Dynamical downscaling of the GENMOM climate model over Western North America from the Last Glacial Maximum to present

We downscaled global climate simulations we previously conducted with the GENMOM coupled atmosphere ocean general circulation model at T31 resolution (~3.71° latitude × 3.75° longitude) by progressively nesting 50- and 15-km configurations of the hydrostatic version of the RegCM5 regional climate model over Western North America (WNA). The extent of the 50-km model domain captures key circulation features over WNA, and the smaller 15-km domain captures important topographic controls of WNA climate.

The global simulations were run to equilibrium at 3000-yr intervals from 21 ka to 0 ka (PI) using time-dependent boundary conditions comprising insolation, atmospheric greenhouse gas concentrations (CO2, CH4, and N2O), ice sheet extent and height, and sea level. We first ran RegCM5 over a 50-km horizontal grid with 23 vertical atmospheric levels coupled with the Biosphere-Atmosphere Transfer Scheme (BATS1e) surface physics model. We prescribed the same boundary conditions in the regional model as those in the global simulations but used the ICE6G Laurentide and Cordilleran ice sheet reconstructions and changing paleo vegetation. Reconstructed vegetation is specified in the model by cross referencing 17 physical characteristics (e.g. albedo, aerodynamical roughness, fractional cover, stomatal resistance, soil texture and color) with the 20 available land-use classes in BATS1e.

The simulations were driven by 105 years of continuous 6-hr lateral boundary conditions (vertical profiles of atmospheric temperature, wind, humidity, surface pressure, and sea-surface temperature) derived from the GENMOM simulations. We then ran the 15-km model over a smaller domain for the same 105 years using boundary conditions derived from the 50-km run. We use the last 100 years of the RegCM5 simulations to exclude model spin-up.

We also downscaled ECMWF Reanalysis v5 (ERA5) product using the same 50-km domain and 15-km nesting from 1970-2021 to allow for an evaluation of model bias added in the downscaling process. As with the GENMOM simulations, we include 1975-2021 for evaluation and exclude the first five years as model spin-up.
 
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