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

This study examined a slow-moving long-wave trough that brought four Atmospheric Rivers (AR) episodes within a week to the U.S. West Coast from 28 November to 3 December 2012, bring over 500 mm to some coastal locations. The highest 6- and 12-hourly rainfall rates (131 and 195 mm, respectively) over northern California occurred during Episode 2 along the windward slopes of the coastal Santa Lucia mountain range. Surface observations from NOAA's Hydrometeorological Testbed (HMT) sites in California, available 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 seven combinations of observation nudging, grid nudging, and 3DVAR to improve the upstream moisture characteristics during this event.

Results during the 6-hrly rainfall maximum period in Episode 2 revealed that the models underestimated the observed 6-hrly rainfall rate maximum on the windward slopes of the Santa Lucia mountain range. The grid-nudging simulations smoothed out finer-mesoscale details in the inner domain that may affect the final QPFs. The non-grid-nudging 3DVAR simulations with COSMIC GPS RO data demonstrated results with the least mean absolute error in the time evolution of accumulated rainfall for both the north central Coastal Range and the windward Sierra Nevada regions. The surface nudging experiment showed the least error in the north Central Coastal Range for the final rainfall accumulation forecast. The 3DVAR cycling simulation's accumulated rainfall forecast was the closest to observations for the windward Sierra Nevada. Results from this study further suggest that observation nudging time windows and 3DVAR cycling intervals less than 3 h would be more beneficial for short-to-medium range mesoscale QPFs during high-impact AR events over northern California.