Characteristics of Tropical Cyclone Formation as Depicted by the Marsupial Paradigm Automated Lagrangian Okubo-Weiss Tracker for the GFS

In the summer of 2017, Blake Rutherford of NorthWest Research Associates (NWRA) and colleagues at the Naval Postgraduate School (NPS) began running an automated Lagrangian Okubo-Weiss (OW) tracker based upon principles of the marsupial paradigm. While Eulerian OW is effective at identifying vortex cores at a given place and time, it has limitations in weak disturbances and time-dependent flow. By integrating the eigenvalue of the velocity gradient tensor along particle trajectories, we define the Lagrangian OW parameter as essentially a time-smoothed version of the Eulerian OW parameter that is then mapped to a horizontal grid to create the Lagrangian OW field.

The 00 UTC forecast of the NCEP GFS 0.25^o resolution global field is assessed every day and posted online, currently at http://96.127.43.56/global.php. Basin maps are produced daily for three basins: North Atlantic, Eastern/Central North Pacific, and Western North Pacific. An important lesson learned during the 2017 northern hemisphere tropical season was the determination of appropriate minimum 700-hPa Lagrangian OW and moisture values in order to warrant the display of a particular vortex during any particular GFS forecast. Enhancements to the code made during the season allowed for the identification of merging circulations, an identification of a lower-level 850-hPa vortex associated with the 700-hPa vortex to know of a circulation’s vertical organization, and the plotting of a pouch-defining manifold to describe recirculating flow aiding in convective development. By the end of the season, an appropriate balance resulted in a minimal number of false alarms without allowing many misses.

Our hypothesis is that vortices that meet these requirements depict tropical depressions in GFS when they have both a minimal amount of 700-hPa moisture as well as Lagrangian OW values of 13 or 14. Further intensification to tropical storm status is characterized by a central Lagrangian OW maximum of 15-30 along with a ring of strain-dominant negative Lagrangian OW, called a “shear sheath”, that surrounds the storm center, thereby shielding it from detrimental environment conditions. Hurricanes tend to have Lagrangian OW maximum values greater than 30. Upon reaching a developed state, vortex identifiers change color on the website’s posted basin plots, providing a useful tool for forecasters. Refined performance of how these GFS forecast vortices compared with reality, with regards to timing, structure, and intensity at the time of tropical cyclone formation, will be presented. Because the Lagrangian OW tracker operates year-round, up-to-date performance, including southern hemisphere circulations, will also be presented.