Assessing the Representation of the Sea Ice Floe Size Distribution Within Arctic Sea Ice Models

Abstract:

Sea ice is composed of discrete units called floes. The size of these floes can determine the strength and nature of interactions between the sea ice, ocean and atmosphere including lateral melt rates, momentum exchange, and surface moisture flux. Sea ice models have traditionally assumed floes adopt a constant size if floes are explicitly considered at all. Observations have shown that floes can adopt a range of sizes spanning orders of magnitude, from metres to tens of kilometres. These observations of the floe size distribution (FSD) are generally fitted to a power law with a negative exponent. There have been several recent efforts to develop FSD models for use within sea ice models of varying complexity.

In this study we compare two alternative approaches to modelling the FSD within the CICE sea ice model. The first assumes floes follow a power law distribution with a constant exponent. Parameterisations of processes that influence the FSD are calculated using a variable FSD tracer. The second is a prognostic floe size-thickness distribution where the shape of the distribution is an emergent feature of the model and is not imposed. We firstly demonstrate the need to include in-plane brittle fracture processes in prognostic FSD models. We then show that neither FSD model results in a significant improvement in the ability of the sea ice model to simulate pan-Arctic metrics, however larger impacts can be seen at regional scales in the sea ice concentration and thickness. We use case studies to understand how the differences in the impacts of the two FSD models emerge, including the different spatial and temporal variability of these impacts. Finally, we conclude with a discussion of the advantages and disadvantages of each FSD modelling approach.