterministic sheets with planar interfaces, with the number of refrozen layer interfaces determined by radar observations. Firn density and correlation length parameters used in forward modelling to match measured 0.5–2 GHz brightness temperatures in Greenland show consistency with similar parameters in CFM predictions. Model predictions also are in good agreement with multi-angle 1.4 GHz vertically and horizontally polarized brightness temperature measured by the SMOS satellite at DOME C, Antarctica. This work shows that co-located active and passive microwave measurements can be used to infer polar firn properties that can be compared with predictions of the CFM. In particular, 0.5–2 GHz brightness temperature measurements are shown to be sensitive to long scale firn density fluctuations with density standard deviations in the range 0.01–0.06 g/cm3 and vertical correlation lengths of 6–20 cm.

Haokui Xu et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-698', Anonymous Referee #1, 26 Sep 2022
    • AC1: 'Reply on RC1', Haokui Xu, 09 Dec 2022
  • RC2: 'Comment on egusphere-2022-698', Anonymous Referee #2, 10 Oct 2022
    • AC2: 'Reply on RC2', Haokui Xu, 09 Dec 2022

Haokui Xu et al.

Haokui Xu et al.


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Short summary
The density profiles of polar ice sheet is a major unknown in estimating the mass loss using lidar tomography methods. In this paper, we shows that combing the active radar data and passive radiometer data can provide a estimation of density properties using the new model we implemented in this paper. The new model includes the short, long time scale variations in the firn and also the refrozen layers which are not included in the previous modelling work.