The Effect of Mesoscale Frontal Waves on Landfalling Atmospheric Rivers and Their Associated Precipitation Over the Green River Watershed, Washington

In this study, the effect of mesoscale frontal waves (MFWs) on extreme precipitation (EP) in the Upper Green River Watershed (UGRW), Washington is investigated. 205 EP days (>95th percentile) are identified in the UGRW between the years 1980 and 2021. To characterize the range of large-scale meteorological conditions associated with EP days, the self-organizing map (SOM) approach is used to cluster daily integrated water vapor transport (IVT) on EP days. To further diagnose the meteorological drivers, composites of several other diagnostic fields are constructed for each SOM node and the preceding days. Together, these results illustrate common orientations of IVT capable of producing EP in the UGRW. Out of 205 EP days, an AR was present during 188 days, 91.7%. A twenty-year catalog of mesoscale frontal waves (MFWs) that formed on U.S. West Coast landfalling ARs from 1999 to 2019 is created. The catalog is used to identify ARs that caused EP in the UGRW (n=56). The ARs are divided into two groups -- those without MFWs (n=30) and those with MFWs (n=26). The groups are compared to determine a statistically significant difference in total-event precipitation, maximum daily-accumulated precipitation, storm duration, maximum IVT, and a widely-used categorical AR scale. When an MFW was present, total-event precipitation increased by 34.1 mm, duration increased by 25.7 hours, max IVT increased by 138 kg m-1 s-1, and categorization on the AR scale increased by an average of 0.9. There was no difference in maximum daily-accumulated precipitation between groups. Five case studies are conducted on ARs with associated MFWs that resulted in the greatest total-event precipitation levels in the UGRW. This research highlights the important role MFWs play in modulating AR intensity and duration.