# Investigations into Frost Flower Physical Characteristics and the C-Band Scattering Response
## 3 Results and Discussion
### Stage 2: Initial Frost Flower Formation and Spatial Proliferation  
Beginning on the morning of 13 January, widespread frost flower formation occurred and their proliferation proceeded spatially from 10% coverage in the morning to cover \(\sim\)70% of the ice surface by mid-afternoon (14:40 on 13 January). This significant change in the characteristics of the newly-formed sea ice surface resulted from changes in the physical properties of the sea ice and micrometeorological variables, which is reflected in changes of the NRCS time series data until 14:40 on 13 January. The initiation of rapid frost flower growth (04:40 on 13 January) was immediately followed by a three-hour period (05:40 to 08:40) during which the surface temperature declined rapidly from \(-\)5 \({}^{\circ}\)C to \(-\)15 \({}^{\circ}\)C before stabilizing at its minimum level for the experiment. Frost smoke was observed above the newly-formed sea ice surface at 05:40. The near-surface wind speed increased after 06:00 (Figure 2), which, in combination with the brine expulsion, began to roughen the previously extremely smooth sea ice surface. With observed frost smoke indicating evaporation of liquid at the surface, frost flowers began to form via the typical mechanism [12; 14]. At 08:40 on 13 January, the ice thickness had reached 3.7 cm with a bulk salinity of 20.3 PSU. A Kimwipe(r) was gently touched to the surface and liquid indicated that the sea ice surface was brine-wetted at this point. The presence of liquid brine on the sea ice surface is indicated in Figures 2 and 4 by the solid horizontal orange bar in each panel. Frost flowers sampled at 08:40 were saline with heights of 5 \(\pm\) 1 mm and surface coverage remained \(\sim\)10%. Differential air-surface temperature plateaued at 15 \({}^{\circ}\)C for the remainder of the experiment with the surface temperature time series closely following the air temperature for the remaining duration. Figure 6: Backscatter measurements during the process of frost flower development for incidence angles from 25\({}^{\circ}\) to 45\({}^{\circ}\). Polarizations are indicated as VV (solid blue), HH (dashed red), and HV (solid black). The solid horizontal yellow bar indicates when brine was present on the sea ice surface. The maximum near surface wind speed during this widespread frost flower proliferation stage was 2 m/s, with a mean of 1.74 m/s (Figure 2c), and did not vary much. The relative humidity 70 cm above the surface of the ice remained high (mean = 79% during this period). The sky was overcast for a few hours (approximately 12:00 noon to 14:00) and the sea ice temperature remained stable when the ice continued to grow thicker (Figure 2b). By 13:40, the ice thickness had reached 5 cm and had a bulk salinity of 18.9 PSU (Figures 3 and 4) and the sea ice surface below the frost flowers remained brine-wetted. After 14:00, the temperature of the surface and the ice volume warmed and then cooled again (Figure 2b), which corresponded to direct illumination of the surface by solar radiation for a few hours after 14:00. The surface salinity had increased to 49.8 PSU at 14:40 on 13 January (by a factor of 3.3 since 08:40). At 14:40, the bottom 2-3 mm of the frost flowers (still 5 mm tall) was still immersed in a brine-wetted layer and their bulk salinity was 90.9 PSU (including the brine-wetted portions). The depth of the brine-wetted layer was measured at multiple points within the physical sampling area using a ruler. Frost flower salinities observed in this experiment were comparable with frost flower salinities in the literature (see Figure 4) and were always greater than the observed sea ice surface salinity, which indicates that there was a concentration of salt and other impurities upwards into the frost flowers. Frost flower salinities measured here were much larger than a previous SERF experiment [19], which indicates substantial variability in this process. Other frost flower salinity measurements include those by [PERSON] et al. [16] (52 to 107 PSU), [PERSON] et al.