Observations of the size distribution of frazil ice in an Ice Shelf Water plume

Abstract

The size distribution of frazil ice is currently unconstrained in ice shelf cavity modeling.Here we observe the time-dependent behavior of the number and size of frazil ice particles in an Ice Shelf Water plume. A novel acoustic scattering inversion was used to infer frazil ice crystal diameters, assuming a log-normal distribution. Observation sites were on land-fast sea ice approximately 13 and 33 km from the front of the McMurdo Ice Shelf, Antarctica. The water column from the ice-water interface to 30 m below mean sea level was monitored over 3 weeks in November of 2016 and 2017. At 15 m below sea level the mean frazil crystal diameter was ∼1 mm. Fractional ice volume, derived from frazil crystal size and number density, correlates with in situ supercooling (up to 50 mK at 15 m below sea level). The data presented here provide valuable input for model initiation and evaluation.Plain Language Summary For the first time we have observed the number and sizes of tiny, disc-like, crystals that appear beneath the springtime sea ice of McMurdo Sound, Antarctica. They are generated by melting at the base of gigantic floating glaciers that surround the Antarctic continent, and are carried out beneath the sea ice in water that is just below its freezing point. From sonar measurements we have found that at 15 m below sea level, there is about one disc-shaped ice crystal with an average diameter of approximately 1 mm in each 10 cm 3 of sea water. Previously, there have been no observed sizes of these ice crystals to guide modeling of the interaction between glaciers and the ocean, and our new results provide valuable input for model initiation and evaluation.

The Effect of Pseudofrazil Particle Entrainment on Salinity Measurements

“…In earlier measurements, Purdie (2012) found likely frazil ice concentration in McMurdo Sound from acoustic backscatter to be between ∼2.2 × 10 −7 m 3 m −3 to 2.2 × 10 −5 m 3 m −3 (0.2 g m −3 to 20 g m −3 ), another one to three orders of magnitude lower than the concentrations from Frazer et al. (2020).…”

“…Only the most extreme frazil concentration values result in overestimates of supercooling higher than the highest observed level of supercooling in some publications. This shows how important it is to be able to quantify particle concentrations precisely (e.g., Frazer et al., 2020) in order to be able to quantify any possible effects of frazil on conductivity measurements. Our values for the maximum freezing point offset to be expected from naturally occurring frazil ice concentrations are lower than the supercooling threshold of 10 mK derived by Skogseth et al.…”

“…The release of latent heat by ice formation decreases supercooling and at the same time removes freshwater in the form of ice formed on surfaces including on scientific equipment (anchor ice; e.g., Mager et al., 2013; Robinson et al., 2020), or “fine spicules or plates of ice, suspended in water” (JCOMM Expert Team on Sea Ice, 2015), frazil ice (see Daly, 1984, 1994, for reviews on frazil ice properties). Frazil crystals have been observed during episodes of supercooling in studies using both conductivity, temperature, depth (CTD) and acoustic sensors (e.g., Drucker et al., 2003; Frazer et al., 2020; Ito et al., 2015; Leonard et al., 2006).…”

“…A large body of literature exists on field measurements of frazil‐ice properties in fresh water (e.g., Ghobrial et al., 2013; Gosink & Osterkamp, 1983; Marko & Jasek, 2010; Marko et al., 2015; McFarlane et al., 2015, 2017, 2019; Morse & Richard, 2009), mainly due to the engineering challenges to infrastructure such as hydro dams (e.g., Gebre et al., 2013). Field measurements of frazil‐ice properties in seawater are less common (e.g., Frazer et al., 2020; Ito et al., 2015; Pegau et al., 1996; Penrose et al., 1994; Purdie, 2012).…”

“…Frazer et al. (2020) assumed a log‐normal distribution of particle sizes in the surface ocean of McMurdo Sound, Antarctica, and used an inverted backscattering model to derive ice volume fraction from acoustic backscattering of a four‐frequency acoustic profiler. Results suggest an average frazil diameter of 1 mm and an ice volume fraction of ∼1 × 10 −6 m 3 m −3 to 1 × 10 −4 m 3 m −3 , at least an order of magnitude smaller than the maximum frazil volume fraction observed in laboratory experiments (Schneck et al., 2019).…”