Lagrangian Flow Boundaries, Transport, and Vortex Cores in Pre-genesis Disturbances

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

This research builds on the "marsupial paradigm" to examine which pre-genesis disturbances eventually become tropical storms and hurricanes. A favorable environment most typically exists within African Easterly Wave (AEW) critical layer flows as an essential precursor environment for the dynamics (wind) and thermodynamics (heat and moisture) to support development. Though a protected kinematic environment is necessary for development, typically less than 20% of these waves actually produce hurricanes. Classic methods to evaluate the near storm environment use an Eulerian framework in which the storm is studied as it flows through a fixed domain. The proposed research seeks to study these flows through the use of Lagrangian Coherent Structure (LCS) analysis in which the diagnostic tools follow the air flowing into the developing pre-storm environment. Influx of moisture, dry air and vorticity (spin in the atmosphere) across closed streamlines of the steady flow disrupt the dynamics that support development in an otherwise protected environment. Intellectual Merit The use of Lagrangian Coherent Structures in diagnosing the pre-genesis storm environment may lead to: (1) An improved understanding of the lateral flow-following transport and exchange of air masses interior and exterior to the wave pouch (2) An improved description of transitions from tropical depression to tropical storm including diagnosis of the kinematic protection due to the shear-sheltering of the vortex core (3) Extending the kinematic analysis to study vertical alignment of Lagrangian boundaries and consequences of alignment or misalignment for cyclogenesis (storm spin-up) (4) Addressing predictability of cyclogenesis by applying these methods to ensemble forecasts Broader Impacts A more complete understanding of the role of flow-following (Lagrangian) boundaries and vortex cores may aid in improved forecasting of storm spin-up with greater lead-time and lower false alarm rates.