Calculating the Propagation Speed of Landfalling Atmospheric Rivers in Coastal California

I was fortunate to take Mesoscale Dynamics at the University at Albany with Professor Keyser where we explored the kinematic and thermodynamic processes responsible for frontal evolution. This research and presentation are inspired by the relatively simple, yet profoundly complex question posed in that class: “What is the expression for the speed of the movement of an isentrope on an isobaric surface?” In other words, what is the speed of a front?

This research is motivated by landfalling atmospheric rivers (ARs) along the U.S. West Coast. ARs owe their formation and evolution in part due to frontal dynamics as they transport water vapor along low-level jet streams ahead of surface cold fronts. The landfall of an AR is often accompanied by orographic precipitation in the Western U.S. where the storm-total precipitation is often driven by both the intensity and duration of upslope water vapor flux. The dependence of storm-total precipitation on AR duration motivates a rather similar question to that posed above: “What is the speed of an AR?”

ARs are often defined within a Eulerian framework using integrated water vapor transport (IVT), yet their movement is driven by propagation of water vapor in tandem with the evolution of mid-latitude cyclones and their fronts. Herein we identify the north-to-south propagation speed of IVT within landfalling ARs along the California coastline, where slow propagation speeds lend itself to the potential for longer duration and higher impact precipitation. The period from October 1960 through February 2023 contained 561 landfalling ARs following the identification of Ralph et al. (2019) whose core of maximum IVT magnitude crossed the San Francisco Bay Area (either from north to south or south to north). In the six-hour window of that “crossing” the average meridional propagation speed of these ARs was –8.0 m/s (from the north at 28.9 km/h or ~0.8 degrees of latitude per 3 hours). A majority (91%) of propagation speeds were negative capturing the landfalling characteristic of southward propagation along the coast. The distribution of landfalling AR propagation speeds with a mean of –8.0 m/s and standard deviation of 6.8 m/s aligns well with published frontal speeds within theoretical frameworks and case studies (e.g., Smith and Reeder 1988, among others). With the northwest-to-southeast orientation of the coastline in this region, the –8.0 m/s meridional propagation speed corresponds to a southward coastal propagation speed of ~9.2 m/s.