A recent study by Sime et al. (1984) suggests that numerical simulations of coronal mass ejections in which the energy source is an input of thermal energy may not reproduce several general features of looplike coronal mass ejections observed in white light coronagraphs. In this and the following paper (Steinolfson and Hundhausen, this issue) we examine why the models used in the comparison by Sime et al. were inadequate and show how modifications of such models to include a more realistic preevent atmosphere can improve agreement with observations. For models utilizing a localized energy source, the ambient atmosphere becomes very important in determining the white light signature of a mass ejection. The significance of the initial magnetic field configuration and the coronal outflow have been illustrated in previous publications. We emphasize the role of the thermal structure as it defines the fast-mode speed in the initial corona. In this paper, we set the stage for detailed calculations in the following paper by considering three different ambient atmospheres. Two of these have already been used as initial states for results which have appeared in the literature. The third is new in that it contains an atmospheric heating term. Through comparison of all three atmospheres, we show that the important difference in the heated atmosphere is the larger fast-mode speed near the equator. ¿ American Geophysical Union 1988