The role of convective organization and the low level jet in the overland reintensification of Tropical Storm Erin (2007)

During the early morning hours of 19 August 2007, approximately 60 hours after making landfall along the central Texas coastline, the remnant vortex associated with Tropical Storm Erin dramatically reintensified across west-central Oklahoma, including the development of a low-level eye-like feature and achievement of an intensity far more intense than it ever did over water. While the work of Arndt et al. (2009, BAMS) detailed synoptic- and mesoscale conditions associated with this evolution, the precise physical and dynamical mechanisms behind this reintensification have as of yet to be conclusively determined in the literature and are an active subject of our research. In this work, we present results from a subset of this work, focusing upon the structure and influence of deep convection and the nocturnal lower tropospheric jet (LLJ) upon the reintensification process.

Using output from a high resolution, convection-permitting Advanced Research Weather Research and Forecasting (WRF-ARW) model simulation of the reintensification period, it is found that surface-based convection develops in an abnormally moist, unstable environment along a convergence axis that develops between the southwesterly flow around the vortex and southerly flow associated with the nocturnal LLJ to the east of the remnant vortex. This moist, unstable environment is supported by the maintenance of boundary layer moisture from the Gulf of Mexico along inflowing trajectories aligned with the LLJ as they pass over abnormally wet soils wetted by the remnant cyclone on 16-17 August. Convective cells, each exhibiting significant mesovortices, are shown to have a structure akin to those found in studies of convectively-active MCVs (e.g. Yu et al. 1999, MWR) and tropical cyclone rainbands (e.g. Hence and Houze 2008, JGR-A). As the remnant vortex moves eastward through Oklahoma, into the axis of the LLJ, convection organizes and vorticity axisymmetrizes around the center, resulting in the development of the eye-like feature and accordant deepening of the vortex.

Analyses will be shown highlighting three key components of the convective contribution to the reintensification. First, the role of convection in generating and enhancing lower tropospheric vorticity in an environment of pre-existing thermodynamic instability and cyclonic vorticity will be shown (e.g. Montgomery et al. 2006, JAS). Circulation budget analyses will be presented to precisely show how this affects the intensity of the vortex. Secondly, the contribution of convective cells to the strength of the LLJ and, presumably, vortex will be highlighted (e.g. Hence and Houze 2008, JGR-A). Finally, quantification of how the latent heat release associated with the deep convection can contribute favorably to the development of the vortex will be presented (Nolan et al. 2007, JAS). Comparisons will be drawn between said role of convection in the reintensification process and its role in the evolutions of more traditional continental mesoscale convective vortices (MCVs).