Extreme Surface Winds During Landfalling Atmospheric Rivers: The Modulating Role of Near-Surface Stability

28 Atmospheric rivers (ARs) are long and narrow regions of strong horizontal water vapor29 transport. Upon landfall, ARs are typically associated with heavy precipitation and strong surface30 winds. A quantitative understanding of the atmospheric conditions that favor extreme surface31 winds during ARs has implications for anticipating and managing various impacts associated with32 these potentially hazardous events. Here, a global AR database (1999–2014) with relevant33 information from MERRA-2 reanalysis, QuikSCAT and AIRS satellite observations are used to34 better understand and quantify the role of near-surface static stability in modulating surface35 winds during landfalling ARs. The temperature difference between the surface and 1 km MSL (DT;36 used here as a proxy for near-surface static stability), and integrated water vapor transport (IVT)37 are analyzed to quantify their relationships to surface winds using bivariate linear regression. In38 four regions where AR landfalls are common, the MERRA-2-based results indicate that IVT39 accounts for 22-38% of the variance in surface wind speed. Combining DT with IVT increases the40 explained variance to 36-52%. Substitution of QuikSCAT surface winds and AIRS DT in place of41 the MERRA-2 data largely preserves this relationship (e.g., 44% compared to 52% explained42 variance for USA West Coast). Use of an alternate static stability measure–the bulk Richardson43 number–yields a similar explained variance (47%). Lastly, AR cases within the top and bottom44 25% of near-surface static stability indicate that extreme surface winds (gale or higher) are more45 likely to occur in unstable conditions (5.3%/14.7% during weak/strong IVT) than in stable46 conditions (0.58%/6.15%).