An analysis of the dynamical coupling in the development of persistent landfalling atmospheric rivers over the eastern North Pacific

Abstract:

Landfalling atmospheric rivers (ARs) are linked to severe flooding and precipitation events, most notably along the western coast of North America. The duration and magnitude of landfalling ARs is correlated to their hydrological impacts on land. The forecast of these hydrologically significant landfalling events can be improved through a better understanding of the mechanisms leading to their formation and evolution prior to landfall. In order to investigate these mechanisms, an analysis of the interaction and feedback between intense lower-level moisture transport and associated upper-level dynamics is performed using simulations from the Weather Research and Forecasting (WRF) model. 

A subset of persistent landfalling AR events is identified using 3hrly MERRA reanalysis and validated against observational datasets. These events are identified as lower tropospheric wind and moisture features with extended geometry that persist over the coastline for longer than two days. A composite analysis of persistent landfalling events shows an eastward extended, perturbed upper-level jet and anticyclonic overturning of potential vorticity contours, indicating anticyclonic Rossby wave breaking. The role of lower-level moisture transport in the development of these upper-level characteristics and impact on the duration of the event are investigated.