Teorías y movimientos hacia la calidad y servicio

Platelet ice accumulates beneath a solid sea ice cover as a result of the oceanic heat flux associated with the outflow of supercooled ISW from ice shelf cavities. This results in the formation of two additional ice types; platelet ice, which becomes part of the mechanical integrity of the solid sea ice cover and a sub-ice platelet layer. This process has been documented in great detail in McMurdo Sound (see Gough et al., 2012 and references there in) and at other locations in the Antarctic (Langhorne et al., in prep). Both ice types originate from the same process but in regard to freeboard conversion to ice thickness require separate treatment. The additional growth of the ‘solid’ sea ice cover from incorporation of platelet crystals and eventual consolidation causes an increase in freeboard. As this additional mass has a similar density to the overlying ice cover, the ice thickness estimate via freeboard is not erroneous. However, the sub-ice platelet layer still produces a buoyant influence but has a very different density (Gough et al. 2012 and Chapter 4 here). Using in situ measurements of sea ice freeboard, thickness and snow, with their respective densities, a solid fraction of 0.16 ± 0.17 was calculated using the hydrostatic equilibrium assumption. This value was at the lower range of estimates previously presented in the literature and its calculation was highly sensitive to the value used for sea ice density. The spread of measured sea ice density in the area (915 to 935 kgm-3) results in a range of solid fractions from 0.03 to 0.36. Upon application of a GNSS surface elevation survey, it was

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noted that sea ice thickness can be overestimated by up to 19 % with a mean deviation of 12 % when converting surface elevation measurements to sea ice thickness when this influence was not accounted for. Such a finding justifies the inclusion of such influences in error budgets for satellite altimetry derived sea ice thickness in the Antarctic. The mean overestimation of 12 % reported here is primarily driven by the study areas proximity to the ice shelf edge. The thickness of the sub-ice platelet layer quickly tails off in the seaward direction, and it is unlikely to influence measurements discussed here beyond 100 km. The separation of the influence of platelet ice and the sub-ice platelet layer is not possible without in situ validation.

The identification of such freeboard/thickness anomalies presents an opportunity to achieve an ambitious, but sought-after geophysical conclusion. Shown here, and in other work, it is clearly established that ice shelf expulsion of supercooled water increases the freeboard of fast sea ice. This can be driven by formation of platelet ice and/or a sub-ice platelet layer. Assuming similar ice shelf margin properties, it is reasonable to suggest that satellite altimeters could provide an insight to the presence of supercooled water via sea ice freeboard/thickness anomalies. This may provide a way of mapping ISW advection into the Southern Ocean. Such advection is indicated as being influential upon larger scale sea ice processes (Bintanja et al., 2013). This ability will only be permitted if certain technical and methodological constraints are overcome. The snow cover must be accurately measured or modelled to reveal the true increases in sea ice freeboard alone. This will require advances in instrumentation or improvement of the accuracy of current techniques which suffer from interference from wet snow and surface roughness (Markus et al., 2011, Zwally et al., 2008, Markus and Cavalieri, 1998). Even then, the measurement of increases in ice freeboard will require highly precise operational instrumentation. Freeboard anomalies associated with increases in sea ice thickness and accumulation of a sub-ice platelet layer are typically less than 0.05 m in the most extreme cases (i.e. within a few kilometers of the ice shelf margin). The measurement of such increases are currently at the limits of contemporary and antecedent satellite altimeter capabilities. Environmental conditions may also restrict such investigations as fast ice areas that are too large (McMurdo Sound is likely at the limit) will be located too far from open water and have insufficient sea surface tie points to retrieve freeboard accurately.

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While undertaking in situ measurements for this work, it was proposed that the loading of equipment and personnel near drill-hole sites could suppress the local freeboard. This was checked in a rudimentary fashion during in situ validation in 2013, which indicated such an affect was occurring. This surface suppression can lead to overestimates of sea ice density using in situ measurements and the hydrostatic equilibrium assumption of 5 kgm-3. It is recommended that loading is not permitted in the immediate vicinity of measurements and that multiple measurements are carried out over an area with a radius of at least 15 m. A larger survey area will also reduce the effect of surface undulations on sea ice density estimates using the hydrostatic equilibrium assumption. Such undulations can lead to the inclusion of freeboard and thickness measurements that are not representative of the local hydrostatic balance and cause a spread in the estimated sea ice density. Whether the hydrostatic equilibrium assumption is fulfilled in sea ice investigations has become a topic of debate. There is no doubt that a sea ice floe, or fast ice cover is in hydrostatic equilibrium, however, disturbance from loading of personnel and equipment as described here, and interference from ships (Hutchings et al., 2014) are cause for concern. In addition the spatial scales that are assessed will also be influential as surface conditions are variable. There is a particular need for improved comparison between satellite sensors, airborne instruments and in situ validation which all represent different spatial scales.