# Investigations into Frost Flower Physical Characteristics and the C-Band Scattering Response
## 1 Introduction
Microwave propagation and scattering in sea ice are governed by the dielectric properties of the constituent materials, the size and shape of inclusions, and interface roughness. Since the dielectric constants of liquid brine and seawater are substantially greater than pure ice, the presence of liquid greatly influences electromagnetic wave interaction with the bulk material. Changes in the physical state due to environmental forcing affect the dielectric constant, which allows microwave scattering measurements to be linked to changes in the sea ice thermo-physical state. Frost flowers have been the subject of remote sensing studies at scales ranging from satellite to on-site Arctic and laboratory studies under controlled conditions [19; 20]. However, the ephemeral nature of frost flower initiation and growth has made a cross-comparison for scaling the lab and field studies to the satellite level quite challenging. The formation of frost flowers on sea ice results in an increase in microwave backscatter at C-band. This has been documented by field studies (e.g., a difference of 6 dB for frost flower covered sea ice versus bare sea ice [17]). Some of the earlier discussions and analysis of scattering from frost flowers on sea ice are summarized and presented in References [21; 22]. Laboratory studies and mesocosm studies have demonstrated the increase in the backscatter as a function of frost flower formation on sea ice (e.g., increase of 6 dB, [20]). From a satellite perspective, previous analysis of ERS-1 synthetic aperture radar (SAR) C-band backscattershowed increases of 5-10 dB [23] and 10-15 dB [24] when frost flowers appeared. Further progression of the backscattering values is highly dependent upon environmental conditions (e.g., wind, snowfall) and sea ice dynamics [24]. In a recent campaign focused on newly-forming sea ice (including frost flowers in Arctic leads), satellite-based SAR measurements at L-band, C-band, and X-band data were studied [25]. They suggested that L-band, due to greater penetration depth and lower sensitivity to surface structures, could be more useful than the C-band and X-band scenes for identifying regions of frost flowers [26]. Previous work at our sea ice mesocosm known as the Sea-ice Environmental Research Facility (SERF) showed that a near-surface based C-band polarimetric radar system could identify several stages of frost flower growth and development [19]. In the experiment, frost flower initiation resulted in a major increase in backscatter for all incidence angles and polarizations. Brine expulsion covering the sea ice surface with liquid was clearly captured by the radar data as a local minimum in the time-series scattering. The experiment continued with a local backscatter maximum followed by a slight decrease of backscatter as the frost flowers size and the areal coverage increased. A final stage of frost flower decimation (i.e., collapse and coalescence) was only identifiable in the backscatter results at low incidence angles (25\({}^{\circ}\)) where the co-polarization ratio reached near unity. Recognizing that rapid changes in the thermodynamic and physical state of the surface were the primary causes of observed scattering behavior of frost-flower-covered sea ice (e.g., [19]), we planned our experiment to occur during atmospheric and sea ice physical conditions that would be conducive to frost flower growth. The overarching objective of this paper is to present the results from our experiment in January 2017 at SERF, which focused on the observation and analysis of the physical processes and C-band scattering characteristics of a frost-flower-covered sea ice surface over a 36-h period. Throughout this work, we compare and contrast the physical and microwave backscattering evolution of frost flowers of this second dedicated experiment with results from Reference [19] by highlighting the differences in the processes, frost flower formation, and scattering behavior. Specifically, we address the following three objectives. 1.