Mesoscale Modification of Precipitation during Landfalling Atmospheric Rivers by Frontal Cyclogenesis

Two atmospheric rivers (ARs), each with maximum onshore integrated vapor transport exceeding 1000 kg m^-1 s^-1, caused heavy precipitation and flooding in the Russian River watershed of California in Dec 2014 and Feb 2019. The river crested several feet above flood stage at Guerneville, CA during each event. Though several other strong and flood-producing ARs have impacted the same region during the last five years, these two events share an important feature: a secondary cyclone that developed along the offshore portion of the AR and strengthened throughout the AR landfall period. This secondary frontal cyclogenesis contributed to significant short-term forecast uncertainty in both cases, a portion of which can be understood from the deterministic ensemble paradigm. In other words, uncertain initial conditions led to rapid growth of errors during the period of rapid cyclone growth. There have been many studies focusing on the growth of forecast errors in deterministic ensembles during cyclogenesis, and we will not revisit that topic here. Here, we focus on the role poorly-resolved processes at the meso-alpha scale and smaller played in the heavy precipitation reaching the Russian River basin. For example, we will discuss the sensitivity of upslope water vapor flux to offshore diabatic heating in the developing cyclone, rain shadowing by mesoscale frontal circulations, and the non-orographic contributions to vertical motion. We will also discuss the implications these poorly-resolved processes have on precipitation and river forecasting during similar events.