Comment on wcd-2022-60

Abstract

Arctic cyclones are the most energetic weather systems in the Arctic, producing strong winds and precipitation that present major weather hazards. In summer, when the sea ice cover is reduced and more mobile, Arctic cyclones can have large impacts on ocean waves and sea ice. While the development of mid-latitude cyclones is known to be dependent on boundary layer (BL) turbulent fluxes, the dynamics of summer-time Arctic cyclones and their dependence on surface exchange processes have not been investigated. The purpose of this study is to characterise the BL processes acting in summer-time Arctic cyclones and understand their influence on cyclone evolution. The study focuses on two cyclone case studies, each characterised by a different structure during growth in the Arctic: (A) low-level dominant vorticity (warm-core) structure, and (B) upper-level dominant vorticity (cold-core) structure, linked with a tropopause polar vortex. A potential vorticity (PV) framework is used to diagnose the BL processes in model runs from the ECMWF Integrated Forecasting System model. Both cyclones are associated with frictional Ekman pumping, and downward sensible heat fluxes over sea ice. However, the frictional baroclinic PV generation process (governed by the angle between the lower tropospheric thermal wind vector and the surface wind) acts differently in A and B due to the different cyclone structures. Positive PV is generated in A around the bent-back warm front, like in typical mid-latitude cyclones. However, the same process produces negative PV tendencies in B, shown to be a consequence of the vertically-aligned columnar vortex structure. This frictional process also acts to cool the lower troposphere, reducing the warm-core anomaly in A, and amplifying the cold-core anomaly in B. Both cyclones attain a vertically-aligned cold-core structure that persists for several days after maximum intensity, which is consistent with cooling from frictional Ekman pumping, frictional baroclinic PV generation, and downward sensible heat fluxes. This may help to explain the longevity of isolated cold-core Arctic cyclones with columnar vorticity structure.