Physical properties of snow Objectives

Physical snow properties are highly variable even on small horizontal scales. These spatial and temporal variations in the snow pack characteristics (e.g. temperature, density, stratigraphy) and its dimension have a crucial impact on the energy and mass budget of Antarctic sea ice.

Therefore, the snow pack on different ice floes is characterized in detail.

Snow stratigraphy will be used as ground truth for the interpretation of retrieved snowmelt signatures from passive microwave data.

Work at sea

The work on physical snow properties was performed on six of the sampled ice floes. The work can be subdivided in two parts: Snow pits to describe essential physical snow parameters and its stratigraphy, and SnowMicroPen measurements deriving a high-resolution density profile of the prevalent snowpack.

Snow pits

Snow pit measurements were taken on the undisturbed shaded working wall of the snow pit.

At first, the temperature was measured every 1 to 5 cm from the top (snow-air interface) to the bottom (snow-ice interface) with a hand-held thermometer (Testo). In a next step the different

Fig. 6.6: Exemplary temperature and salinity profiles from sea-ice core retrieved on 22 February 2018 at

station PS111_SIP_14

layers in the snow pack and its stratigraphic parameters were described. For each layer the snow grain size and type (e.g. rounded crystals, facetted crystals, depth hoar) is determined by the magnifying glass and a 1-to-3-mm grid card. In addition, every layer was characterized by its hardness with the following categories: fist (F), 4 fingers (4F), 1 finger (1F), pencil (P), and knife (K). Afterwards, the density of each layer was measured volumetrically by removing a defined snow block with a density cutter of snow from each layer (density cutter weight: 155 g, volume: 100 ml) and weighting it with a spring scale. In addition, measurements of liquid water content (in Vol. %) were performed with a Snow Fork (Toikka, Finland) through the di-electrical properties of the snowpack. Snow Fork measurements were performed twice every 2 cm from the top to the bottom. Moreover, snow samples were taken from the bottom snow layer at three measurements sites for salinity measurements on board. At two sites, additional snow samples were taken from the top of the snowpack for later isotope analysis back at AWI, Bremerhaven.

Overall, 16 snow pits were analyzed on 6 different ice floes. Table 6.6 summarizes the explicit conducted measurements for each snow pit.

Tab. 6.6: Overview of all sampled snow pits. Possible conducted measurements are:

temperature (T), density (r), stratigraphy (STR), liquid water content (LWC), and salinity (S). X indicates done measurements PS111_SIP_2-1 PS111_SIP_2-1_SPIT01 2018-02-08 14:25 -75.1063 -41.8088 51 X X X

PS111_SIP_3-2 PS111_SIP_3-2_SPIT01 2018-02-11 13:15 -74.9868 -59.6174 33 X X X PS111_SIP_3-2 PS111_SIP_3-2_SPIT02 2018-02-11 14:30 -74.9851 -59.6106 50 X X X PS111_SIP_3-2 PS111_SIP_3-2_SPIT03 2018-02-11 15:25 -74.9839 -59.6018 38 X X X PS111_SIP_7-1 PS111_SIP_7-1_SPIT01 2018-02-15 13:36 -75.3471 -41.1960 26 X X X X PS111_SIP_7-1 PS111_SIP_7-1_SPIT02 2018-02-15 14:20 -75.3452 -41.1774 14 X X X X PS111_SIP_7-1 PS111_SIP_7-1_SPIT03 2018-02-15 14:52 -75.3438 -41.1641 21 X X X X PS111_SIP_8-4 PS111_SIP_8-4_SPIT01 2018-02-16 14:49 -75.9713 -36.4672 21 X X X X PS111_SIP_8-4 PS111_SIP_8-4_SPIT02 2018-02-16 15:32 -75.9710 -36.4836 14 X X X X PS111_SIP_10-3 PS111_SIP_10-3_SPIT01 2018-02-18 12:28 -76.8575 -35.5574 16 X X

PS111_SIP_10-3 PS111_SIP_10-3_SPIT02 2018-02-18 13:12 -76.8497 -35.5542 25 X X PS111_SIP_10-3 PS111_SIP_10-3_SPIT03 2018-02-18 13:45 -76.8488 -35.5543 26 X X PS111_SIP_14-3 PS111_SIP_14-3_SPIT01 2018-02-22 12:35 -76.9238 -34.2591 15 X X X

PS111_SIP_14-3 PS111_SIP_14-3_SPIT02 2018-02-22 13:08 -76.9212 -34.2519 24 X X X X PS111_SIP_14-3 PS111_SIP_14-3_SPIT03 2018-02-22 13:44 -76.9189 -34.2439 16 X X X X PS111_SIP_17-1 PS111_SIP_17-1_SPIT01 2018-03-03 13:05 -72.8199 -27.6185 69 X X X X

SnowMicroPen

The SnowMicroPen (SMP) is a high-resolution snow penetrometer. It measures the bonding force between snow grains, with high spatial resolution and high speed. During the measurement, the SMP is pressed down to the snow surface while the rod is driven into the snow pack. A piezoelectric force sensor measures penetration resistance as function of depth.

The measured data is displayed on the controller and stored in binary format on a SD card.

6.4 Physical properties of snow

During PS111 the SMP was used in two different ways. On the one hand, 4 to 7 SMP measurements were conducted close to each snow pit as reference measurement to the manual measurements. On the other hand, we performed transect measurements on straight lines with two measurements per 0.5 to 2 m.

An overview of all measurements is given in Table 6.7.

Tab. 6.7: Overview of all SMP measurements, subdivided into measurements close to snow pits (_SPIT) and on transect lines (_TRANS). Time and position indicate the starting point of the respective measurement sequence. PS111_SIP_2-2 PS111_SIP_2-2_SMP_TRANS01 2018-02-08 14:19 -75.1064 -41.8093 19 0.5 m PS111_SIP_2-2 PS111_SIP_2-2_SMP_SPIT01 2018-02-08 14:54 -75.1070 -41.8128 7

PS111_SIP_3-3 PS111_SIP_3-3_SMP_TRANS01 2018-02-11 13:12 -74.9868 -59.6177 101 0.5 m PS111_SIP_3-3 PS111_SIP_3-3_SMP_SPIT01 2018-02-11 14:28 -74.9851 -59.6107 6

PS111_SIP_3-3 PS111_SIP_3-3_SMP_SPIT02 2018-02-11 15:28 -74.9838 -59.6014 5 PS111_SIP_3-3 PS111_SIP_3-3_SMP_SPIT03 2018-02-11 15:56 -74.9835 -59.5954 6

PS111_SIP_7-2 PS111_SIP_7-2_SMP_TRANS01 2018-02-15 13:42 -75.3469 -41.1928 46 2.0 m PS111_SIP_7-2 PS111_SIP_7-2_SMP_SPIT01 2018-02-15 14:27 -75.3448 -41.1744 5

PS111_SIP_7-2 PS111_SIP_7-2_SMP_SPIT02 2018-02-15 14:48 -75.3440 -41.1657 5 PS111_SIP_7-2 PS111_SIP_7-2_SMP_SPIT03 2018-02-15 15:15 -75.3428 -41.1551 5

PS111_SIP-8-5 PS111_SIP_8-5_SMP_TRANS01 2018-02-16 14:49 -75.9737 -36.4850 70 1.0 m PS111_SIP-8-5 PS111_SIP_8-5_SMP_SPIT01 2018-02-16 15:29 -75.9711 -36.4837 5

PS111_SIP-8-5 PS111_SIP_8-5_SMP_SPIT02 2018-02-16 15:36 -75.9707 -36.4834 5

PS111_SIP_10-4 PS111_SIP_10-4_SMP_TRANS01 2018-02-18 12:20 -76.8581 -35.5570 85 2.0 m PS111_SIP_10-4 PS111_SIP_10-4_SMP_SPIT01 2018-02-18 13:26 -76.8520 -35.5558 5

PS111_SIP_10-4 PS111_SIP_10-4_SMP_SPIT02 2018-02-18 13:48 -76.8495 -35.5542 6 PS111_SIP_10-4 PS111_SIP_10-4_SMP_SPIT03 2018-02-18 13:55 -76.8488 -35.5543 5 PS111_SIP-17-2 PS111_SIP_17-2_SMP_SPIT01 2018-03-03 13:05 -72.8196 -27.6164 4

PS111_SIP-17-2 PS111_SIP_17-2_SMP_TRANS01 2018-03-03 13:13 -72.8190 -27.6127 14 1.0 m

Preliminary (expected) results

Fig. 6.7 shows an example snow pit data set from station PS111_SIP_3. The snow pit was sampled in a representative area of the floe with a snow depth of 38 cm. The snow pack contained of 6 different layers (from top to bottom): thin wind slab crust (1), followed by a soft layer of rounded ice grains (2), a hard wind slab layer (3), again a soft layer of rounded ice grains (4), and a layer of fine-grained old snow (5). Finally, the bottom layer shows typical depth hoar structures (6). Throughout the sampled snowpack the grain sizes increase, which correlated with the strong temperature gradient from -6.9°C (30 cm) to -3.2°C (bottom). Due to clear sky conditions and associated high shortwave solar radiation fluxes, the layer below the crust is slightly heated up.

In addition to the traditional snow pit measurements, about 404 SnowMicroPen (SMP) measurements have been conducted. Those were done, on the one hand, close to all sampled snow pit sites (Fig. 6.7, gray lines), and, on the other hand, on transect lines with a spacing of 0.5 to 2 m. Therefore, Fig. 6.8 shows an extract of 10 SMP profiles (covering 10 m) from a 23-m transect line at station PS111_SIP_3. The measurement site of the transect was chosen

as representative for the ice floe with a homogenous surface appearance. Measurements were taken twice at every measurement point. During post-processing, corresponding measurement profiles were averaged. Even though, the measurement site was visually homogeneous, the internal structures differ significantly on the shown 10-m transect. While the snowpack is rather homogeneous between meter 6 and 10, an increased layering becomes obvious throughout the end of the profile line. This transect line is therefore an excellent example for small scale variabilities on vertical and horizontal scales in the snowpack on Antarctic sea ice.

Fig. 6.8: Extract from a 23-m SMP transect line at station PS111_SIP_3-3_SMP. Plotted vertical profile lines are calculated mean values of two measurements per measurement point. Horizontal lines

indicate the respective snow depth.

Data management

Data from all snow pit and SMP measurements will be delivered to PANGAEA within two years after the cruise.

Fig. 6.7: Example of a snow pit analysis from station PS111_SIP_3_2_SPIT02.

Temperature measurements are marked in red, density measurements with the

density cutter in black, and density measurements with the SMP in gray.

Horizontal lines indicate the different layer interfaces. Below the lines grain type classifications for the respective layer are

given.