Atmospheric Rivers in the Northwestern Pacific: Source Regions and Observation Impact

Significant progress has been made in understanding the characteristics of Atmospheric Rivers (ARs) in the Northeastern Pacific and improving the prediction of ARs and their impacts over western North America. Much of this progress has been facilitated through the international AR Reconnaissance (AR RECON) Research and Operations Partnership. AR RECON observations, including both dropsondes and surface pressure observation from drifting buoys (drifters), have been shown to be very impactful, likely due to the fact that these observations are taken in regions of very few other in-situ observations, and, in the case of the dropsondes, often in cloudy regions where satellite-based observations may be less accurate. Recently, AR RECON has expanded to consider the Northwestern Pacific, in recognition of the fact that this region also has very few in-situ observations and errors here may propagate to North America over just a few days. In this study, we use Forecast Sensitivity Observation Impact (FSOI) from the Navy Global Environmental Model (NAVGEM) data assimilation and forecast system to evaluate the impact of the surface pressure observations from drifters in the Northwestern Pacific on forecast error reduction for January through March 2024, including the AR RECON drifters deployed by the Air Force 53^rd Weather Reconnaissance Squadron out of Guam in mid January. Preliminary results indicate that the drifters with the most impactful surface pressure observations are located in the Philippines Sea and over the tropical Western Pacific. Earlier research has shown that surface pressure observations in the Northeastern Pacific are most beneficial when they occur in regions of high integrated vapor transport, large pressure gradients, or when they are relatively isolated. We will perform similar analyses to see if this is also true for the Northwestern Pacific drifter observations. We will complement the FSOI analysis with AR climatologies produced using ERA5 reanalysis and backward trajectory techniques to discern if high-impact surface pressure observations are occurring in regions of AR inflow or genesis regions. Back trajectories are initiated every 6 hours from within individual AR filaments that become spatially co-located with the regional monthly AR climatological maximum. Air parcel quantities (e.g., altitude, temperature, moisture content) and along-path surface parameters (e.g., surface fluxes) are traced along trajectory segments over a multiday period in order assess upstream contributions and source regions. These techniques are used to assess AR hot spots and source regions in the Northwestern Pacific and other regions.