Characterization of Snow Geophysical Changes on Arctic First-Year Sea Ice Using Multi-Frequency Scatterometry during the Late-Winter to Early-Melt Transition

The Arctic is on the path to a new climate regime influenced by thinner first-year ice (FYI). The Arctic has recently observed an extensive loss of multi-year ice (MYI) replaced by FYI, and decline of spring snow depth. A warming Arctic delays sea ice freeze-up which could lead to thinner FYI, thereby decreasing adequate time for snow accumulation on FYI. Thinner snow covers in a warming Arctic may become more saline due to the amplified vapor and thermodynamic gradients between the atmosphere and ocean. Snow electro-thermophysical properties on FYI change temporally over hourly to seasonal time scales throughout the FYI evolution regime, from freeze-up to advanced melt. Snow accumulation and redistribution of snow on FYI exhibits high spatiotemporal variability, and plays a central role in the mass and energy exchange across the ocean-sea ice-atmosphere (OSA) interface by modulating sea ice growth and decay processes.  Active microwave remote sensing have been proven to be an efficient tool widely used to characterize the thermodynamic state of snow covered FYI, where snow cover plays a pivotal role in microwave propagation and scattering within the snow/sea ice system. During freeze-up and early winter seasons, in homogeneous dry (cold) snow conditions (with almost zero incoming shortwave radiation), microwaves largely penetrates through the snow pack with little attenuation from the basal snow layers, and/or snow/sea ice interface, primarily contributed due to presence of brine. However, during early melt season, for a given snow thickness, more complexity in microwave interactions is added due to snow pack metamorphism (rapid kinetic snow grain growth), air pressure, persistent cloud cover, and diurnal oscillations in the air temperature (due to varying shortwave and longwave incoming/outgoing radiation) affecting snow layer temperatures and brine volume. These fluctuations in snow thermodynamic and geophysical properties can lead to complex microwave scattering mechanisms, increased absorption of microwave energy, hence underestimating the snow thickness retrievals on FYI.

Understanding the sensitivity of multi-frequency microwave scattering to changes in snow geophysical properties driven by atmospheric forcing on Arctic FYI requires investigation. We present a case study based on a time-series observational Ku-, X- and C-band surface-based fully-polarimetric microwave scatterometer system (Fig: 1(a) to better understand the sensitivity of varying snow thermodynamics on microwave scattering, from late-winter to early melt. The study site is a 16 cm highly saline snow cover over smooth FYI, near Resolute Bay, Nunavut, Canada. The in-situ snow property measurements (snow temperature, salinity, density and grain radius), hourly on-ice air temperature and air pressure were acquired from 17^th to 24^thMay 2012 (Fig: 1(b, c). Modeled surface radiation measurements obtained from NCEP North American Regional Reanalysis (NARR) data are also used to understand codependence of atmospheric forcing on warming snow geophysical and thermodynamic properties, in turn affecting the microwave backscatter at Ku-, X- and C-bands. Along with co-polarized microwave scattering coefficients and co-pol ratio (CPR), we introduce dual-frequency ratio (DFR) and normalized difference frequency index (NDFI) as new additional parameters to quantify the influence of atmospheric forcing on snow geophysical properties, influencing microwave interactions at all three frequencies.  

During the warming period from 21^st to 24^th May, preliminary results suggest strong influence of warming air temperatures, low air pressure, down welling shortwave radiation and reflected shortwave, influencing increase in microwave backscatter for all three frequencies. All three frequencies show substantial differences in co-polarized backscatter throughout the incidence angle range, with C-band exhibiting the strongest change. Observable increase in snow temperatures (Fig: 1(d)) on the highly saline snow cover facilitates an increase in brine volume and snow dielectrics, thereby increasing the backscatter for all three frequencies. Strong statistical correlation (~90%) was observed between upwelling shortwave against all microwave parameters for all three frequencies, suggesting the presence of liquid water content in the saline snow pack, during the warming phase. Moist saline snow cover facilitates increase in VV and HH backscatter (Fig: 1(e)) throughout the incidence angle range, and exhibit sensitivity to CPR, DFR (Ku/C and X/C) and NDFI (Ku,C), especially at mid- and far-range incidence angles. Warming air temperatures exhibit an “out of phase” relationship with backscatter for three frequencies, suggesting a lag effect of air temperature on backscatter, owing to low thermal diffusivity of snow cover. DFR and NDFI exhibit stronger sensitivity to cloud cover (down welling longwave) than other microwave parameters, especially at mid-range incidence angles. Air pressure shows a strong negative correlation with co-pol backscatter and CPR. This could be due to enhanced brine wicking from the ice surface into the basal layer of the snow, under a low pressure system, promoted with an increase in brine volume under a warming atmosphere. On the other hand, DFR and NDFI show strong positive relationship, which could provide additional information on the frequency sensitivity to atmospheric forcing, which drives the snow geophysical and thermodynamic properties. However, this requires further investigation. Based on our initial observations, the thermal dependence of multi-frequency microwave backscatter over highly saline snow on FYI shows promise to provide an initial foundation to understand its sensitivity to atmosphere-snow interface, necessary for snow thickness estimation on FYI.