Identifying and Characterizing Atmospheric Rivers Impacting Southern California

Abstract:

Atmospheric rivers (ARs) are channels of high water vapor flux within the low atmosphere that transport moisture towards midlatitudes on synoptic timescales. For areas with coastal mountainous terrain such as North America’s west coast, ARs often produce high intensity precipitation due to orographic forcing. Regional AR studies focus on the Pacific Northwest as this is where ARs landfall most frequently. For Southern California (SCA) there are relatively few AR landfalls per year, however, ARs that do landfall in SCA provide a majority of the area’s annual total precipitation as well as some of the region’s highest intensity rainfall. As SCA is prone to both drought as well as precipitation-induced hazards, and because the area is dependent on relatively few precipitation events to provide the bulk of annual rainfall totals, any changes to storm frequency or intensity may dramatically impact the region. It imperative to understand the characteristics and mechanisms behind high-impact ARs events that landfall in SCA in order to properly forecast and prepare for future occurrences, particularly as these events are important for water management and hazard mitigation.

We develop an algorithm that uses daily total precipitable water fields from reanalysis to identify AR activity impacting North America’s western coast from 1979 to 2013 and categorizes identified AR events according to landfall region. Additional reanalysis fields are used to create composites of atmospheric variables prior to, on the day of, and after AR landfall in order to determine and differentiate the defining characteristics for ARs impacting these varying regions. This allows us to characterize the atmospheric makeup behind ARs impacting SCA, including possible indications as to the mechanisms behind their initiation as well as trajectory.