The OCTET modeling system has been designed to simulate the atmospheric dynamics, microphysics and scavenging above hypothetical large city fires with energy release rates on the order of 10-100kW/m2 over areas of tens to hundreds of square kilometers. It simulates the three-dimensional, moist, nonhydrostatic circulations of natural and fire-driven convective clouds and the microphysical interactions among hydrometeors and aerosols in these clouds. In order to validate the model, simulations of planned forest and slash burns have been performed and results compared with available observations. In this paper we briefly describe the OCTET modeling system and present simulations for two planned forest burns in Ontario, Canada. The Hardiman fire was fairly well observed and includes microphysical data taken from aircraft; it involved only liquid hydrometeors. The Battersby fire penetrated well above the freezing level, and there is evidence of frozen hydrometeors. Comparisons of the numerical results with the observations of cloud dynamics and microphysics confirm the ability of the model to simulate these clouds and suggest that the model can provide insight into the dynamical, microphysical and scavenging processes that occur in clouds. © by the Society of American Foresters. Abstract reproduced by permission.