Measurements of the aerosol and ice crystal populations in tropical stratospheric cumulonimbus anvils

Abstract

A pair of particle size spectrometers was flown aboard the NASA U‐2 operating from the Canal Zone to make measurements of the aerosol and ice crystal budgets in cirrus produced by thunderstorms in the tropics. Measurements indicate that fairly large crystals up to 1 mm in size are injected into the stratosphere during cumulonimbus activity. Ice water contents range from a few thousandths to a few hundredths of a gram per cubic meter. Because the ambient temperature is typically around −80°C the mass of the larger crystals largely returns to lower altitudes before evaporating. Aerosol size distributions indicate a curious narrow growth mode between 0.15 and 0.2 µm in cirrus anvils which is absent outside these clouds. The presence of this narrow mode is attributed to near water saturation produced at cloud top. Nucleation of new ice crystals as well as aerosol is hypothesized.