Cloud particle measurements in thunderstorm anvils and possible weather threat to aviation

Since 1990, there have been at least 10 known incidents where jet aircraft have experienced loss of thrust in one or more turbofan engines while maneuvering in the anvil region near the central core of a thunderstorm. The exact cause of the uncommanded thrust reduction, commonly called engine rollback, is still under investigation. It appears that the rollback incidents may be associated with ingestion of high mass concentrations of ice particles, snow, and possibly small concentrations of supercooled liquid water in the anvil region. The characteristics of cloud particles in thunderstorm anvils have not been extensively studied. Results from analysis of aircraft observations in the anvils of midlatitude and tropical thunderstorms are discussed. Aircraft and limited radar observations show that most anvils associated with small, garden-variety thunderstorms contain low (<∼0.4 g m^-3) mass concentrations of ice particles. In larger, more intense midlatitude storms, anvils may contain ice water contents from 1 to 3 g m^-3. The mean of the maximum particle dimension in the anvil region of the more intense storms showed a strong modal size of about 2 mm. The particles themselves appear to be ice crystals and aggregates of ice crystals, i.e., snowflakes. The mass concentration of ice particles usually decreases rapidly away from the center of thunderstorms, falling off to less than half its peak value within about 10 km of the central region of the storms. The data suggest that the ice water content is well below 1 g m^-3 at a distance of ∼50 km away from the central region of a thunderstorm, i.e., the region with high radar reflectivity.