Characterising Frazil Ice Populations using Acoustic Techniques

Abstract

There is a demand for quantitative frazil ice size observations in order to fully model an Ice Shelf Water (ISW) plume describing the outflow of supercooled melt water from underneath ice shelves. These outflows have significant impacts on the growth of the sea ice surrounding ice shelves. Here, we develop a method to fully characterise a frazil population size distribution using acoustic techniques. Previous work in the field has thus far almost entirely been focused on frazil ice in rivers and lakes using spherical scattering in an equivalent sphere assumption, and often using only one or two acoustic frequencies. In this thesis, an oblate spheroid model is combined with a log-normal size distribution for the diameters of frazil crystals to create an analytic formula for the total scattering of a frazil population. Acoustic observations were collected across four deployments in November 2016 and November 2017 from McMurdo Sound, all of which are believed to be on the path of the northwards flowing ISW plume from the McMurdo/Ross Ice Shelf cavity. The optimisation approach was successfully applied to these four-frequency data, producing time series of believable population characteristics. A deployment from 4-9 Nov 2017 is analysed in detail, with median diameter (exp(μ)) of the population for analysed results spanning 0.16mm to 0.50mm, and number densities N of 320m^(-3) to 32 000m^(-3). The optimised spread parameter, σ, returned higher values than expected (~1.3) for one of the two favoured solutions of the optimisation process, which produces results of physical interest. These parameters correspond to frazil populations with mean diameters ranging from 0.37mm to 1.16mm with standard deviations from 0.8mm to 2.4mm. Unlike smaller equivalent sphere assumptions that cannot accurately measure larger sizes, the broad dimensions of the parameter optimisation have plausibility for the inclusion of rarer, large particles as have occasionally been qualitatively observed in photographs and videos under the sea ice in the McMurdo Sound ISW outflow. Supporting oceanographic evidence showed strong positive correlations between the fractional ice volume calculated from the optimised parameters and supercooling in the water column.