# Investigations into Frost Flower Physical Characteristics and the C-Band Scattering Response
## 1 Introduction  
The Arctic iescape is transitioning from a persistent cover of multi-year sea ice to predominantly first-year ice [1; 2; 3]. Along with this transition to thinner ice, sea ice mobility is increasing [4; 5], which results in the potential for more frequent lead openings in sea ice during the winter season [4]. Frost flowers are a transient phenomenon that occur on newly-forming sea ice, which have received increased attention in recent years due to their influence on physical, radiative, and biogeochemical characteristics of the ocean-sea ice-atmosphere interface [6]. Frost flowers are fragile ice crystals that may grow into different structural forms depending on the atmospheric and boundary layer conditions that are present during their initial formation [7]. Thermodynamically, they act as an insulating layer at the ice-atmosphere boundary, which modifies the heat and radiative fluxes and influences the sea ice growth rate [8]. They also exhibit very high bulk salinity, which is thought to be caused by liquid brine being wicked up from a skim on the underlying sea ice surface. They continue to be the subject of field studies by atmospheric chemists. For example, [PERSON] et al. [9] recently evaluated the presence of sea salts, frost flowers, atmospheric aerosol particles, and proposed sea-salt cycles. Frost flowers have also received attention for their role as a potential microbial habitat [10] and their impact on carbon cycling, which the newly-forming sea ice provides as a source of CO\({}_{2}\) to the atmosphere [11]. Certain atmospheric conditions near the surface and in the far-field favor the initiation and growth of frost flowers on new sea ice [12; 13; 14]. Micrometeorological conditions that favor frost flower formation include a large temperature difference between the relatively warm, newly-forming sea ice surface and the cold \"far-field\" atmosphere (e.g., where the \"far-field\" indicates the temperature at a theoretically infinitely large distance from the ice-atmosphere interface imposed as a boundary condition [12]). Additionally, frost flowers may grow when a low near-surface wind speed permits a water vapor pressure gradient to form between the surface (saturated air with respect to water) to a \"far-field\" height from the surface (sub-saturated). The low wind speeds and laminar surface flow preserves the shape of that gradient above the surface [14]. While the above micrometeorological conditions favor widespread frost flower initiation and spatial proliferation, they are not sufficient for producing frost flowers. Previous work [7; 13; 14] indicated that frost flowers require a vapor source, such as a liquid brine skim on the sea ice surface developed due to brine expulsion, which creates evaporative conditions at the surface due to super-saturation at the air-liquid brine skim interface. [PERSON] et al. [14] further observed that mm-scale surface nodules are raised above the mean sea ice surface with lower surface temperatures than the brine skim surface, which induced frost flower initiation by providing condensation points for the supersaturated near-surface air. This agrees with the suggestions of Reference [15]. Once frost flower crystals have formed, high salinity brine on the sea ice surface is subsequently wicked up into the frost flower structure. The formation, growth, and eventual decay of frost flowers is a transient event that occurs over a time frame of 12 h to 48 h based on previous research [13; 14; 16; 17]. Microwave remote sensing is used to monitor and evaluate thermodynamic and physical changes in sea ice from a local scale to a hemispheric scale [17; 18]. Physically, sea ice is a complex, inhomogeneous substance consisting of a pure ice background embedded with brine and air inclusions. Its surface can be electromagnetically rough and/or covered with a layer of snow or frost flowers.