Measurements of high number densities of ice crystals in the tops of tropical cumulonimbus

Abstract

Imaging and light scattering instruments were used during the January/February 1987 STEP Tropical Experiment at Darwin, Australia, to measure ice crystal size distributions in the tops of tropical cumulonimbus anvils associated with tropical cyclones and related cloud systems. Two light scattering instruments covered particles from 0.1‐μm to 78‐μm diameter. Particles larger than 50‐μm diameter were imaged with a two‐dimensional Grey optical array imaging probe. The measurements were made at altitudes ranging from 13 to 18 km at temperatures ranging from −60° to −90°C. Additional measurements made in continental cumulonimbus anvils in the western United States offer a comparative data set. The tropical anvil penetrations revealed surprisingly high concentrations of ice crystals. Number densities were typically greater than 10 cm−3 with up to 100 cm−3 if one includes all particles larger than 0.1 μm and can approach condensation nuclei in total concentration. In order to explain the high number densities, ice crystal nucleation at altitude is proposed with the freezing of fairly concentrated solution droplets in equilibrium at low relative humidities. Any dilute liquid phase is hypothesized to be transitory with a vanishingly short lifetime and limited to cloud levels nearer −40°C. Homogeneous nucleation of ice involving H2SO4 nuclei is attractive in explaining the high number densities of small ice crystals observed near cloud top at temperatures below −60°C. The tropical size distributions were converted to mass using a spherical equivalent size, while the continental anvil data were treated as crystalline plates. Comparisons of the ice water contents integrated from the mass distributions with total water contents measured with NOAA Lyman‐alpha instruments require bulk densities equivalent to solid ice for best agreement. Correlation between the two data sets for a number of flight passes was quite good and was further improved by subtraction of water vapor density values ranging between ice and water saturation. Ice water contents up to 0.07 g 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 μm. With rare exceptions, particles larger than 100 μm were not observed near the cloud tops. In continental cumulonimbus anvils, much larger plate crystals approaching 1 mm in size account for the majority of the ice water. Most of the ice crystal mass lofted to anvil altitudes falls to lower levels prior to evaporating. The anvils can experience strong radiational heating as well as cooling depending upon lower cloud cover, particle size distribution, and time of day.