The Landfall and Inland Penetration of a Flood-Producing Atmospheric River in Arizona. Part 2: Impacts of WRF Resolution on water vapor transports and precipitation

Atmospheric rivers (ARs) are a dominant mechanism for generating intense wintertime precipitation along the west coast of continents in the midlatitudes, including the U.S. West Coast. While studies over the past 10 years have extensively explored the impact of ARs on the temperature and precipitation in, and west of, California's Sierra Nevada and the Pacific Northwest's Cascade Mountains, their influence on the weather across the intermountain west remains an open question. Following the detailed observational analysis of Part 1 of an intense AR storm that caused heavy precipitation across AZ and the Four Corners region of the United States, we utilize high-resolution Weather Research and Forecast model (WRF) simulations of this storm to investigate water vapor pathways into the Intermountain West. Four sets of simulations are produced using WRF to downscale the Climate Forecast System Reanalysis with different combinations of horizontal and vertical resolutions. A control run with 3-km grid spacing and 40 vertical levels is first validated with observations from Part 1 of this study, and then used to assess the quality of the sensitivity runs. We then test the overall sensitivity to horizontal resolution on both simulated precipitation and water vapor transports through the region by generating simulations identical to the control run except with grid spacings of 9, 27, and 81 km. Next we test precipitation sensitivity to the resolution of water vapor transport across coastal terrain by coarsening grid spacing over the coastal terrain to 9, 27, and 81 km while retaining 3 km grid spacing over AZ and the Four Corners region. Finally, both sets of simulations are repeated with coarser vertical resolution to assess the relative improvement from increased horizontal grid spacing compared with improvement from increased vertical resolution. This two-part study is part of a larger project aimed at identifying moisture sources for heavy precipitation events across the Intermountain West.