Meteorology surrounding the Roselawn accident

Following the tragic commuter aircraft accident at Roselawn, IN on 31 October 1994, the authors were involved in a detailed study of the meteorological conditions that existed at the time of the accident. It was concluded that supercooled large droplet icing was a factor in the accident; however, these supercooled large droplets did not result from the classic melting of snow with the resulting rain or drizzle falling into a subfreezing layer. Instead, there appeared to be a coalescence process taking place near cloud top which was made possible by an area of lower and warmer cloud tops which advected into the region during the final 11 to 12 minutes that the aircraft was flying in a holding pattern. Weak embedded convection in the area of the holding pattern provided a lifting, cooling and selective growth mechanism that allowed the cloud water droplets to continue to grow to supercooled large drop sizes near the relatively warm cloud top. The droplets remained in the liquid state in sub-freezing temperatures until the aircraft impacted them. The warm cloud tops did not support the growth of ice crystals that would have depleted the cloud liquid water by converting it to ice. This analysis demonstrates the small-scale, transient nature of supercooled large droplet conditions in this case. The analysis also highlights detectable meteorological parameters associated with supercooled large droplets and points out some of the meteorological conditions that should trigger an elevated level of awareness by forecasters and pilots to the possible existence of supercooled large droplet, severe icing conditions.