NORTHWESTERN WEDDELL SEA OBTAINED BY IN-SITU AND SATELLITE MEASUREMENTS
4.3 Snow thickness distribution
Snow on sea ice plays a key role within the atmosphere-ice-ocean system, since it modifies heat flux from the ocean to the atmosphere, and the sea ice mass balance.
Furthermore, all electromagnetic ice thickness measurements (see above) give total (snow + sea ice) thickness only. Hence, snow thickness needs to be measured as well, to compute sea ice thickness from both data sets. Therefore, on ANT-XXIII/7 we have paid particular attention to extensive observations of representative snow thickness distributions.
Snow thickness was measured in three ways:
1 and 2) Direct thickness readings were obtained with a metal ruler (snow stick) a) along tape measure profiles with a spacing of 1 m or 2 m, or b) in concert with EM31 ice thickness surveys where snow thickness was measured approximately every 20 m. Tape measure profiles were mostly 200 m long (13 out of 25), with shorter lines when floe size was limited. EM31 profile lengths reached up to several kilometres (see Table 4.1).
4. Regional variability of sea ice properties and thickness in the northwestern Weddell Sea obtained by in-situ and satellite measurements
Fig. 4.18: Ice station locations with SAR scene from 19th September in the background. Marker colour coding shows mean snow thickness obtained from direct, snow stick profiles.
3) (GSSI SIR-3000 with 400 Mhz antenna) was pulled over ice floes in a pulka (sledge) along the tape measure profiles as well as following or coincident with the EM31 transects. The contrast in dielectric permittivity between snow (~1.5) and sea ice (~ 2) allows to obtain radar snow thickness profiles with a radar antenna moved along the snow surface. Lateral radar snow thickness sampling is roughly 1 cm, leading to high-resolution snow thickness profiles.
In total a sum of 25 tape measure profiles, 14 EM31 transects and 12 radar surveys have been conducted cumulating to profile lengths of approximately 3.6 km, 17 km and 5 km respectively. Figure 4.18 shows a summary of most ice stations during ANT-XXIII/7 along with dates and mean snow thickness (colour code). The SAR scene in the background was acquired on September 19.
4.3 Snow thickness distribution
4.3.1 Ruler stick measurements
Figure 4.19 provides an overview on the snow thickness statistics (snow stick, tape measure profiles), mean snow thicknesses of all floes are summarized in table 4.1.
As outlined in figure 4.23 for sea ice types, the same three different regions can be distinguished for snow regimes, too: I) moderate thickness of 0.34 m in the MIZ, II) moderately to very thick (mean: 0.53 m), highly variable snow cover on the band of SYI and FYI in the central part of the study area and III) very small snow thickness of 0.09 m (mean of ruler measurements) in the Larsen Polynya area (south-west).
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a) Mean thickness b) Modal thickness c) Standard deviation
Fig. 4.19: Snow thickness statistics obtained from direct, snow stick measurements. Larsen Polynya stations are not shown for plot scale reasons. The point-size legend is valid for a) to c).
The thick snow on SYI indicates that part of the snow has remained from the last summer. The thin snow cover of less than 0.12 m in the Larsen area is probably due to a) low precipitation b) high evaporation and c) the fact, that the sea ice is comparably young and did not accumulate large snow masses during the observed winter time d) strong winds causing snow drift. These aspects might be discussed including δ18O measurements, which were taken from the snow samples (see below).
4.3.2 Ground penetrating radar (GPR) measurements
Snow thickness profiling on sea ice with off-the-shelf geophysical radar devices is a rather innovative approach to snow thickness studies. There is no evidence in literature for any operational application of GPR for snow thickness on sea ice besides our own experiments on ARK-XIX and ARISE 2003. Thus, first results from this cruise set a milestone in the development of radar as a standard geophysical tool in sea ice research.
4. Regional variability of sea ice properties and thickness in the northwestern Weddell Sea obtained by in-situ and satellite measurements
Fig. 4.20: Radar profile from 9 September. Upper panel shows processed radar section along with snow stick results (vertical bars). Picked radar snow thickness along with snow stick thickness is
shown in the lower panel.
In eleven out of twelve acquired radar profiles a clear reflector could be identified as the snow/ice interface. The single failure on 22 September was presumably caused by a very rough ice surface scattering the radar pulse in combination with several internal ice layers in the snow pack (see chapter 4.5). Figure 4.20 shows a typical radar result after several processing steps. The main processing challenge is to clean the data from the direct wave, travelling inside the antenna from transmitter (Tx) to receiver (Rx) dipole. As the Rx-Tx distance is 15 cm in the used antenna, the direct wave interferes with the snow/ice reflection, arriving at similar times as the direct wave. Once the direct wave is successfully removed, the radar section shows one dominant reflection along with several multiples (Fig. 4.20). For snow thickness smaller than ~ 15 cm the reflection is not as clear or totally lacking due to the mentioned interference of direct wave and snow/ice reflection. Thus radar snow thickness profiles are biased towards thicker snow, if thin snow layers are present.
Figure 4.21 underlines this constraint. On 8 September, 2 and 4 October, a significant part of the snow stick profile contained a very thin snow layer, resulting in a residual between snow stick- and radar- derived mean snow thickness. The shift between mean thicknesses on 30 September arises from not coinciding radar- and snow-stick profiles on that day. The radar profile on this day covers a much larger area (~400 m) than the 170 m long tape measure profile.
4.3 Snow thickness distribution
Fig. 4.21: Comparison of a) Radar and snow stick snow thickness statistics as well as b) Radar-velocity-derived dielectric permittivity and permittivity measured with the “Snow fork”. Error bars show
the respective standard deviation for lateral snow thickness profiles (a) and vertical snow fork permittivity sections (b). Snow-fork* in b) delineates snow pits on the same floe as the radar sounding,
whereas otherwise the radar antenna was placed exactly over the snow pit.
To compute snow thickness from radar wave travel times, the speed of light in snow (radar wave velocity, determined by the dielectric permittivity) must be known. The dielectric permittivity of snow is governed by its density and wetness parameters that are measured by the “snow-fork” discussed in chapter 4.5. Thus one method to derive the permittivity of the snow pack is to run an average over the 5 to 10 cm snow-pit sections described in chapter 4.5. Resulting snow-fork permittivities are shown in figure 4.21b. Additionally, radar soundings over known snow thickness (e.g.
over the snow pit) allow determining the radar velocity and thus the dielectric permittivity of the underlying snow. Figure 4.21b provides a comparison of permittivities, either from coinciding radar measurements and snow pits (24.9., 26.9.
& 30.9.) or days when snow pit and radar calibration site where in proximity (Snow-fork*). The variability within the snow pack (standard deviations in Fig. 4.21b) has a similar magnitude as the deviations between radar and snow-fork permittivity.
Generally dielectric permittivies of snow on sea ice are consistent around 1.55 ± 0.1.
The resulting radar velocities vary from 23 to 25 cm/ns, corresponding to a precision of ± 3 cm for 80 cm thick snow. This means that also without a prior knowledge of the floeʼs snow properties, GPR is able to retrieve accurate snow thickness estimates, assuming a wave speed of ~ 24 cm/ns.
4. Regional variability of sea ice properties and thickness in the northwestern Weddell Sea obtained by in-situ and satellite measurements