Understanding California Mountain Wintertime Hydroclimate Trends using an Ensemble of High-Resolution Variable-Resolution CESM Simulations

Topography plays a central role in the capture (e.g., orographic uplift) and storage (e.g., snowpack) of usable water in California — approximately 60 percent of the state's water supply originates from the Sierra Nevada. Thus, the grid-scale and sub-grid-scale resolution of topography in climate models is critical to accurately resolve winter season hydroclimate trends, such as precipitation distribution, intensity, and phase and the snowpack life cycle. Due to computational constraints, even today's most cutting-edge global climate models rarely push to resolutions higher than 28km, which makes them unable to adequately resolve local topographic features that can have a pronounced impact on precipitation. Recent advances in climate modeling with variable-resolution affords researchers the ability to reach these scales. Therefore, to understand how CESM responds to more realistic orographic forcing, resolves critical surface properties (e.g., forest cover and soil distributions) and simulates key teleconnections (e.g., atmospheric rivers), variable-resolution CESM (VR-CESM) simulations at 28km, 14km, 7km, and 3.5km have been performed over the Sierra Nevada to assess mountain range to watershed level spatial scales over a near-term historical timeframe (1999-2015). This VR-CESM resolution ensemble was used to understand systematic bias in both the atmospheric (CAM) and land-surface model (CLM) and constrain the resolution dependency of these biases. The effects of grid-resolution and resolved topography on elevational profiles, orographic uplift and windward/leeward ratios in the Sierra Nevada were assessed against observational and reanalysis datasets including PRISM, MODIS and NOAA weather stations.