Impact of Sea-Ice Bottom Topography on the Ekman Pumping - Towards an Interdisciplinary Approach in Earth System Science - page 130

laser altimeter (as described above) gives the distance of the instrument to the
surface (ice or snow), hence the thickness is obtained by the difference between
laser and EM measurements. The sea-ice thickness is sampled at 10 Hz, which leads
to an average point spacing of

4 m. The footprint of the instrument is about 40 m
(Kovacs et al. 1995 ; Redi et al. 2006 ). Ice thickness samples thinner than 0.1 m are
considered as open water. Since sea ice surface and underside pro

*

les are recorded
at the same time we can have a complete description of the ice topography on the
surface and underneath the ice.
In order to obtain information about the bottom topography of the ice, in each
point where both measurements are available, the thickness recorded with the
EM-bird is subtracted from the filtered laser altimeter pro les. The spatial resolu-

tion is lower than the one for the upper surface (30 40 cm) since the thickness

-

measurements are recorded every 4 m. A routine is then applied to the filtered

*

pro le to select minima that are deeper than a certain threshold value. The proce-

dure is the same as used in other studies for the detection of sails from surface ice
pro les (e.g. Von Saldern et al. 2006 ; Rabenstein et al. 2010 ). Values ranging from

− 9 m can be used as thresholds for keel detection (Davis and Wadhams 1996 ;

Williams et al. 1975 ). Here we assume a sail height to keel depth ratio of 4 (Timco
and Burden 1997 ). In the studies by, e.g., Rabenstein et al. ( 2010 ), Von Saldern
et al. ( 2006 ) a sail height of 0.8 m is used as threshold value for the identi

5to -

cation of
sails. This gives a threshold value for the keel depth equal to − 3.2 m. Since the EM-
bird underestimates the real thickness of ridges by up to 50

60 % (Haas 2004 ), we

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Fig. 2 An example of a sea ice pro le of ca. 35 km length. The black line shows the upper surface
pro le obtained by the filtered laser altimeter data. The light blue line represents the bottom pro le
of the ice obtained by subtracting the thickness from the laser altimeter pro les. The circles
represent the detected keels. The depth of the keels is then multiplied by a factor of 2 as described
in Sect. 2