Diabatic Contributions to the Formation and Evolution of Mesoscale Frontal Waves in Atmospheric River Events along the U.S. West Coast

Atmospheric rivers (ARs) often provide necessary water resources to regions such as the western United States; however, ARs can also be responsible for damaging flood events due to the production of extreme precipitation. The severity of the flooding impact associated with an AR event is determined by its strength and persistence. A factor in the maintenance of ARs is the presence of mesoscale frontal waves (MFWs), which can increase the longevity of ARs and therefore, threat of extreme precipitation. To improve precipitation forecasts, it is necessary to understand the dynamics of the interactions between ARs, mesoscale frontal waves, and extreme precipitation events.

To build on this growing area of research, we have simulated 13 AR events based on the presence of MFW features during the evolution of the event using the Model for Prediction Across Scales (MPAS). We utilized the 10–60-km variable resolution mesh, centering the high-resolution area just north of Hawaii to encompass the majority of the North Pacific Basin and Western United States. Our control simulations are initialized 36-h prior to MFW formation and integrated through the end of each event; results are verified using the ERA5 reanalysis and observations (e.g., satellite-derived radar reflectivity, dropsonde profiles) when available. Applying potential vorticity (PV) inversion techniques allows us to quantify the relative contributions of baroclinic and diabatic processes in each event, and therefore, assess the importance of latent heating to MFW formation and evolution.

Previous work suggests that while diabatic processes play a significant role in the formation and/or maintenance of frontal waves, sufficient upper-air support is often needed for the wave to intensify into a secondary cyclone. In the context of AR events, however, not all MFWs result in secondary cyclogenesis. Nevertheless, the presence of MFWs have a large impact on the intensity and position of ARs and consequently, the landfalling precipitation. Therefore, by sampling a variety of AR events with various MFW evolutions, we aim to identify key mechanisms in determining the structure of MFWs in the context of ARs with the hopes of improving the predictability of high-impact precipitation events.