Near-Coastal Sea Surface Temperature Variability and Its Impact on Landfalling Atmospheric Rivers in California

The precipitation distribution over land within a landfalling atmospheric river (AR) can be highly diversified by known geophysical influences, such as topography and bulk moisture transport. However, less is known about the coupling of ARs and ocean surface temperatures, especially the near coastal antecedent sea surface temperature (SST) variability. This work expands a completed modeled case study of an AR making landfall in Southern California in February, 2017 using a specialized version of the WRF model (called West-WRF) in which a localized area coastal SSTs were warmed by a maximum of 1.5°C. The SST anomalies were generated by applying a smooth bell-curve anomaly to the SST analysis with a spatial extent approximately equal to the typical width of an AR. The mean areal precipitation in a 3-day period over the Santa Barbara, Santa Ynez, and Santa Maria watersheds was not significantly affected by the warm anomaly offshore, but the precipitation distributions over the ocean highly varied depending on the the planetary boundary layer scheme used in the WRF model. In this paper, we will further explore the dynamics of the heat transport from SSTs by varying amplitudes and sizes of SST anomalies for 5 other AR cases in Northern California in 2017. By using different scales and amplitudes of antecedent variability in the SST, we will investigate the advective time scales necessary to influence onshore energy and moisture transport in ARs. In addition to the onshore precipitation analysis over the Russian River watershed, this analysis will be combined with NEXRAD radar data, where available, to analyze the variance of precipitation over the ocean due to the planetary boundary layer schemes.