User:Robbie Mallett/sandbox

The sea ice snow cover is a key feature of the polar climate systems. The snow cover shields the underlying ice it from incoming sunlight and insulates it against the cold atmosphere, with wide-ranging impacts on the polar climate and ecosystems. Despite its significant role in polar climate change, its spatial distribution and properties are relatively poorly understood, and in 2019 was identified by the IPCC as a key knowledge gap.

=Snow in the Sea Ice System=

The Nature of Arctic Ocean Snowfall
The nature of snowfall on Arctic sea ice varies significantly by region. In the Atlantic Sector, snow is generally deposited by storm systems moving up from the Norwegian and Greenland seas. Elsewhere it is deposited much more gradually and consistently (Boisvert et al., 2018). The gradual nature of Arctic snowfall makes its representation a challenge for atmospheric reanalysis products, especially as the daily balance of precipitation and sublimation may be relatively fine (Andreas et al., 2002; Liston and Sturm, 2004). Boisvert et al. (2018) carried out an intercomparison of reanalysis precipitation over the Arctic ocean and found that some reanalyses show excessively large precipitation events, and by comparison with drifting buoys produce overly frequent precipitation events. Barrett et al. (2020) compared reanalysis products with records from Arctic Ocean drifting stations (Sect. 2.5.1), finding that some reanalyses are significantly wetter than others, but finding interannual variability to be similar at both a basin-wide and regional scale. The spread of snowfall amounts present in reanalyses was recently narrowed significantly through calibration with CloudSat data (Cabaj et al., 2020; Edel et al., 2020).

The Physical Nature of the Arctic Ocean Snowpack
Snow on Arctic sea ice is distributed in an extremely variable way due to a combination of underlying ice topography and the chaotic fluid dynamics of wind-flow (Iacozza and Barber, 1999; Shalina and Sandven, 2018; Moon et al., 2019). Its depth is weakly coupled to the sea ice thickness distribution over thin ice due to spatially varying thermal heat flux from the ocean, and strongly coupled over thick ice due to the ridges and rubble (Liston and Sturm, 2004; Kwok et al., 2011). The stratigraphy of snow on sea ice can be characterised at the broadest scale by two layers of distinctive snow grains: depth hoar and overlying wind pack (Sturm et al., 2002b; Domine et al., 2012; Merkouriadi et al., 2017a,b).

The formation of these characteristic layers relies on vapour gradients at the snowpack and snow-grain level respectively. These phenomena are often explained in terms of a ‘vapour equilibrium’. An ice surface at non-zero humidity is in a mass balance equilibrium, with water molecules constantly accreting from the air onto the surface, and sublimating off from the surface to the air. A change in conditions or surface geometry will shift this equilibrium either positively or negatively. A positive shift (say, from an increase in humidity), will result in ice growth. A negative change (say, from an increase in ice temperature) will cause more sublimation and ice ablation.