Impact of Terrain on Supercells According to Idealized Simulations with Actual Terrain

This study examines the impacts of terrain on supercell thunderstorms in a hybrid real-data/idealized approach. The actual terrain from the Alabama/Georgia/Tennessee "triple point" is used for the lower boundary in a series of simulations using the CM1 numerical model. An observed sounding from a tornadic outbreak day is used to define a horizontally homogeneous initial state. Thunderstorms are then initialized with an updraft nudging technique, which is placed in different locations relative to the terrain in a series of simulations. Radiative processes and boundary-layer evolution are neglected so that the atmospheric conditions remain nearly steady and can be controlled via changes to the initial sounding. This methodology was designed to isolate the possible effects of the actual topography in a controlled manner, and to evaluate the relative effects of terrain as compared to changes in atmospheric conditions (e.g., shear and instability). Simulations in this framework are in progress, but preliminary results show that terrain can either inhibit or enhance the intensity of low-level mesocyclones, and that small variations in the environmental wind profile have a larger impact on supercell intensity and structure.