The Influence of the Low-Level Jet Upon the Overland Reintensification of Tropical Storm Erin (2007)

During the early morning hours of 19 August 2007, three days after landfall, the remnant circulation associated with Tropical Storm Erin dramatically reintensified over west-central Oklahoma. Previous studies in the peer-reviewed literature have documented the structure and environment of the reintensifying tropical cyclone; its contributions to a predecessor rainfall event across the Upper Midwest of the United States; and the contributing role of abnormally moist soil moisture conditions on synoptic-to-seasonal time scales to the reintensification of the tropical cyclone.

In the latter of these studies, it was demonstrated that the maintenance of boundary layer moisture along inflowing air parcels from the northwestern Gulf of Mexico was a critical contributor to Erin's reintensification. These inflowing air parcels were embedded within a southerly low-level jet feature between the remnant Erin circulation and a subtropical ridge of high pressure centered over the southeastern United States. However, the precise kinematic and thermodynamic influences of this low-level jet upon Erin's overland reintensification remain uncertain.

In this research, we utilize a thirty-member ensemble of 4 km WRF-ARW v3.4.1 numerical model simulations to investigate the influence of the low-level jet upon Erin's overland reintensification. The Ensemble Kalman filter implementation provided by the WRF-DART data assimilation system is utilized to provide perturbed initial and lateral boundary conditions for these simulations. The ensemble is initialized at 1800 UTC 17 August 2007, while numerical simulations begin at 0000 UTC 18 August 2007 and extend forward until 1800 UTC 19 August 2007. Physical process parameterizations are selected as appropriate for the modeling of tropical cyclones.

The thirty-member ensemble produces a diverse range of solutions for Erin's simulated intensity and track evolution. Ensemble sensitivity and part correlation analyses are conducted to examine the relationship between Erin's overland reintensification and the low-level jet. Composite atmospheric fields from the simulations containing the strongest and weakest simulated Erin vortices are analyzed to further examine this relationship. Key research findings will be discussed and implications will be drawn toward the possible influence(s) of lower- to middle-tropospheric jets, such as the African easterly jet, upon tropical cyclogenesis.