Multiscale Ensemble Sensitivity Analysis of Two Landfalling Atmospheric River Events during January and February 2017

Atmospheric rivers (ARs) are narrow corridors of enhanced water vapor transport in the lower atmosphere that contribute significantly to the hydrological cycle and regional climate. ARs occur in midlatitude regions across the globe and often make landfall on the west coasts. The U.S. West Coast is frequently impacted by ARs from the Pacific Ocean, which contribute 30-50% of its total annual rainfall. On the one hand, the precipitation ARs bring constitutes a crucial source of fresh water over the semiarid western U.S. On the other hand, extreme ARs can lead to severe flooding events and other damage (e.g., strong winds, landslides, debris flows). For example, a strong AR made landfall over the U.S. West Coast on 8-9 January 2017, and brought over 300 mm precipitation over large portions of the Sierra Nevada Mtns, and several locations over the CA Coastal Mountains. On February 7 2017, the extreme water flows associated with landfalling ARs created a massive hole in the primary spillway of the Oroville dam at California, the tallest dam in the U.S. It caused an emergency evacuation of about 190,000 people living downstream of the dam. Given the large environmental and societal impacts of the landfalling AR events, it is vital to forecast them as accurately as possible by numerical weather prediction (NWP), including in the short range (1-2 days), the medium range (3-7 days), and even longer if possible. In this study, ensemble sensitivity analysis, which employs a linear correlation and regression between a chosen forecast metric and the forecast state vector, is employed to analyze the uncertainty development for both short- and medium-range forecasting of the two landfalling AR events mentioned above. The ensemble sensitivity results will be checked by the perturbed simulations using the WRF-ARW system tailored to western U.S. weather and hydrological extremes.

The verification periods for the two AR events are January 7-9 and February 7-10. To compute ensemble sensitivity, we have used the 1-7-day ensemble forecasts from the 50-member European Center for Medium-Range Weather Forecasts (ECMWF) model retrieved from the TIGGE archive. The most significant findings of this study are: 1) the area averaged precipitation over northern California is more associated with the low level moist process in short-term forecasts; 2) both the precipitation area and amount are affected by the upstream Rossby wave packets from western Pacific in the medium range; 3) the high-latitude ridge over the North Pacific is the key upstream system changing precipitation forecasts in the first case, while in the second case the deep low crossing the central Pacific from southern Japan is the key system modifying the precipitation forecast. Overall, the AR ensemble forecasts are more affected by the upper-level flow patterns in the medium range while the moisture and the low-level jet are more important in the short term. The results from perturbed initial condition experiments around the upstream ridge and the deep low using the WRF model will also be shown.