Modeling Studies of Landfalling Atmospheric Rivers and Orographic Precipitation over Northern California

Atmospheric Rivers from eastern North Pacific winter storms making landfall on the California Coast can cause heavy orographic precipitation on the windward side of the Coastal Range and the Sierra Nevada mountains. Lack of observations over the eastern North Pacific Ocean can result in poorly resolved microphysical characteristics of the upstream airflow, adding more errors into mesoscale model initial conditions and ultimately short-term quantitative precipitation forecasts (QPFs). This study investigated a very slow-moving long-wave trough that brought three ARs within a week to the U.S. West Coast from November 28th to December 04th 2012. This event caused flood warnings along the Russian and Napa Rivers and flood watches in many Northern California counties. Surface observation data from NOAA's Hydrometeorological Testbed (HMT) sites and GPS Radio Occultation (RO) vertical profiles from the Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) satellite mission were both assimilated into WRF-ARW via a combination of observation nudging and 3DVAR to improve the upstream moisture characteristics during this event. Precipitation distributions and surface results from these WRF simulations are compared with NCEP Stage IV precipitation analysis and HMT rain gauge datasets to evaluate any possible improvement in the QPFs. The goal of this study is to quantify the uncertainty of such weather events and to improve the predictability of precipitation over coastal mountainous regions.