The Telescope Array Surface Detector (TASD) in western Utah is designed for the measurement of Ultra-High-Energy Cosmic Rays (UHECRs) with characteristically low flux. During its ten years of operation, however, TASD recorded at least 24 bursts of gamma-ray showers, each consisting of two or more TASD events within a single millisecond of lightning activity. After implementation of specialized lightning detectors, the bursts were further correlated with lightning leaders preceding each stroke. We conclude that these bursts are downward Terrestrial Gamma Flashes (TGFs), similar to those observed by satellite. These data constitute the majority of all data on such events recorded from the ground. We are recreating observations using Geant4 simulations to better understand the properties of downward TGFs and the acceptance statistics of TASD. Acceleration mechanisms, for example, are poorly understood. Spectra from models such as Relativistic Runaway Electron Avalanche (RREA) and Fast Negative Breakdown (FNB) are used to generate a detector response to replicate empirical properties, especially the unique waveform timing and footprints seen at TASD. This also helps characterize the TASD acceptance and aperture for TGFs in a manner similar to cosmic ray studies.