A journal article entitled The Landfall and Inland Penetration of a Flood-Producing Atmospheric River in Arizona. Part II: Sensitivity of Modeled Precipitation to Terrain Height and Atmospheric River Orientation by M. Hughes, K. M. Mahoney, P. J. Neiman, B. J. Moore, M. Alexander, and F. M. Ralph was published in the October 2014 issue of the Journal of Hydrometeorology.
Following the observational case study of a landfalling, flood-producing atmospheric river (AR) across Arizona in late January 2010 led by P. Neiman, this study uses numerical models to explore the sensitivity of precipitation amounts during that AR to upstream terrain height and atmospheric river orientation.
To test sensitivity of precipitation across Arizona to upstream terrain and model grid spacing, a series of simulations are run using the Weather Research and Forecasting (WRF) Model and compared against a 3km control simulation. The control simulations are compared with eight WRF sensitivity experiments in which 1) model or terrain grid spacing decreases sequentially from 81 to 3 km and 2) upstream terrain is elevated. The drying ratio, a measure of airmass drying after passage across terrain, increases with Baja’s terrain height and decreases with coarsened grid spacing. Subsequently, precipitation across Arizona decreases as the Baja terrain height increases, although it changes little with coarsened grid spacing. Northern Baja’s drying ratio is much larger than that of southern Baja. Thus, ARs with a southerly orientation, with water vapor transports that can pass south of the higher mountains of northern Baja and then cross the Gulf of California, can produce large precipitation amounts in Arizona.
Then, using a linear model (LM) of orographic precipitation for a central-Arizona-focused subdomain, sensitivity to AR incidence angle is tested. The actual incidence angle of the AR (211°) is close to the optimum angle for large region-mean precipitation. Changes in region-mean precipitation amounts are small (~6%) owing to AR angle changes; however, much larger changes in basin-mean precipitation of up to 33% occur within the range of physically plausible AR angles tested. The diagram at right summarizes the results: The actual angle of the Jan 2010 AR was favorable for large precipitation amounts both because it was southerly enough to cross southern Baja’s somewhat lower terrain, and westerly enough to cause an extreme local response in the Verde river basin.
Contact: Mimi Hughes