Representing sea-ice heterogeneities and the Arctic boundary-layer using a thermal heterogeneity parameter

Sea-ice cover exerts important controls on the Arctic climate and may form horizontally heterogeneous patterns, especially in the marginal ice zone (MIZ). Earth System Models (ESMs) represent the sea-ice heterogeneity within a grid cell as an ice fraction. The heterogeneous sea-ice cover, however, causes complex nonlinear surface-atmosphere interacting processes that cannot be quantified appropriately using solely the ice fraction. Among the nonlinear interacting processes are the secondary circulations in the atmospheric boundary layer (ABL) that are driven by the sea-ice and ocean water surfaces and their thermal contrast. An effective representation of the surface-atmosphere momentum, temperature and moisture exchanges for a grid cell of an ESM should accommodate for the occurrence of secondary circulations. This is of particular relevance when leads evolve in the sea ice. These elongated cracks in the sea-ice cover expose local regions of open ocean water with surface temperatures much higher than the surrounding sea ice. As a result, convective plumes develop above leads. Even if leads occupy a small areal fraction only, their impact on the regional temperature, atmospheric stability over sea ice, and surface-atmosphere fluxes in winter is disproportionally large.

To quantify and parameterise secondary circulations related to leads, we extend a thermal heterogeneity parameter [1], which defines the ratio between buoyancy effects of surface thermal contrasts to the inertia of the mean flow. This extension incorporates factors such as temperature difference between the sea-ice and water surfaces, the angle between geostrophic wind and lead orientation and typical length scales. Data are used from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) campaign and from the BACSAM II flight campaign, where turbulence was measured at two different heights simultaneously: on an aircraft and 60 m below the aircraft using a passive trailing body called T-bird. The aircraft data are analysed with a wavelet transform, enabling a multiscale decomposition to extract a mesoscale contribution to the fluxes. Surface temperature characteristics are obtained from the Modis global Level-2 product (resolution: 1 km). A case study reveals a strong correlation between thermal heterogeneity parameters and mesoscale flux contributions for 20 km subintervals with 1 km rolling steps along the flight legs. The correlation is enhanced for leads oriented normal to wind, and when fetch-dependent downstream effects are included.

[1] Margairaz, Fabien & Pardyjak, Eric & Calaf, Marc. (2020). Surface Thermal Heterogeneities and the Atmospheric Boundary Layer: The Thermal Heterogeneity Parameter. Boundary-Layer Meteorology. 177. 1-20. 10.1007/s10546-020-00544-7.