The results of the particle size spectrometer experiment on the Pioneer Venus sounder probe are presented. The vertical cloud structure is found to consist of three primary cloud regions of approximately 20 km total thickness suspended within an ubiquitous aerosol haze which extends more than 10 km above and below it. The three cloud regions are separated by sharp transition regions where both particle chemistry and microphysics exhibit change. The size distributions are multimodal in all cloud regions. Three size modes are observed in the middle and lower cloud region which are composed of aerosol, H2SO4 droplets, and crystals. The crystals likely could be either sulphates or chlorides. We provide interpretations of the sources, growth characteristics, and fate of the particle species through a partitioning analysis of the LCPS size distribution data.
An improved forward scattering spectrometer probe, the FSSP‐300, was developed for the Airborne Arctic Stratospheric Expedition. The 300 measures particles in the size range 0.3 μm to 20 μm and has a greater sensitivity and faster time response than its predecessor, the FSSP‐100X. An intensive characterization of this probe's operating characteristics has been made and its limitations evaluated. Measurements from this probe are affected by Mie scattering ambiguities and index of refraction uncertainties, nonuniform laser intensity, uncertainties in sample volume definition, and time response roll‐off. Correction algorithms have been developed to account for some of the probe limitations. After applying these corrections, the uncertainties in number and mass concentration are on the order of 25% and 60%, respectively.
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.
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