Arctic Cyclone Cloud and Boundary-Layer Features Producing Thermodynamic and Dynamic Impacts on Arctic Sea Ice During MOSAiC

MOSAiC was a rare opportunity to obtain observational data on Arctic cyclones (ACs), with at least 22 ACs sampled during four of the five legs.  This presentation will illustrate how some of the AC cloud and boundary-layer characteristics produce both thermodynamic and dynamic impacts on the sea ice in the non-melt seasons.  Deep clouds associated with the ACs enhanced the downwelling radiation, with the surface radiation dependent on cloud height, temperature, and liquid water layers.  Underneath the warm fronts ahead of the ACs, this effect directly warmed the surface, sometimes to the extent of destabilizing the near-surface boundary layer.  Synoptically induced low-level jets (LLJs) found within AC warm sectors between the surface warm front and cold front often also provided warm-air advection down to a few hundred meters above the surface, which, in many cases, produced a stable lower boundary layer and enhanced downward sensible heat flux in these AC regions.  These effects in some mid-winter ACs produced near-surface temperature increases of 20° C or more compared to the surface temperatures prior to the ACs, with some, but not all, winter cases also producing strong thermal waves penetrating through the snow and ice and reducing sea-ice growth. 

In some ACs, a quasi-axisymmetric LLJ near the top of the boundary layer in the warm sector and then wrapping around the low center as a “bent-back” feature produces the appearance of two successive LLJs with greatly differing wind directions at the MOSAiC site.  The rapid change in wind speed and direction between these LLJ pairs produces a rapid change in the surface stress vector and imparts strong forcing on the sea ice.  During MOSAiC, strong ice deformation, lead formation, etc., was observed during the time periods between the two LLJs.  Hence, these LLJs not only play a role in the atmospheric thermal impacts on sea ice from ACs, but also on the dynamic impacts.

The broad relevance of Arctic Cyclone mesoscale features, such as low-level jets, warm-sector warm-air advection, and cloud macro and microstructure, to the sea-ice thermodynamic and dynamic environment during the MOSAiC field program will be illustrated with MOSAiC case-study observations, including basic meteorological measurements, rawinsondes, ARM remote sensors, surface energy budget measurements, and ice motion measurements.