Sam Lillo-February 15

The Rossby Wave Roadmap and Implications on Medium-Range Predictability

Despite decades of improvements in the performance of NWP, there are still times when forecast models experience errors that are drastically greater than the mean. These periods of poor performance are referred to as dropouts, or busts, and are characterized by a verifying atmosphere that is significantly different from the forecast. Often these situations are noticed by the public as manifested through perceptible differences from anticipated temperatures or precipitation. Of even greater importance are occasions when these busts involve a failed forecast of a high impact weather event.

Given the potential societal impacts of bust events, there is a special need to systematically investigate these cases. This current study works toward providing insight into flow regimes associated with low inherent predictability by identifying situations featuring an elevated frequency of forecast failures associated with high impact events. These events are often consequence of high amplitude Rossby waves, attributed to one or more Rossby wave packets (RWPs). Two methods of diagnosing RWPs and their waveguides are employed using wave activity flux and ray tracing. Through analysis of the background flow fields that modulate the track of RWPs, and identification of the critical lines that define waveguides, connections can be drawn between subseasonal and seasonal variability and RWP track and frequency. While the existence of long-tracking RWPs can increase intrinsic predictability, the associated waveguide can act as a channel for amplifying and communicating errors around the globe. The first step in anticipating high impact forecast failure potential in the one to two-week range then becomes identifying the nature of the waveguide, and potential wave activity sources in the vicinity.