MEASUREMENTS OF HIGH NUMBER DENSITIES OF ICE CRYSTALS IN THE TOPS OF TROPICAL CUMULONIMBUS

Imaging and light scattering instruments were used during the January/ February 1987 STEP Tropical Experiment at Darwin, Australia, to measur e ice crystal size distributions in the tops of tropical cumulonimbus anvils associated with tropical cyclones and related cloud systems. Tw o light scattering instruments covered particles from 0.1-mum to 78-mu m diameter. Particles larger than 50-mum diameter were imaged with a t wo-dimensional Grey optical array imaging probe. The measurements were made at altitudes ranging from 13 to 18 km at temperatures ranging fr om -60-degrees to -90-degrees-C. Additional measurements made in conti nental cumulonimbus anvils in the western United States offer a compar ative data set. The tropical anvil penetrations revealed surprisingly high concentrations of ice crystals. Number densities were typically g reater than 10 cm-3 with up to 100 cm-3 if one includes all particles larger than 0.1 mum and can approach condensation nuclei in total conc entration. In order to explain the high number densities, ice crystal nucleation at altitude is proposed with the freezing of fairly concent rated solution droplets in equilibrium at low relative humidities. Any dilute liquid phase is hypothesized to be transitory with a vanishing ly short lifetime and limited to cloud levels nearer -40-degrees-C. Ho mogeneous nucleation of ice involving H2SO4 nuclei is attractive in ex plaining the high number densities of small ice crystals observed near cloud top at temperatures below -60-degrees-C. The tropical size dist ributions were converted to mass using a spherical equivalent size. wh ile the continental anvil data were treated as crystalline plates. Com parisons of the ice water contents integrated from the mass distributi ons with total water contents measured with NOAA Lyman-alpha instrumen ts require bulk densities equivalent to solid ice for best agreement. Correlation between the two data sets for a number of flight passes wa s quite good and was further improved by subtraction of water vapor de nsity values ranging between ice and water saturation. Ice water conte nts up to 0.07 m-3 were observed in the tropical anvils with over 0.1 g m-3 in continental anvils. The size distributions in tropical anvils generally reveal mass modes at sizes of 20-40 mum. With rare exceptio ns, particles larger than 100 mum were not observed near the cloud top s. In continental cumulonimbus anvils, much larger plate crystals appr oaching 1 mm in size account for the majority of the ice water. Most o f the ice crystal mass lofted to anvil altitudes falls to lower levels prior to evaporating. The anvils can experience strong radiational he ating as well as cooling depending upon lower cloud cover, particle si ze distribution. and time of day.