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

started to form over open water at the leading edge of this MCAO, and a comma-shaped cloud signature was clearly identifiable in IR images by 25 March at 0200 UTC (not shown). Like many PLs in the Nordic Seas [e.g., 48], it moved southeastward as it deepened (Fig 2B and 2C). The PL hit land in Nordland county of Norway after 0900 UTC (Fig 3A). By 1200 UTC, it had reached a large part of the Norwegian coast (Figs 2C and 3B). The winds associated with the PL reached an observed maximum speed of 24.8 m s-1. The PL started to dissipate at around 1800 UTC (Figs 2D and 3C). The lifetime of this PL was 20 hours (Table 1), in agreement with climatologies of PLs in the Nordic Seas [e.g., 15]. The estimated size of the PL at its mature stage was 586 km in diameter, which is larger than the typical diameter of PLs [e.g., 48]. With an average speed of 15 m s-1, the PL travelled 1,070 km. Both the average speed and distance travelled are larger than those of most PLs [48]. 3.2 Verification of the track, size and intensity of the simulated PL Fig 4 shows the SLP isobars of the eight simulations and ERA5 on 25 March 2019 at 1500 UTC, when the mature PL was affecting the Norwegian coast. The large-scale features of the SLP field are similar for all simulations, whereas the simulations notably differ from each other at the mesoscale, in particular near the location of the PL. Overall, these spaghetti plots show that most simulations fail to represent the PL. Only 23d12h and the latest initialised simula- tions 24d12h and 24d18h represent a PL.

The PL in the the simulations and ERA5 forms and dissipates on 25 March. The simulations 24d12h and 24d18h represent well the size and lifetime of the observed PL, as well as the the Fig 3. AVHRR channel 4 images showing the PL on 25 March (a) before it hits land, (b) when it is over a great part of the Norwegian coast, and (c) when it starts to dissipate. The yellow isolines represent the ERA5 SLP field (contours every 2 hPa). The blue outlining represents the coastline. The dataset used to plot the geographic contours has been obtained from the GSHHG [33] under a CC BY license, with permission from Dr. Paul Wessel. https://doi.org/10.1371/journal.pone.0292250.g003 Table 1. Lifetime, translation speed and distance travelled by the PL. Dataset Start hour [UTC] End hour [UTC] Size [km] Average speed [m s-1] Distance [km] Observations 0100 2100 586 15 1,070 23d12h 1200 2000 402 14 395 24d12h 0000 2000 585 12 892 24d18h 0000 2100 561 15 1,113 ERA5 0500 1900 561 13 636 https://doi.org/10.1371/journal.pone.0292250.t001 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 8 / 28