Laboratory study of the properties of frazil ice particles and flocs in water of different salinities

Abstract

Abstract. Measurements of the size and shape of frazil ice particles and flocs in saline water and of frazil ice flocs in freshwater are limited. This study consisted of a series of laboratory experiments producing frazil ice at salinities of 0 ‰, 15 ‰, 25 ‰ and 35 ‰ to address this lack of data. The experiments were conducted in a large tank in a cold room with bottom-mounted propellers to create turbulence. A high-resolution camera system was used to capture images of frazil ice particles and flocs passing through cross-polarizing lenses. The high-resolution images of the frazil ice were processed using a computer algorithm to differentiate particles from flocs and determine key properties including size, concentration and volume. The size and volume distributions of particles and flocs at all four salinities were found to fit log-normal distributions closely. The concentration, mean size, and standard deviation of flocs and particles were assessed at different times during the supercooling process to determine how these properties evolve with time. Comparisons were made to determine the effect of salinity on the properties of frazil ice particles and flocs. The overall mean size of frazil ice particles in saline water and freshwater was found to range between 0.52 and 0.45 mm, with particles sizes in freshwater ∼13 % larger than in saline water. However, qualitative observations showed that frazil ice particles in saline water tend to be more irregularly shaped. The overall mean size of flocs in freshwater was 2.57 mm compared to a mean size of 1.47 mm for flocs in saline water. The average growth rate of frazil particles was found to be 0.174, 0.070, 0.033, and 0.024 mm min−1 and the average floc growth rate was 0.408, 0.118, 0.089, and 0.072 mm min−1 for the 0 ‰, 15 ‰, 25 ‰, and 35 ‰, respectively. Estimates for the porosity of frazil ice flocs were made by equating the estimated volume of ice produced based on thermodynamic conditions to the estimated volume of ice determined from the digital images. The estimated porosities of frazil ice flocs were determined to be 0.86, 0.82, 0.8 and 0.75 for 0 ‰, 15 ‰, 25 ‰ and 35 ‰ saline water, respectively.

Continuous in situ measurements of anchor ice formation, growth and release

“…Newly formed anchor ice accumulations likely have higher porosities because they often do not maintain their structural integrity when sampling is attempted. The porosity of frazil ice flocs has been estimated to be approximately 0.80 (Schneck et al, 2019) and this may represent a reasonable upper limit for the porosity of newly deposited anchor ice. Values for the accretion rate γ in the literature range from approximately 10 -6 to 10 -3 (Malenchak, 2011) and measurements of the volumetric concentration of frazil Cv reported in the literature vary from 10 -6 to 10 -2 (McFarlane et al, 2019).…”

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

“…where N is the number density of scatterers and g is the probability distribution of scatterers' diameter. Following and and supported by recent observations (McFarlane et al, 2015(McFarlane et al, , 2017(McFarlane et al, , 2019Schneck et al, 2019) and theoretical considerations (Crow & Shimizu, 1988), we choose a log-normal distribution, g ∼ Λ( , ). We associate Σ th bs ( ) with the measured backscattering cross section Σ obs bs ( ).…”

“…Only Schneck et al (2019) have made laboratory measurements of frazil size distributions in salt water and shown they again follow a log-normal distribution. The ice crystal diameters are ∼13% smaller than in fresh water, with a mean diameter of 0.45 mm, standard deviation 0.31 mm, while flocs have a mean of 1.47 mm and standard deviation of 1.28 mm (Schneck et al, 2019). McFarlane et al (2017McFarlane et al ( , 2019 also summarize the methods of detection of suspended frazil in laboratory and river studies.…”

“…Suspended frazil ice crystals form in turbulent fresh or salt water that is colder than its salinity‐ and pressure‐dependent freezing temperature, a state referred to as in situ supercooled (e.g., Daly, 1984; Martin, 1981; Schneck et al, 2019; Tsang & Hanley, 1985). In natural water bodies in situ supercooling can be generated in numerous ways (Martin, 1981), but two are of particular importance in the Southern Ocean.…”

“…Quantitative observations of the shape and size of individual frazil ice particles in salt water of ocean salinity are sparse, with suspended ice crystal diameters ranging 1–3 mm in laboratory experiments (e.g., Martin, 1981; Schneck et al, 2019; Smedsrud, 2001) and an upper bound of 10–25 mm in the ocean (Dieckmann et al, 1986; Gough et al, 2012; Penrose et al, 1994). Only Schneck et al (2019) have made laboratory measurements of frazil size distributions in salt water and shown they again follow a log‐normal distribution. The ice crystal diameters are ∼13% smaller than in fresh water, with a mean diameter of 0.45 mm, standard deviation 0.31 mm, while flocs have a mean of 1.47 mm and standard deviation of 1.28 mm (Schneck et al, 2019).…”