Study of a polar low in Norwegian Sea using COMBLE observations and numerical modeling

Polar lows (PLs) are intense mesoscale low pressure systems that develop over open water during cold-air outbreaks over high-latitude areas. They may cause hazardous weather conditions to ocean vessels and coastal sites. PLs are not only difficult to forecast by numerical weather prediction models but also inadequately documented and characterized. Most observational studies of polar lows are based primarily on satellite imagery and very little is known about the fine-scale vertical structure of PLs. During the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) campaign, a PL developed within the Norwegian Sea in a post-frontal cold-air mass on 19 March 2020. Satellite imagery shows that this PL developed rapidly from a small disturbance to a comma-shaped cloud structure. Later it transitioned to a spiraliform PL with an eye-like center, under a shallow tropopause (~5 km) with a weak tropopause cyclonic vortex. Landfall on the island of Andøya occurred at 12:30 UTC on 20 March over the COMBLE instrument site. The PL dissipated quickly after the landfall making for a lifetime of around 24 hours. The vertical cloud structure of the PL shows strong convection before the eye passed over the observation site, whereas clouds are more stratiform following landfall. Observations are used to estimate convective mass flux and latent heating rate. The mesoscale structure and the development of the PL are reproduced reasonably well by the operational convection-resolving weather-prediction model, the Arome-Arctic model, which has a horizontal grid cell spacing of 2.5 km. However, validation against the COMBLE observations reveals that Arome-Arctic does not capture the intensity of the PL convection in the PL; therefore, we simulate this PL using the Weather Research and Forecasting (WRF) Model with a horizontal grid cell spacing of 1 km. The higher-resolution WRF model output is able to resolve more features of the PL, allowing us to further characterize its mesoscale structure.