Laser profiling of the ice surface topography during the Winter Weddell Gyre Study 1992

Abstract

A laser profiler mounted on a helicopter was used to measure the surface topography of sea ice in the Weddell Sea. The measurements were carried out during the cruise Antarktis X/4 of R/V Polarstern as part of the Winter Weddell Gyre Study 1992 (WWGS′92). The total length of the measured surface profiles amounts to 793 km. The observed mean ridge heights and mean ridge spacings are fairly well correlated. Therefore the ridging intensity R1, which is the ratio of ridge height and spacing, was used as a quantitative classifier in order to arrange the laser data into different groups. At a cutoff height of 0.8 m the average ridge height and spacing are 1.16 m and 160 m at the lowermost range of ridging intensities (R1<0.02), 1.2 m and 50 m at the intermediate range (0.02<R1<0.04), and 1.3 m and 26 m at the uppermost range (R1>0.04), respectively. The best fits to the observed ridge height distributions are achieved by negative exponentials, at lower ridging intensities as a function of ridge height, at the regime of highest ridging intensity as a function of the ridge height squared. The distributions of ridge spacing are well described by a lognormal distribution function. A simple model is used to estimate the average thickness hd* and the areal fraction of ridged ice from the laser data. The values of hd* range from 3.1m with an areal coverage of 5% at the lowermost range of R1 to 3.7 m with an areal coverage of 31% at the uppermost range. If the ridged ice is distributed as a uniform layer over a unit area, the corresponding effective thickness values are 0.14 m and 1.16 m, respectively. Mean ridge heights and spacings are determined for different cutoff heights in order to enable comparisons to other measurements. The effect of ridges on the surface stress over the ice is estimated using the drag partition theory, which is applicable to the regime of lowest ridging intensity. The contribution of ice ridge form drag to the total wind drag for an average ridging intensity of <R1> = 0.007 and a cutoff height of 0.8 m is about 35 to 40% at neutral stability. If it is assumed that the aerodynamic roughness length z0 of the intervening ice surface varies between 3.3×10−4 m and 5.5×10−4 m, the drag coefficient Cdn ranges from 2.08×10−3 to 2.24×10−3 at a reference height of 10 m.