Lagrangian Vortices in Developing Tropical Cyclones
Abstract
Tracking pre-genesis tropical cyclones is important for earlier detection of developing systems as well as targeting potential locations for dropsondes in field experiments. The use of a reference frame moving with the disturbance gives a more accurate depiction of streamlines and closed circulation than the Earth-relative frame. However, identification of recirculating regions does not require a choice of reference frame when marked by the Galilean invariant Eulerian OkuboWeiss (OW) parameter. While the Eulerian OW parameter is generally effective at identifying vortex cores at a given place and a given time, it has its limitations in weak disturbances and in time-dependent flows. Integrating the eigenvalue of the velocity gradient tensor along particle trajectories provides a time smoothing of the Eulerian OW parameter, and provides earlier detection with fewer false alarms. We refer to this integration along trajectories as the Lagrangian OW parameter. When mapped to a horizontal grid it becomes a Lagrangian OW field. The Lagrangian OW field has advantages over the Eulerian OW field in the detail of additional flow structures that it identifies. The Lagrangian OW field shows the Lagrangian boundaries that are present as a disturbance develops from an easterly wave, and a shear sheath that forms when a disturbance becomes self-sustaining, typically at tropical storm strength. Since all of these structures are Lagrangian, they are advected with the flow field, and display the continuous evolution of coherent flow features as the fluid evolves.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 25, 2015
- Accession Number
- AD1001949
Entities
People
- B. Rutherford
- Michael T. Montgomery
- T. J. Dunkerton
Organizations
- Naval Postgraduate School