Between 6 September and 10 September 2001 an interesting tropical cyclone merger event between Gil and Henriette occurred in the Eastern Pacific. The event was well-observed by geostationary satellite and by the SeaWinds instrument aboard NASA's QuikSCAT satellite. We have analysed this merger event using absolute vorticity fields computed from the QuikSCAT surface winds. We have then used these vorticity fields, and ensemble perturbations of them, to initialize the adaptive multigrid barotropic tropical cyclone track model of Fulton (MWR, January 2001). The model results show the sensitivity of the merger process to the relative size and strength of the vorticity fields of the two storms. In addition we have run the model, which normally is equivalent barotropic and has the full effects of the earth's sphericity, in its beta-plane, f-plane, and pure barotropic forms to test the sensitivity of the merger process to these simplifications.