Uncertainties in a regional climate model in the midlatitudes due to the nesting technique and the domain size

Deficiencies of the nesting technique for regional models and their internal variability represent a significant source of uncertainties in regional model outputs. Presented numerical experiments on four different‐size domains, two of them being midlatitude channel domains, show that the spatial structure and magnitude of uncertainties strongly depend on the domain size. The experiments are performed by using the Weather Research and Forecasting (WRF) model nested into the operational European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses on the same horizontal resolution 0.25°× 0.25° and by nesting WRF into a larger WRF domain with the same resolution. Uncertainties are quantified by the root‐mean‐square differences (rmsd) between the WRF results and their driving lateral boundary fields. The results from the midlatitude channel domain show that uncertainties in the tropospheric wind associated with the imperfect nesting method are amplified in the baroclinically active regions of the Atlantic and Pacific. The zonal wind rmsd have a dipole structure in the Atlantic in both the midlatitude channel and the half‐channel simulations nested into ECMWF. The dipole is absent when WRF is nested into itself. On the contrary, the maximal rmsd for the meridional wind are always located in the domain center. When the domain centered on Europe excludes the western Atlantic and North America, the simulated uncertainties become spatially nearly homogeneous, and the magnitude of rmsd due to the imperfect nesting technique greatly reduces. Key Points Model differences influence uncertainties due to the nesting technique Pure nesting uncertainties are largest in the Atlantic In typical RCM domain for Europe, internal variability is nearly homogeneous