# Investigations into Frost Flower Physical Characteristics and the C-Band Scattering Response
## 3 Results and Discussion
### Late Stage  
During the third stage of this experiment, from 14:40 on 13 January to 08:40 on 14 January, the sea ice surface was covered with frost flowers (Figures 7 and 8) that were no longer developing vertically or increasing in spatial coverage. Kimwipe(r) tests indicated that the sea ice surface was still brine-wetted during this stage. From 14:40 to 20:40 on 13 January, the air and surface temperature marginally increased by 1-2 \({}^{\circ}\)C (Figure 2). Near surface wind speed remained low (mean wind speed for the six-hour period was 2.07 m/s) and the relative humidity increased from 80% to 85%. For a short period of time \begin{table}
\begin{tabular}{c c c} \hline \hline
**Incidence Angle (\({}^{\circ}\))** & \(h\) **(cm)** & \(L_{c}\) **(cm)** \\ \hline
20 & 0.23 & 0.68 \\
25 & 0.23 & 0.72 \\
30 & 0.26 & 0.80 \\
35 & 0.25 & 0.90 \\
40 & 0.26 & 0.99 \\
45 & 0.25 & 0.80 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Evaluated surface roughness parameters derived from LiDAR measurements. Figure 9: LiDAR Roughness Parameters. (**a**) Correlation Length and (**b**) Root Mean Square Height. (approximately from 14:40 to 15:40), the sun directly illuminated the sea ice surface, which accounts for the brief temperature increase in the air, sea ice surface, and sea ice volume temperatures over that hour (Figure 2). At 20:40 on 13 January, the average height of the frost flowers remained 5 mm (Figure 3), the frost flower bulk salinity was 98.5 PSU (Figure 4), and the morphology had not substantially changed. A Kimwipe(r) was able to pick up liquid brine from the surface. The surface salinity had increased to 56 PSU, the sea ice thickness reached 6 cm, and the bulk sea ice salinity was 19.5 PSU. Overnight, from 20:40 on 13 January to 08:40 on 14 January, the air and surface temperatures decreased and near surface wind speed decreased to 0.58 m/s and remained \(\sim\)0.5 m/s for the remainder of the experiment (Figure 2). The sea ice temperature in the upper portion of the sea ice slightly increased in the evening as a result of energy accumulated at the surface during the daylight hours and then followed the cooling trend of the atmosphere (Figure 2). At 08:40 (sunrise) on 14 January, we observed that the frost flowers were a mix of stellar dendritic and hexagonal plate crystals. The surface salinity had decreased to 49.7 PSU and the frost flower salinity had decreased to 82 PSU. The lower part of the frost flowers were brine-wetted, which was distinct from the underlying solid sea ice. This portion was included in the frost flower samples. The average sea ice thickness was 8 cm. Observations of the sea ice crystal structure showed that the upper 2 cm were randomly oriented crystals, consistent with the frazil ice growth, which was followed by a 1 cm transition layer (both of which are indicative of extremely quiescent growth conditions) and a 5 cm layer of congelation growth. Scatterometer measurements exhibited differences throughout this time-period (i.e., 14:40 on 13 January to experiment end), which is dependent upon the incidence angle (see Figure 6). For 25\({}^{\circ}\) incidence angle, the co-polarized backscatter decreased from 14:40 on 13 January to 00:40 on 14 January with a commensurate increase in cross-polarized backscatter. The co-polarized ratio reached approximately unity from 21:00 to 23:00 on 13 January. This is coincident to the maximum air temperature, surface temperature, and relative humidity. Assuming that surface roughness did not change appreciably during this short time, these results suggest that volume scattering was enhanced. In reality, the scattering process is very complex since the sea ice surface roughness and underlying ice may also contribute to backscatter. At 00:40 on 14 January, the co-polarized backscatter reached a local minimum for a 25\({}^{\circ}\) incidence angle and increased, which reached a local maximum at the end of the experiment. From 05:40 on 14 January to the end of the experiment, the co-polarized ratio was \(<\)1 dB.