The Interaction of Moisture Fluxes and Orographic Precipitation over Northern California Associated with a Landfalling Atmospheric River.

Abstract

Atmospheric rivers emanating from the tropics are responsible for the majority of the meridional transport of water vapor in the Northern Hemisphere, and have also been linked to episodes of heavy orographic precipitation along the mountains of the U.S West Coast. As moist air flow impinges on mountain ranges, orographic lifting converts water vapor to precipitation and can greatly reduce the moisture content of the airmass. The nearly along-coast parallel orientation of the Coastal and Sierra Nevada Ranges in Northern California, and the proximity of the Petaluma Gap to the south along the coast yield a geography, where moisture may enter the Sacramento Valley from multiple locations, complicating the quantification of airmass transformation over the region. Limitations of surface and satellite observing networks further complicate these calculations. In this study, the Weather Research and Forecasting Model (WRF) Version 2.2 is used to investigate the moisture flux and three-dimensional airmass transformation over northern California associated with the 29-31 December 2005 atmospheric river. Moisture flux analysis of the storm reveals that moisture enters the Sacramento Valley by both flowing over and around the Coastal Range. A large portion of the flow-over moisture is converted to precipitation along the windward slopes. Flow-around moisture enters through the Petaluma Gap, and then a significant portion is deflected northward by the strong barrier jet associated with the Sierra Nevada range. Moisture convergence and orographic lifting enhance precipitation along the slopes of the Sierra Nevada and Siskiyou Ranges. A drying ratio, or moisture reduction, of nearly 55% is found for the entire mountain complex, with 30% and 25% for the Coastal and Sierra Nevada Ranges, respectively. In a model sensitivity test where the Coastal Range is removed, the amount of moisture reaching the Sierras is only slightly increased compared to when the Coastal Range is present. When all terrain is removed, there is little reduction of moisture flux by the ocean/coast boundary, and the atmospheric river is able to penetrate deep into the western U.S.