Quasigeostrophic Forcing during Landfalling Atmospheric Rivers over Northern and Southern California

Evidence has shown that atmospheric rivers (ARs) in different synoptic-scale flow patterns leads to variations in moisture source regions, orientations, intensities. The variety of synoptic-scale flow regimes that can produce landfalling ARs over California can also result in differences in quasi‐geostrophic (QG) forcing located over the terminus of the concomitant AR, which may impact the resulting precipitation distributions in numerous ways. For example, an AR that is present during QG forcing for ascent may result in a higher percentage of storm total precipitation that falls at lower elevations. An AR that produces a higher percentage of precipitation at lower elevations will have different hydrologic impacts compared to an AR that produces precipitation that is more confined to higher elevations.

This research utilizes 38 years of National Center for Environmental Prediction’s Climate Forecast System Reanalysis data to identify and compare the QG forcing present during landfalling ARs over the Russian River watershed in Northern California and the Santa Ana River watershed in Southern California. Preliminary results suggest that extreme precipitation is driven by both a favorable orientation of integrated water vapor transport relative to watershed topography and QG forcing for ascent co-located over the terminus of the AR.