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

ERA5 remains positive most of the time (not shown). The time series of the RMSE of ERA5 follows closely that of 24d18h (Fig 11B). The correlation coefficient of ERA5 shows less vari- ability than that of the simulations, remaining between 0.58 and 0.84 (not shown). 3.3.7 1-h accumulated precipitation. The aggregate statistics indicate that 24d18h has the most skill at representing precipitation (Table 8). The ME of the simulations is positive, with 23d12h and 24d18h having the lowest mean bias (0.1 mm). The latter also has the lowest RMSE (1.3 mm) and the highest correlation coefficient (0.53), indicating modest correlation with the observations. The other simulations have virtually no correlation with the observations, except for 24d12h, which shows weak correlation with them. During most of the time, the ME of all the simulations ranges from -1 to 1 mm (not shown). The highest values of RMSE are found at 16:00 and 17:00 UTC, just after the observed SLP minimum is attained (Fig 11C). There is a large spread of the correlation coefficients of the simulations (not shown). In contrast with the simulations, ERA5 has a negative ME (-0.1 mm) (Table 8). The RMSE of ERA5 (1.2 mm) is lower than that of all the simulations, although it is only slightly lower than that of 24d18h. ERA5 shows modest correlation with the observations (0.62). Most of the time, ERA5 shows a somewhat smaller RMSE compared to the simulations (Fig 11C), and its correlation coefficients are higher than those of most of the simulations (not shown). The

ERA5 1-h accumulated precipitation never exceeds 2.5 mm, which is likely due to its relatively low resolution. In contrast, the maximum simulated precipitation is 9.5 mm, corresponding to 24d12h, and the maximum observed precipitation is 20.6 mm. This agrees with the finding of Hu and Franzke [50] that ERA5 underestimates the daily precipitation extremes observed by weather stations in Germany. Table 7. Aggregate statistics computed to verify the simulated and ERA5 wind gusts against the observations from surface stations. ME [m s-1] RMSE [m s-1] r 23d00h -0.8 6.2 0.51 23d06h -1 7.5 0.37 23d12h 0.1 6.3 0.38 23d18h -1.2 6.1 0.53 24d00h -0.1 5.1 0.65 24d06h 0 5.4 0.52 24d12h 1 4.9 0.74 24d18h 1 4 0.8 ERA5 1.9 3.9 0.81 https://doi.org/10.1371/journal.pone.0292250.t007 Table 8. Aggregate statistics computed to verify the simulated and ERA5 1-h accumulated precipitation against the observations from surface stations. ME [mm] RMSE [mm] r 23d00h 0.2 1.6 0.22 23d06h 0.3 1.8 0.22 23d12h 0.1 1.6 0.35 23d18h 0.2 1.5 0.34 24d00h 0.5 1.8 0.19 24d06h 0.5 1.8 0.23 24d12h 0.2 1.6 0.42 24d18h 0.1 1.3 0.53 ERA5 -0.1 1.2 0.62 https://doi.org/10.1371/journal.pone.0292250.t008 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 20 / 28