Assessment of simulations of a polar low with the Canadian Regional Climate Model

same region and shows the same depth. The incipient PL in 24d12h and 24d18h with a well defined SLP minimum is located on the right side of this mid-tropospheric through, whereas in the other simulations only a weak (low-level) through within the SLP field in this area is observed. The 1000-500 hPa thickness field shows that the cold air tongue associated with the MCAO has a northwest-southeast orientation in all simulations. Since the atmospheric conditions aloft are similar in all simulations during the genesis time of the PL, they cannot explain why it has only been correctly captured by 24d12h and 24d18h (with respect to both observations and reanalysis data). Therefore, the difference between the simulations must be in the lower atmosphere. Fig 13 shows the geopotential height, tempera- ture and horizontal wind at 900 hPa on 24 March at 1900 UTC in the region where the low- level through preceding the genesis of the PL started to form (i.e. 5 hours before the PL shown in Fig 12). All simulations show a strong northwest-southeast temperature gradient to the west of Jan Mayen, close to the sea ice edge. In contrast with the other simulations, the northerly cold air advection and winds in 24d12h and 24d18h are more intense and more widely extended; therefore, the cold airmass moves further south in these two simulations. At the same time, on the east side of this cold air, a warm front pushes northward in these two simula- tions, with a more widely defined and stronger warm air advection in this area than the other

simulations. These results indicate that, in the presence of a baroclinic environment, only the low-level atmospheric conditions with a well defined cold/warm air temperature advection present in 24d12h and 24d18h lead to baroclinic instability, which is involved in the genesis of the PL. It is also clear in Fig 13 that the low-level pressure deepens or vorticity (wind rotation) starts to develop in these two simulations, i.e. small scale features corresponding to the PL development phase. Since 24d12h and 24d18h are the latest initialised simulations, their atmospheric fields dur- ing the hours preceding the PL formation are more similar to those of ERA5, the driving data, compared to those of the other simulations. Thus, the fact that the other simulations except for 23d12h do not represent the PL is due to forecast error growth and missing small-scale features during the initial stage of the PL formation. Nevertheless, the question remains about why 23d12h represents a PL at a later moment in time. In 23d12h, a strong low-level baroclinic zone forms a few hours before the PL forms in this simulation (Fig 14), and the PL shows baro- clinic development. This PL makes landfall shortly after being formed, thus dissipating before it can reach a larger size. Fig 14 reveals also clearly that stronger winds over both cold (west) and warm (east) near the developed PL induce small scale conditions (i.e., temperature advec- tions) favourable to strengthen low-level baroclinity and cyclogenesis in the latest initialised simulations, not present in the other simulations.

In conclusion, on the 24–25 March, the simulated environmental conditions are favourable for PL development, with a low-level baroclinic environment and an upper-level through, but the different evolution of the low-level circulation and small-scale features explains why a few simulations capture the PL whereas the others do not. 4. Conclusion Compared to low-resolution models, convection-permitting models provide a better represen- tation of physical processes [20]. Therefore, they are a powerful tool to study mesoscale phe- nomena, including PLs. This study has focused on a PL that made landfall in Norway in 2019, and the aim was to analyse the impact of the initial conditions on the simulation of the PL, and to analyse the skill of the CRCM6/GEM4 at reproducing it. The main limitations of this study is that the available conventional observations mostly cover the mature and dissipation stages of the PL, and that they are irregularly distributed in space. PLOS ONE Assessment of simulations of a polar low with the canadian regional climate model PLOS ONE | https://doi.org/10.1371/journal.pone.0292250 October 5, 2023 22 / 28