The Development and Evolution of Two Atmospheric Rivers in Proximity to Western North Pacific Tropical Cyclones in October 2010

The landfall of an atmospheric river (AR) produced >200 mm of rainfall over interior and coastal mountainous regions of northern California in late October 2010. The high-impact AR was the amalgamation of two ARs that developed far upstream within regions of water vapor that were transported directly from western North Pacific tropical cyclones (TCs) into the Midlatitudes. Synoptic-scale analyses, Lagrangian air parcel trajectory analysis, and an AR water vapor budget demonstrate that both the water vapor source regions and the physical processes associated with increasing or decreasing integrated water vapor (IWV) values along the ARs changed during the lifecycles of the two ARs as they cross the North Pacific Ocean.

The development of the two ARs occurred in association with decreasing integrated water vapor (IWV) values in an environment characterized by the transport of water vapor along the AR corridor from TC source regions, troposphere-deep air parcel ascent, and large precipitation rates. The development of the two ARs occurred within the equatorward entrance region of an intensifying North Pacific jet stream (NPJ) and evolved similar to mature predecessor rain events (PRE) ahead of TCs. The subsequent evolution of the two ARs occurred in association with constant IWV values in an environment characterized by the transport of water vapor along the AR corridor from extratropical and subtropical source regions, strong frontogenesis, large precipitation rates and large amounts of IWV flux convergence. An AR water vapor budget demonstrates that decreases in IWV via precipitation along an AR may be offset by the horizontal aggregation of water vapor along the AR via IWV flux convergence in the presence of frontogenesis. In the present study, the results indicate that the PRE-like development of the ARs may have facilitated the intensification of the NPJ and generation of strong frontogenesis that supported the subsequent maintenance of the ARs prior to landfall.