Monday, 7 July 2014: 4:30 PM
Essex Center/South (Westin Copley Place)
Learjet, C-130, DC-8 and GV research aircraft measurements of microphysical and dynamical properties of convective clouds studied in ICE-T (tropical maritime), SEAC4RS (maritime and Southeast US continental), DC3 (mid-latitude continental), HIPLEX and CCOPE (historical mid-latitude continental) field campaigns are compared. Cloud base temperature and drop size distribution appears to be a controlling factor in the development of millimeter-diameter drops observed in strong updrafts at the freezing level. Tropical maritime clouds in the Caribbean (ICE-T), which have cloud-base droplets out to 80 microns in diameter, form relatively high concentrations of millimeter-diameter drops at 0 C; convective clouds observed over Alabama and Mississippi form relatively low concentrations of millimeter diameter drops at 0 C, and no millimeter-diameter drops are observed in mid-latitude continental convective clouds, which have relatively high, cold cloud bases with narrow droplet distributions. The formation of millimeter-diameter drops in updrafts leads to rapid glaciation by the time air in the updraft reaches -15 C, and precipitation develops in the form of graupel particles that fall and melt into raindrops. Mid-latitude continental clouds that do not form millimeter-diameter drops also eventually form graupel particles, but transport a relatively higher percentage of ice condensate into the anvil region. Observations supported by the a 1-D cloud model with detailed microphysics suggest that the rapid glaciation is largely the result of freezing of millimeter-drops upon collision with small ice particles. The model suggests that the differential in fall velocities between the large drops and small ice particles, which are apparently the result of a primary nucleation process, results in rapid freezing (and perhaps splintering) of the large drops. The corroborative results between model and observations have indications for improved parameterizations of rainout and convective transport of ice into the upper troposphere.
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