Influences of the diurnal cycle and the low-level jet on the inland reintensification of Tropical Cyclone Erin (2007)

Tropical Cyclone Erin (2007) made landfall along the Texas coast as a weak tropical storm, and after slowly recurving poleward over Texas and Oklahoma, it briefly reintensified to tropical storm intensity with observed sustained surface winds of 25 m s^-1. This study examines the evolution of the vortex as it progressed inland, recurved and reintensified, with a particular focus on diurnal variations in the strength of the circulation. Two potentially important mechanisms are explored: 1) the position and intensity of the low-level jet (LLJ) in relation to the vortex, and 2) the fluxes of heat and moisture from the land surface. First, observations and model analyses are used to demonstrate diurnal variations in the LLJ and how it alters the intensity and distribution of convection around the vortex. This analysis shows invigoration of convection during the overnight hours of both 18 and 19 August, in conjunction with enhanced low level inflow, convergence, and lifting at the exit region of the LLJ. These factors are compared with recent work that described interactions between the LLJ and continental mesoscale convective vortices.

Next, a multi-member ensemble of WRF-ARW V3.0.1.1 model convection-permitting simulations is used to understand the role of underlying land surface conditions. Ensemble members are created by objectively varying properties of soil moisture, soil temperature, and rainfall impacts to the land-surface model. Results suggest that the presence of deep tropical moisture across the domain, coupled with a favorable orientation of the simulated cyclone with respect to the LLJ, combine to result in more intense and better focused deep moist convection near the center of the cyclone. Soil temperature appears to have little to no impact upon the evolution. Differences between this evolution and that described by Emanuel et al. (2008, MWR) will be presented. Further sensitivity to model initialization time and conditions as well as planetary boundary layer parameterizations will be presented alongside the ensemble results. A companion presentation by Evans et al. will examine the role of deep convection near the cyclone center in the observed and simulated reintensification.