Variations in the microstructure of saline ice during its growth and decay: Evidences from an experimental study

Abstract. Ice physics are highly sensitive to the ice temperature and are primarily determined by the distribution of inclusions such as gas bubbles and brine pockets within ice. However, a detailed understanding of their distributions and evolution patterns during ice freezing and melting is lacking. To address this issue, in situ experiments were conducted to collect detailed information on the variations in the microstructure of ice using continuous sampling and a high-resolution imaging system. The results revealed a 5- to 10-fold increase in the volume fraction and a 2-fold increase in the size of gas bubbles during the melting phase of ice. Moreover, the size of brine pockets in the ice surface, middle, and bottom layers clearly increased for different reasons. The nearly 30 % increase in gas bubbles observed in the middle layer was thermally driven, while the increase in the surface layer was influenced by the net shortwave radiation. Additionally, the variation in the inclusion size distribution was attributed to the merging process, most of which occurred among smaller inclusions rather than among larger inclusions. The changing ice temperature is a significant factor in the merging process of the middle layer but not for the surface or bottom layers. This study could enhance the understanding of the effect of the transfer of energy between the atmosphere and water beneath ice on the ice microstructure.