Sea ice thickness retrieval algorithms based on in situ surface elevation and thickness values for application to altimetry

In situ measurements of sea ice thickness (I), snow depth (S), and snow freeboard (F_sn) from drilling profile lines from 15 cruises into the Southern Ocean, Antarctica, were analyzed. I was calculated from in situ F _sn and S using an isostatic approach. I was also directly estimated from F_sn as can be obtained from laser altimetry. The root-mean-square difference (RMSD) between observed and calculated I reduces, and the correlation between F_sn and I increases substantially, when (1) using values averaged over the survey lines (∼50 m) instead of single drill hole measurements (∼1 m) and (2) treating positive and negative sea ice freeboard (F_i) separately. For small F_i, however, S approximates F_sn pointing toward an isostatic balance also between S and I. Our linear regression analysis between the in situ measurements suggests a direct conversion of F_sn into I using a region-specific set of equations. RMSD values are similar to those obtained employing isostatic balance models and treating positive and negative F_i separately. However, more data would have been needed to obtain significant differences between most of the various models suggested. Still our new approach gives a viable alternative for Antarctic I retrieval from altimetric measurements of F _sn alone. Correlation between in situ observations of F_sn and S is high. RMSD between observed and calculated S is small. This suggests estimation of S from altimetric F_sn measurements. Such S has an estimated precision of ∼5 cm, and is neither affected by snow wetness or grain size nor limited to S < 50 cm.