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CR prepress abstract   -  DOI:

Backward trajectories analysis of southern California atmospheric rivers

Sarah M. Harris*, Leila M. V. Carvalho

*Corresponding author:

ABSTRACT: Atmospheric rivers (ARs) are filamentary channels of high water vapor flux that transfer moisture horizontally through the atmosphere at low-levels. ARs are often responsible for large annual rainfall totals as well as high-intensity storms due to orographic forcing. ARs are important features when predicting hazardous events such as flooding and are vital components of many regional water budgets. This is especially true for drought-prone areas such as southern California (SCA), which experiences relatively few storms per season, many from AR events. Here we use a Lagrangian model to create backward air parcel trajectories of 159 AR events that made landfall on the U.S. west coast from December 2004 to December 2015. Trajectories are used to examine the lifecycles and movements of these ARs and to differentiate ARs that made landfall in different regions. Prior to landfall, SCA ARs share similarities to but also have distinct differences from other ARs. At 1000 m above mean sea level (MSL) SCA AR trajectories travel shorter distances over the same 72-h time frame than trajectories for ARs that made landfall farther north. Additionally, along-trajectory measurements for SCA ARs tend to be warmer and have higher specific humidity values. This applies to both the 1000 m and 2000 m MSL levels. These results imply that SCA ARs move slower and have the potential to produce higher-intensity storms at landfall. An analysis of a case study event of an extreme AR that made landfall in SCA on February 17, 2017 confirms these results.