Hydrometeor Spectra and Cloud Electrification

Microphysical characteristics and cloud electrification of convective clouds in 1991 Convection and Precipitation/Electrification (CaPE) have been examined in order to investigate the patterns of hydrometeor size distributions and evolution of clouds and precipitations and the relation between the ice development and cloud electrification. Observations from two spiral descents in a squall line anvil stratiform regions conducted during Convective Oklahoma Precipitation Study (COPS) in 1991 are also presented to investigate the vertical structure of precipitation and electrification in stratiform clouds. Temperatures, vertical and horizontal wind field, cloud liquid water content, PMS 2-D hydrometeor images, replica images, hydrometeor size spectra, cloud electric field and aircraft charging have been analyzed. The study compared observed precipitation size distributions of a specific pass in CaPE-5 with the predicted size spectra using quasi-stochastic model (QSM) assuming initial conditions for the differential equation is the observed cloud condition in the prior pass. Gamma distribution are obtained replacing k! with Gamma(k + 1) in Poisson distribution. Both drop growth rate by coalescence and ice growth rate by collision are computed for a particular pass in order to examine the precipitation evolution rate. The significant difference of vertical motions between CaPE and COPS clouds is the dominant cause for different microstructure in the two cases. In CaPE cases, vertical updrafts were as high as 25 m s ^{-1}. Supercooled raindrops were observed in the updraft regions as well as graupel particles. In COPS stratiform anvil clouds, the maximum updraft appeared to be less than 2 m s^{-1}. Supercooled raindrops were not observed. Hallett-Mossop secondary ice mechanism is found to be responsible for ice multiplication. Electric charge generation mechanisms are examined for ice growth by vapor deposition and by collision with growing graupel. Correlations between particle size spectra and electric charging generation are derived and examined for both growth processes. Observations show that in most cases positive charges are located on the upper levels and the highest concentration of negative charge is just above the freezing level in clouds (in COPS case, 5000 m). The syntheses from laboratory studies for ice collision and bounce process and ice diffusional growth and evaporation are applied to cloud electric field measured on scale of tens of kilometers or less.