How well can satellite altimetry and firn models resolve Antarctic firn thickness variations?

Abstract. Elevation changes of the Antarctic Ice Sheet (AIS) related to surface mass balance (SMB) and firn processes vary strongly in space and time. Their short-term natural variability is large and hampers the detection of long-term climate trends. Firn models or satellite altimetry observations are typically used to investigate such firn thickness changes. However, there is a large spread among firn models. Further, they do not fully explain observed firn thickness changes, especially on smaller temporal and spatial scales. Reconciled firn thickness variations will facilitate the detection of long-term trends from satellite altimetry, the resolution of the spatial patterns of such trends and, hence, their attribution to the underlying mechanisms. This study has two objectives: First, we quantify interannual Antarctic firn thickness variations on a 10 km grid scale. Second, we characterise errors in both the altimetry products and firn models. To achieve this, we jointly analyse satellite altimetry and firn modelling results in time and space. We use the timing of firn thickness variations from firn models and the satellite-observed amplitude of these variations to generate a combined product (‘adjusted firn thickness variations’) over the AIS for 1992–2017. The combined product characterises spatially resolved variations better than either firn models alone or altimetry alone. We detect highest absolute differences between the adjusted and modelled variations at lower elevations near the AIS margins, probably influenced by the lower resolution, more blurred spatial distribution of the modelled variations. In a relative sense, the largest mismatch between the adjusted and modelled variations is found in the dry interior of the East Antarctic Ice Sheet (EAIS), in particular across large megadune fields. Here, the low signal-to-noise ratio poses a challenge for both models and altimetry to resolve firn thickness variations. The altimetric residuals still contain a large part of the altimetry variance and include firn model errors, such as firn signals not captured by the models, and altimetry errors. Apart from time-variable penetration effects of radar altimetry signals, the residuals disclose patterns indicating uncertainties in intermission calibration.