While mesoscale atmospheric models are useful for the short-term prediction and assessment of real-time weather situations associated with fire danger or fire behavior, another application for which they are well-suited is the simulation of weather conditions associated with strong wildfires in the past. These simulations can be used to better understand the atmospheric contribution to the observed fire behavior. For this study, the MM5 modeling system employed by the EAMC was applied to the Mack Lake fire that occurred on May 5th, 1980 near Mio, Michigan. At the time of the fire, model output was not available on a regular basis to analyze a given weather situation, so fire-weather forecasters relied upon analyses of surface observations and rawinsondes. For this particular fire there were three rawinsonde stations in the region, but none of them fully captured the above-ground conditions that appeared to have contributed to the observed fire behavior. In these sorts of situations, mesoscale model results can be particularly useful for diagnosing and understanding the atmospheric processes that contributed evolution of the fire.
The rawinsonde observations from the region suggested that a weak low-level jet (LLJ) was present in the Great Lakes region. However, the model results indicate that there was a substantially stronger LLJ over central Michigan at the time of the fire than was observed at any of the rawinsonde stations. The model results suggest the potential that kinetic energy and turbulent kinetic energy associated with the LLJ could have been mixed down to the surface just before the fire grew out of control, contributing strongly to fire evolution in this case.
This diagnosis of a mesoscale feature that is often undetectable in surface and radiosonde observations indicates the potential for mesoscale models to improve the fire-weather information available to fire fighters and fire managers. Simulations of severe fire events in other regions indicate that certain LLJ configurations allow fires to interact with the atmospheric mixed-layer such that energy from the LLJ can contribute strongly to rapid fire growth. By using the mesoscale model results, an index that assesses the LLJ structure and the potential for a fire circulation to interact with the LLJ has been developed.