Observations of the Size Distribution of Frazil Ice in an Ice Shelf Water Plume

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 cm3 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. Key Points Acoustic scattering‐derived frazil ice populations have been observed down to 30 m in Ice Shelf Water beneath Antarctic sea ice Assuming a log‐normal distribution, mean frazil crystal diameter is ∼1 mm at 15 m below sea level and ∼13 km from the ice shelf front Model‐derived fractional ice volume correlates with in situ supercooling of up to 50 mK at 15 m below sea level