Simulation and Interpretation of the Genesis of Tropical Storm Gert (2005) as Part of the NASA Tropical Cloud Systems and Processes Experiment

Abstract

Several hypotheses have been put forward for the mechanisms of generation of surface circulation associated with tropical cyclones. This paper examines high-resolution simulations of Tropical Storm Gert (2005), which formed in the Gulf of Mexico during NASA's Tropical Cloud Systems and Processes Experiment, to investigate the development of low-level circulation and its relationship to the precipitation evolution. Two simulations are examined: one that better matches available observations but underpredicts the storm's minimum sea level pressure and a second one that somewhat overintensifies the storm but provides a set of simulations that encapsulates the overall genesis and development characteristics of the observed storm. The roles of convective and stratiform precipitation processes within the mesoscale precipitation systems that formed Gert are discussed. During 21-25 July, two episodes of convective system development occurred. In each, precipitation system evolution was characterized by intense and deep convective upward motions followed by increasing stratiformtype vertical motions (upper-level ascent, low-level descent). Potential vorticity (PV) in convective regions was strongest at low levels while stratiform-region PV was strongest at midlevels, suggesting that convective processes acted to spin up lower levels prior to the spinup of middle levels by stratiform processes. Intense vortical hot towers (VHTs) were prominent features of the low-level cyclonic vorticity field. The most prominent PV anomalies persisted more than 6 h and were often associated with localized minima in the sea level pressure field. A gradual aggregation of the cyclonic PV occurred as existing VHTs near the center continually merged with new VHTs, gradually increasing the mean vorticity near the center. Nearly concurrently with this VHT-induced development, stratiform precipitation processes strongly enhanced the mean inflow and convergence at middle levels, rapidly increasing the midlevel vorticity. However, the stratiform vertical motion profile is such that while it increases midlevel vorticity, it decreases vorticity near the surface as a result of low-level divergence. Consequently, the results suggest that while stratiform precipitation regions may significantly increase cyclonic circulation at midlevels, convective vortex enhancement at low to midlevels is likely necessary for genesis.

An examination of two pathways to tropical cyclogenesis occurring in idealized simulations with a cloud-resolving numerical model

“…This indicates that the boundary layer recovery was often significantly shorter than the diurnal cycle. Both diurnal and semi-diurnal cycles have been found in the areal extent of the upper level cirrus deck of fully developed tropical cyclones (Browner et al, 1977;Muramatsu, 1983;Lajoie and Butterworth, 1984;Steranka et al, 1984;Kossin, 2002). The results presented here for the early stage of a developing system may be relevant to these previous studies, although the existence of substantial convective downdrafts and cold pools in the core of developing tropical cyclones has not yet been established definitively.…”

“…Other numerical modeling studies by H04, M06, Wang et al (2010) and Braun et al (2010), do not show development along pathway Two. The initial conditions used in the studies by N07 and herein, are idealized.…”

Paradigms for tropical cyclone intensification

“…Rauber et al 2008, Holton 2004, Asnani 2005, Ahrens 2008, Holton and Hakim 2012 ), tropical weather briefings and the current literature (Lighthill 1998, Smith 2003, Molinari et al 2004, Nong and Emanuel 2004, Montgomery et al 2006, Terwey and Montgomery 2008, Braun et al 2010. Indeed, the last four authors and others have talked about 'igniting the WISHE mechanism' after the vortex (or secondary maximum in the tangential wind) has reached some threshold intensity.…”

On Polarimetric Radar Signatures of Deep Convection for Model Evaluation: Columns of Specific Differential Phase Observed during MC3E*

“…Caniaux et al (1995); Gray (2000); Lang et al (2003); Mechem et al (2006); Braun et al (2010); Fridlind et al (2012); Zeng et al (2013); Mrowiec et al (2015)). Additionally, contoured frequency with altitude (CFAD) plots of reflectivity are used to evaluate differences in modeled and observed storm microphysics and dynamics (Lang et al 2003;Blossey et al 2007;Matsui et al 2009;Shi et al 2010;Lang et al 2011;Tao et al 2014;Matsui et al 2015).…”