Quantification of the processes driving the overland reintensification of Tropical Storm Erin (2007)
A multi-member ensemble of WRF-ARW V18.104.22.168 convection-permitting numerical simulations is used to understand the role of underlying land surface conditions. Ensemble members are created by objectively varying properties of soil moisture and soil temperature input to the land-surface model at the outset of each simulation. Results show that greater soil moisture content, as observed in August 2007, results in enhanced surface heat and moisture fluxes both along the track of and well removed from the remnant vortex. This in turn leads to a shallower, moister boundary layer, resulting in greater convective potential instability and a reduced potential to support deleterious divergent near-surface convective cold pools. In both the reintensifying and non-reintensifying simulations, convection initiates along a convergence axis aligned with the nocturnal low level jet (LLJ) early on 19 August 2007. This convection exhibits greater updraft magnitudes, vertical mass fluxes, and diabatic heating rates within the reintensifying ensemble members featuring greater soil moisture content. Within a moist environment throughout the depth of the troposphere, this allows for the efficient generation of near-surface relative vorticity. Soil temperature appears to have minimal upon the evolution. Differences between this evolution and that described by Emanuel et al. (2008, MWR) will be presented.
Utilizing high frequency output from the control simulation of the aforementioned multi-member ensemble, the simulated reintensification process is described. Stretching of pre-existing vertical vorticity associated with the remnant vortex by convection starts the reintensification process. Updrafts associated with this convection further tilt ambient horizontal vorticity associated with the LLJ; stretching of this tilted vorticity by the convective updrafts further intensifies the vortex. The initially asymmetric nature to the convection supports growth of the vortex in a manner similar to that described by Nolan et al. (2007, JAS). After this occurs, axisymmeterization of vertical vorticity elements about the remnant vortex results in the formation of an intense, nearly axisymmeteric ring of relative vorticity in the boundary layer. This evolution is qualitatively similar to the tropical cyclogenesis process described by Montgomery et al. (2006, JAS). Circulation and potential vorticity budgets highlighting this evolution will be discussed in conjunction with further discussion about the dynamical role of the LLJ to the observed and simulated reintensification process.