# Investigations into Frost Flower Physical Characteristics and the C-Band Scattering Response
## 3 Results and Discussion  
### Stage I: Initial Sea Ice Growth  
The experiment started on 12 January after the roof had been retracted and the heaters and pumps were turned off at 20:40. Water vapor gradually rose from the water surface at the experimental start time. Frazil ice began to accumulate in the SERF almost immediately. A continuous sea ice surface formed shortly before 23:00 (12 January). The sky was clear, the air temperature was \(<\)\(-25\)\({}^{\circ}\)C, and the surface wind speed was very low (\(<\)1 m/s) (Figure 2). The air temperature decreased overnight and reached a minimum of \(-32.4\)\({}^{\circ}\)C at 05:18 before slowly increasing. The cold overnight air temperature cooled the near-surface volume during the first 12 h of growth (Figure 2b) with the surface temperature reaching a minimum of \(-14.9\)\({}^{\circ}\)C at 06:54. The air-surface temperature difference (i.e., T\({}_{\text{surface}}\)-T\({}_{\text{air}}\), where positive values indicate a warmer surface overlain by colder air) ranged between 17 \({}^{\circ}\)C and 27 \({}^{\circ}\)C until about 06:00 on 13 January when it steadied around 15 \({}^{\circ}\)C. The near-surface wind speed stayed very low throughout the night until 06:00 (13 January) when it began to increase marginally (Figure 2). Physical sampling during this stage occurred overnight at 22:40 (12 January), and at 00:40 (13 January), and 04:40 (13 January). At 22:40 on 12 January, the initial dark nilas sea ice surface grew extremely smooth and remained dry. A Kimwipe(r) was unable to pick up any liquid from the surface. At 04:40 on 13 January, the sea ice was 2.7 cm thick with a bulk salinity of 22.3 PSU (Figures 3 and 4) and the surface was dry again. Frost flower coverage increased from a few sparse flowers at 04:40 to about 10% areal coverage by 07:40 (Figure 5). Although the micrometeorological conditions show that frost flower formation might have been expected during this initial sea ice growth period (i.e., strong temperature gradient and low wind speed), two physical factors were absent, which negated frost flower initiation. Liquid brine was absent from the sea ice surface throughout the overnight period so the near-surface air lacked a water vapor source to saturate it and the sea ice surface was exceptionally smooth and void of any raised nodules that might have served as relatively cold condensation points (compared to the mean ice surface). Photographic inspection (Figure 5) indicated the absence of raised nodules on the surface of the sea ice during this period. In this case, the absence of frost flowers supports the necessity of these physical factors for frost flower formation previously postulated in References [12; 14].  
backscattering was much lower than co-polarized backscattering) [19; 36]. The co-polarization ratio (VV/HH) increased with an increasing incidence angle, which is typical for thin ice types [36].  
Figure 3: Measured sea ice thickness and frost flower heights. Surface liquid brine presence indicated by solid horizontal bar starting at 8:40 a.m.  
Figure 2: Measured environmental conditions. **(a)** Temperature (air and surface); **(b)** thermocouple profile in the sea ice; (c) relative humidity and wind speed.  
Figure 4: Mean salinity measurements during the experiment for bulk sea ice, frost flowers, and the sea ice surface. Surface liquid brine presence indicated by solid horizontal bar starting at 08:40.  
Figure 5: The sea ice surface at 07:40 on 13 January prior to sunrise.