Summarizing Relationships Among Landfalling Atmospheric Rivers, Integrated Water Vapor Transport, and California Watershed Precipitation 1982–2019

June 9, 2022

The paper “Summarizing Relationships Among Landfalling Atmospheric Rivers, Integrated Water Vapor Transport, and California Watershed Precipitation 1982–2019” by Joseph Ricciotti and Jay Cordeira was recently published in the American Meteorological Society Journal of Hydrometeorology. Ricciotti is a former graduate student of Cordeira at Plymouth State University whose M.S. thesis was sponsored by the USACE-funded Forecast Informed Reservoir Operations program and contributes to the goals of CW3E’s 2019-2024 Strategic Plan to support atmospheric river (AR) research and applications.

The study investigates the relationship among landfalling ARs, integrated vapor transport (IVT) associated with landfalling ARs, and watershed mean areal precipitation (MAP) for a 38-year period over California. On average, the daily average IVT magnitude at different coastal locations explains ~34% of the variance in annual watershed MAP across 140 HUC-8 watersheds with large spatial variability across California. Further investigation of the IVT magnitude and direction at coastal locations illustrated that accounting for water vapor transport direction increases the explained variance in annual MAP to an average of 45%, with highest values (~65%) occurring in watersheds over northern and coastal California similar to prior studies by CW3E authors. Similar investigation of the lower-tropospheric water vapor flux vector at 850 hPa and 925 hPa revealed further increases in the explained variance in annual MAP to an average of >50%.

The results of this study (1) emphasize the importance of both IVT direction and water vapor flux altitude to watershed MAP, (2) align well with previous studies for select locations that highlight the importance of upslope (i.e., lower tropospheric) water vapor flux during landfalling ARs and precipitation, and (3) motivate the development of AR-related and watershed-centric forecast tools that incorporate IVT direction and water vapor flux altitude parameters in addition to IVT magnitude.

Figure 6a,b of Ricciotti and Cordeira (2022): California watershed maximum correlation values (r²) for (a) IVT magnitude and watershed MAP and (b) projected IVT and watershed MAP. What does this figure imply? A: Incorporating both IVT magnitude and direction in order to emphasize key upslope processes during landfalling ARs helps explain more of California’s watershed precipitation than IVT magnitude alone, especially across northern California.

Ricciotti, J. A., and J. M. Cordeira, 2022: Summarizing Relationships Among Landfalling Atmospheric Rivers, Integrated Water Vapor Transport, and California Watershed Precipitation 1982–2019. J. Hydromet., Early Online Release: https://doi.org/10.1175/JHM-D-21-0119.1.