UC Berkeley

Abstract The 1997 New Year's flood event was the most costly in California's history. This compound extreme event was driven by a category 5 atmospheric river that led to widespread snowmelt. Extreme precipitation, snowmelt, and saturated soils produced heavy runoff causing widespread inundation in the Sacramento Valley. This study recreates the 1997 flood using the Regionally Refined Mesh capabilities of the Energy Exascale Earth System Model (RRM‐E3SM) under prescribed ocean conditions. Understanding the processes causing extreme events informs practical efforts to anticipate and prepare for such events in the future, and also provides a rich context to evaluate model skill in representing extremes. Three California‐focused RRM grids, with horizontal resolution refinement of 14 km down to 3.5 km, and six forecast lead times, 28 December 1996 at 00Z through 30 December 1996 at 12Z, are assessed for their ability to recreate the 1997 flood. Planetary to synoptic scale atmospheric circulations and integrated vapor transport are weakly influenced by horizontal resolution refinement over California. Topography and mesoscale circulations, such as the Sierra barrier jet, are better represented at finer horizontal resolutions resulting in better estimates of storm total precipitation and storm duration snowpack changes. Traditional time‐series and causal analysis frameworks are used to examine runoff sensitivities state‐wide and above major reservoirs. These frameworks show that horizontal resolution plays a more prominent role in shaping reservoir inflows, namely the magnitude and time‐series shape, than forecast lead time, 2‐to‐4 days prior to the 1997 flood onset. Plain Language Summary The 1997 California New Year's flood event caused over a billion dollars in damages. This storm became a central part in guiding efforts to reduce flood risks. Earth system models are increasingly asked to recreate extreme weather events. However, the ability of Earth system models to recreate such events requires rigorous testing. Testing ensures that models provide value in anticipating and planning for future flood events. This is particularly important given the changing climate. We evaluated the Department of Energy's flagship Earth system model, the Energy Exascale Earth System Model, in its ability to recreate the weather and flood characteristics of the 1997 flood. The model resolution, important for resolving mountain terrain and storm interactions, and forecast lead time, important for storm progression accuracy, are assessed. The multi‐forecast average from the highest‐resolution model best recreates the observed precipitation, snowpack changes, and flood characteristics. Our findings provide confidence that the highest resolution model could be used to study how a 1997‐like flood event would be altered in a warmer world. Key Points Energy Exascale Earth System Model forecasts at 3.5 km grid spacing skillfully recreate the hydrometeorology of California's 1997 flood Horizontal resolution alters the representation of key flood drivers such as the Sierra barrier jet, precipitation extremes, and snowmelt Forecast lead time 2‐to‐4 days prior to the onset of the 1997 flood minimally influences forecast precipitation and snowmelt skill