Close Window

Texas A&M University: "Analysis and simulation of Allison (2001) over Texas"

Final Report


1. Gain an understanding of the mechanisms involved in the genesis of the storm and the nature of the storm;
2. Determine the predictability of this event with the use of a high-resolution mesoscale model; and
3. Determine how effective the MM5 model could have been used to assess flood potential for this situation.

Through numerical simulations and analysis of numerical output, we found evidence that Allison developed within a low-level jet beneath an approaching upper-level trough. In the numerical simulations, we found that Allison was not a single well-defined vortex. Instead, individual vortices developed beneath areas of intense convection and subsequently merged with the overall circulation. A similar process has been described by M. Montgomery in a different storm. In order to gather observational evidence for multiple vortices, we undertook a careful surface mesoanalysis of Allison as it made landfall, and found that two vorticity centers were indeed present. A careful review of radar and satellite imagery stored at the Houston-Galveston NWSO showed clear evidence of two vortices rotating around each other. When the primary vortex was making landfall near Galveston and producing initial flash flooding, NHC (in real-time and post-analysis) was tracking a secondary vortex that had migrated away from the high cloud shield.

Because of the importance of these results, we have focused most of our attention on objective 1 during this project. Objective 2 will be investigated in subsequent months, and objective 3 will be investigated only during the landfalling stage. Initial collaborative analysis indicate that the complexity of the structure of this tropical cyclone may require intense analysis, precluding our capability to reach the 3rd objective stated above.


The discovery of a multiple vortex structure in Allison which MM5 runs may be resolving at 6km resolution was a direct product of visits by university personnel to the NWSO and of collaborative research conducted while there. Preliminary MM5 runs at TAMU indicate high-resolution local modeling may resolve a more common instance of multiple vortices in hybrid tropical cyclones such as Allison (these structures were also likely present in tropical storms Bill and Grace in the 2003 season). These findings may lead to a completely new approach for establishing a mass-weighted center of these cyclones which would be much more representative for position and track forecasts, with correlative improvements in anticipating impacts to life and property.


No presentations or publications during the course of the project. An abstract will be submitted to the 2004 AMS Hurricane Conference, and we shall be working jointly on two journal articles in which the research results will be disseminated.


4.1 (University) The partnership enabled us to draw on the perceptions and experiences of the field forecasters. Through this interaction, we were able to construct research questions that were of direct operational relevance, and to test hypotheses on the basis of personal forecaster experience prior to conducting numerical experiments or diagnoses. The operational significance of the problem leant a special excitement and urgency to the graduate student’s work.

4.2 (Forecaster) As mentioned in section 2 above, research to date on the structure of Allison has been not only informative but had preliminary prognostic value in anticipating developments in similar tropical cyclones (Grace, 2003). Though work on a definitive technique was beyond the scope of the current project. At least a preliminary methodology for reaching a more practical and meaningful positioning and forecasting technique for hybrid tropical cyclones has been suggested by this research.