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Atmospheric rivers (ARs), and frontal systems more broadly, tend to exhibit prominent “V” shapes in time series of stable isotopes in precipitation. Despite the magnitude and widespread nature of these “V” shapes, debate persists as to whether these shifts are driven by changes in the degree of rainout, which we determine using the Rayleigh distillation of stable isotopes, or by post-condensation processes such as below-cloud evaporation and equilibrium isotope exchange between hydrometeors and surrounding vapor. Here, we present paired precipitation and water vapor isotope time series records from the 5–7 March 2016, AR in Bodega Bay, CA. The stable isotope composition of surface vapor along with independent meteorological constraints such as temperature and relative humidity reveal that rainout and post-condensation processes dominate during different portions of the event. We find that Rayleigh distillation controls during peak AR conditions (with peak rainout of 55%) while post-condensation processes have their greatest effect during periods of decreased precipitation on the margins of the event. These results and analyses inform critical questions regarding the temporal evolution of AR events and the physical processes that control them at local scales. 

Abstract. Ice nucleating particles (INP) have been found to influence the amount, phase, and efficiency of precipitation from winter storms, including atmospheric rivers. Warm INP, those that initiate freezing at temperatures warmer than −10°C, are thought to be particularly impactful because they can create primary ice in mixed-phase clouds, enhancing precipitation efficiency. The dominant sources of warm INP during atmospheric rivers, the role of meteorology in modulating transport and injection of warm INP into atmospheric river clouds and the impact of warm INP on mixed-phase cloud properties are not well-understood. Time-resolved precipitation samples were collected during an atmospheric river in Northern California, USA during winter 2016. Precipitation was collected at two sites, one coastal and one inland, that are separated by less than 35km. The sites are sufficiently close that airmass sources during this storm were almost identical, but the inland site was exposed to terrestrial sources of warm INP while the coastal site was not. Warm INP were more numerous in precipitation at the inland site by an order of magnitude. Using FLEXPART dispersion modelling and radar-derived cloud vertical structure, we detected influence from terrestrial INP sources at the inland site, but did not find clear evidence of marine warm INP at either site. We episodically detected warm INP from long-range transported sources at both sites. By extending the FLEXPART modelling using a meteorological reanalysis, we demonstrate that long-range transported warm INP are observed only when the upper tropospheric jet provided transport to cloud tops. Using radar-derived hydrometeor classifications, we demonstrate that hydrometeors over the terrestrially-influenced inland site were more likely to be in the ice phase for cloud temperatures between 0°C and −10°C. We thus conclude that terrestrial and long-range transported aerosol were important sources of warm INP during this atmospheric river. Meteorological details such as transport mechanism and cloud structure were important in determining warm INP source strength and injection temperature, and ultimately the impact of warm INP on mixed phase cloud properties.