Ice‐floe collisions interpreted from acceleration data during LIMEX ‘89

Abstract

Wave-induced ice motions measured during the Labrador Ice Margin Experiment (LIMEX '89) IntroductionThe ice floes encountered during the Labrador Ice Margin Experiment (LIMEX) in March and April 1989 were small by most standards; most floes were less than 15 m across and were approximately 1 m thick. Collisions between the floes are of interest because they influence the lateral deterioration of the floes and contribute to wave attenuation within the ice-pack, particularly when there is continuous contact between adjacent floes within each wave cycle. On a broader scale, collisions between floes have a role in determining ambient noise levels beneath the ice and in the drift of the pack-ice, but neither of these is pursued in the present work. The wave-induced motions and the contact between floes are also of importance, for example, in predicting the dispersal of an oil slick in this environment. Several features of the marginal ice zone environment are expected to influence the nature and frequency of collisions between floes. The present analysis concentrates on the characteristics of the wave field, ambient temperature and local winds. A characteristic feature of small floes is that they tend to follow the motion of the sea surface and are therefore prone to collide on a regular basis according to the wave cycle, in contrast to floes that are larger relative to the wavelength, which have more damped motions.In McKenna and Crocker (1990), the influence of wave amplitude on the relative speed between floes at the point of contact was investigated. Only direct collisions in the direction of wave propagation were considered. The contact was assumed to be inelastic and ceased when the floes drifted apart. Such a situation is likely to cause the largest collision energies for small floes relative to the wavelength (e.g. 10-m floes with a 10-s wave period), but contact with other neighbouring floes is probable when floes are not circular or are offset. Furthermore, collisions are more likely when different wave frequencies and directions are superimposed. For several periods during the LIMEX '89 experiment, the air temperature dropped well below freezing and ice was formed between the floes, occasionally filling the entire space. This meant that there was continuous energy transfer between floes, with each wave cycle, until the ice melted with the heat of the day.Rottier (1991) developed a model for predicting collision frequencies under various wave conditions and floe dimensions in an effort to simulate floe collisions in marginal ice zones. Based on observations in Fram Strait and the Barents Sea, three types of collisions were characterized: (i) a collision that is caused by a direct hit, (ii) compression of broken ice pieces or newly grown ice in the interstitial spaces, and (iii) shear between adjacent floes transverse to the direction of wave propagation. The more irregular the floe sizes and shapes are, the more likely that type (iii) interactions will occur. Collisions may not fall uniquely into...

Wave energy attenuation in fields of colliding ice floes – Part 2: A laboratory case study

“…However, field observations of colliding ice floes are rare, and measurements directly relating collisions to dynamical processes in sea ice and underlying surface layer of the ocean are practically nonexistent. The first studies devoted to ice floe collisions, conducted in the 1980s and early 1990s, were based on measurements with accelerometers placed on the ice Becker, 1987, 1988;Martin and Drucker, 1991;McKenna and Crocker, 1992;Rottier, 1992) or with the help of socalled "strain arrays" (e.g. Hibler III and Leppäranta, 1984).…”

“…Until now, laboratory experiments devoted to wave attenuation in ice concentrated mostly on frazil, grease, and pancake ice or mixtures of those ice types -that is, conditions in which collisions were insignificant (see, e.g. Zhao and Shen, 2015;Rabault et al, 2019;Yiew et al, 2019, and references there). The same is true for most MIZ field studies (e.g.…”

Ice-floe kinematics in the Ross Sea marginal ice zone using GPS and accelerometers

“…An experiment was conducted in the Ross Sea MIZ on 24−25 January 1999, to measure the kinematic behaviour of individual ice floes using global positioning system (GPS) receivers, triaxial accelerometers and magnetoresistive compasses. Similar experiments have been performed in the Greenland Sea MIZ (Martin and Becker, 1987), the east Arctic MIZ (Martin and Drucker, 1991) and the Labrador Sea MIZ (McKenna and Crocker, 1992). The main difference between these experiments and the current one is the use of GPS receivers.…”

“…The ice floes used in this experiment were small compared with the wavelength of the observed ocean swell and, according to McKenna and Crocker (1992), will approximately follow the motion of the sea surface. The frequency of the observed ocean swell was within the low-frequency band.…”

Modeling ocean wave propagation under sea ice covers

“…Focusing on small scale mechanisms, Mckenna and Crocker [83] conducted a study in the Labrador Sea using a tiltmeter/accelerometer package. Since floe collisions represent an energy sink to the wave field, they were interested in the frequency of floe-floe collisions in a wave field.…”