| Summary: | Polar lows in the Nordic Seas have been examined through a case study based on unique observations gathered during a field campaign, numerical simulations, sensitivity experiments with altered orography and sea ice cover, and a climatology based on objective tracking and two reanalysis products. A detailed analysis of a shear-line polar low has been presented using comprehensive observations from a research aircraft, dropsondes, the ASCAT scatterometer, and the CloudSat radar; in conjunction with convection-permitting simulations performed with the Met Office Unified Model. High winds to the north and west were within the cold-air mass and associated with large surface turbulent heat fluxes and convective clouds. This suggested that barotropic instability manifested by mesoscale waves coalescing into polar low’s centre, and diabatic processes, were important for its intensification. The model generally captured the polar low structure well — in particular the thermodynamic fields and the strength of the horizontal shear. The spatial structure of the convective cloud bands was simulated reasonably well, but the model significantly underestimated the liquid water content and height of the cloud layers compared to observations. Through sensitivity simulations of two typical Nordic Sea polar lows, it was found that Svalbard blocked Arctic air masses, and acted as an additional source of cyclonic vorticity aiding polar low development. A decrease in sea ice near Svalbard resulted in a moderate intensification of the polar lows, while an increase in sea ice significantly hindered their growth. These environmental changes modified the polar lows’ tracks and development, but did not eradicate them. A new climatology has been compiled from nine extended winters using two reanalyses: ERA5 and ERA-Interim. Mesoscale cyclones were tracked by an objective vorticity-based method. Compared to ERA-Interim, ERA5 reproduces the spatial distribution of cyclone density more like those from satellite-based studies, as it is able to resolve the wind field gradients with higher fidelity. An increase of polar lows near Scandinavia was found and there is tentative evidence that this is a result of sea ice loss in the northern Nordic Seas.
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