Investigating Atmospheric Rivers Affecting Southern California

Southern California (SCA) is prone to numerous precipitation-induced hazards including flooding and landslides, and on the opposite end of the spectrum is also susceptible to periods of little or no rainfall resulting in droughts as well as fires. Home to millions of people that are generally concentrated along the coast, this region requires accurate forecasting to plan and mitigate for hazardous events as well as for proper water resource management. Unfortunately, most of SCA's annual precipitation total occurs from relatively few storm events per season and any changes to storm frequency and/or intensity may have dramatic regional consequences. As SCA and the state as a whole is in the midst of one of the worst droughts on record, it is imperative to understand the features responsible for any rainfall that may occur as this is essential to accurate forecasting. Atmospheric rivers (ARs) are one such event that bring significant precipitation to SCA and are comparable to highways of water vapor flux. These features transport moisture from tropical to midlatitudes and are responsible for a majority of the planet's meridional water vapor movement. ARs are important to coastal environments, particularly those with mountainous terrain such as in SCA, as they induce high intensity precipitation through orographic forcing.

We develop an algorithm to identify AR events affecting North America's west coast from 1979 to 2013 using daily total precipitable water fields calculated from reanalysis. Landfall latitudes of identified ARs are determined and ARs are categorized into designated landfall regions. Composites of atmospheric characteristics according to landfall area are created to investigate the varying features and spatial distributions between ARs landfalling at different latitudes. In addition, we conduct trajectory analyses of several high intensity AR events to investigate their initiations and life cycles.