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

In this work we conducted a case study of a PL that developed over the Norwegian Sea on 25 March 2019 with two main objectives: 1. To quantify the impact of the initial conditions on the simulation of the PL; 2. To assess the skill of the developmental version of the convection-permitting Canadian Regional Climate Model version 6 (CRCM6/GEM4) at reproducing the observed PL. The main verification method used in case studies of PLs is visual verification, but this type of verification does not quantify the skill of the model [8]. Therefore, we have applied an objec- tive method to verify the simulations of the PL against conventional observations. Since the PL made landfall in Norway, we have been able to use near-surface observations of a wide range of atmospheric variables. Given that more work is needed on the verification of near-surface fields in the polar regions [7], this study will partly contribute to fill in this research gap. The article is organised in four sections. Section 2 provides information about the CRCM6/ GEM4 and the datasets used for the verification of the simulations, as well as a description of the methods used to prepare the datasets and to verify the simulation output. Section 3 pro- vides a description of the life cycle of the PL and includes the analysis of the results. Section 4 summarizes the main conclusions of this study. 2. Data and methods 2.1 Datasets 2.1.1 Simulations. The PL that developed over the Norwegian Sea on 25 March 2019 has been simulated with the developmental version of the convection-permitting CRCM6/GEM4.

The dynamical core of the CRCM6/GEM4 has been developed from the limited-area version of the Global Environmental Multiscale Model [GEM; 26–28]. The CRCM6/GEM4 uses the dynamical core of the version 4 of the GEM model (GEM4), whose detailed description is given by Girard et al. [29]. GEM uses an implicit semi-Lagrangian method for spatiotemporal integration [26, 29]. The model uses a rotated longitude-latitude grid in the horizontal [30]. The vertical coordinate is a hybrid log-hydrostatic pressure coordinate, based on the formula- tion of hydrostatic pressure developed by Laprise [31]. For the spatial discretization, the model uses three-dimensional staggered grids, the Arakawa C grid in the horizontal and the Char- ney-Phillips grid in the vertical. For the lateral driving, GEM employs the nesting technique suggested by Davies [32], which consists of applying a sponge zone around the domain with a relaxation coefficient decreasing from the outside to the inside. For the simulations reported here, the model uses a grid spacing of 0.0225˚ (� 2.5 km), a verti- cal grid with 62 levels, and a time step of one minute. The size of the domain is 1024 x 1024 grid points (Fig 1), including the ten grid point sponge zone around the perimeter of the domain, and the model top is at 2 hPa. The output of the simulations, excluding the sponge zone, therefore cov- ers an area of approximately 2510 x 2510 km2. Such domain is sufficient to capture not only the mesoscale phenomena, but also synoptic-scale features affecting polar low development.

The following subgrid parameterisation schemes have been used for the simulation: the cor- related-k radiation scheme [34], the planetary boundary layer scheme MoisTKE that unifies turbulence and cloudiness [27, 35], the non-convective condensation scheme Predicted Parti- cle Properties [P3; 36], and the land-surface scheme Interactions between Soil, Biosphere and Atmosphere [ISBA; 37]. Since convection is partially resolved, the deep convection scheme is turned off and only the shallow convection scheme Kuo-transient [27] is used. The orographic gravity wave drag and blocking, and non-orographic gravity wave drag schemes are also turned off. 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 3 / 28