The influence of landfalling atmospheric rivers on U.S. west coast precipitation during February 2014

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Kawzenuk, Brian K.
Atmospheric Rivers (ARs) are long narrow corridors of high vertically integrated water vapor (IWV) and high integrated water vapor transport (IVT). The regions of high IWV and IVT are typically < 1000 km wide and >2000 km long. ARs represent the majority of extratropical moisture transport and contribute to extreme precipitation and flooding along the U.S. West Coast. Several ARs made landfall along the U.S. West Coast during February 2014and contributed a significant percentage of water year precipitation during an anomalously dry winter. This study investigates the role that landfalling ARs played in precipitation production throughout the western United States during February 2014 as well as their influence on widespread drought after a long dry period. NCEP-GFS model analyses and NCEP Stage-IV precipitation data were used to monitor the lifecycles of ARs and their effect on precipitation production, respectively. In total, twelve ARs made landfall along the U.S. west coast and produced locally as much as 900 mm of precipitation during February 2014. Strong correlation coefficient values between daily average IWV/IVT and 24-h precipitation amounts were found. The analyses indicate that along with IVT magnitude, IVT vector orientation with respect to orography plays a large role in precipitation production. If IVT vectors were oriented roughly parallel to the upslope terrain gradient then enhanced saturated ascent and topographic enhancement of precipitation occurred, whereas little to no precipitation occurred during events where IVT vectors were oriented perpendicular to the upslope terrain gradient. Analyses of the IWV tendency (IWVT) through the IWV budget of two AR events that made landfall during February 2014 suggested differences in moisture source regions and mechanisms that lead to the development and maintenance of the ARs. The analysis of these two ARs suggest that differences in the IWVT can produce large differences in the structure of ARs prior to landfall and precipitation distributions during landfall.