A diagnostic analysis of high-resolution numerical simulations of the genesis of tropical storm Gert (2005)

The central focus of the tropical cyclogenesis problem in recent years has concerned the dynamical pathway by which a cyclonic mesoscale surface vortex is established, of sufficient strength to initiate the self-sustaining air-sea interaction feedback processes (WISHE) necessary for intensification of the incipient tropical cyclone (TC) vortex to a mature hurricane. Recent studies have introduced a new paradigm for tropical cyclogenesis which emphasize the contribution of deep cumulonimbus convection in the “bottom-up” development of the TC vortex, in contrast to earlier conceptual models which emphasized a “top-down” development from midlevel mesoscale convective vortices (MCVs) generated in the MCS stratiform cloud regions of pre-genesis tropical disturbances. The high-resolution cloud-resolving numerical simulations of genesis in idealized MCV-like environments (Montgomery et al. 2006, Nolan et al. 2007) and realistic case studies (Hendricks et al. 2004, Davis and Bosart 2006) have demonstrated that the construction of a warm-core surface-concentrated TC vortex proceeds via an upscale vorticity cascade process consisting of multiple diabatic mergers and axisymmetrization of “vortical” hot towers (VHTs), i. e. intense small-scale low-level cyclonic vortices generated from individual cumulonimbus clouds.

Although these studies suggest that the VHT route to genesis may operate in the real world, more high-resolution cloud-resolving modeling case studies are needed to determine whether the VHT paradigm is universal. Our diagnostic analysis builds on a companion modeling case study by Braun et al. (2007), in which the transformation of a Carribean tropical wave to a Gulf of Mexico tropical storm (Gert 2005) is investigated from high-resolution WRF simulations, including a statistical assessment of the convective and stratiform contributions to Gert's development. Here, we follow the methodology of the aforementioned modeling studies in which budget analyses are performed to assess the role of VHTs during the key merger events in the formation of tropical storm Gert.