A laboratory study of graupel charging

R. G. PEREYRA; E. E. AVILA; N. E. CASTELLANO; C. P. R. SAUNDERS

JOURNAL OF GEOPHYSICAL RESEARCH

AMERICAN GEPHYSICAL UNION

Año: 2000 vol. 105 p. 20803 - 20812

0148-0227

Measurements have been made of charge transfer when vapor grown ice crystals rebound from a riming target representing a graupel pellet falling in a thunderstorm.  Earlier studies in the laboratory in Córdoba of charge transfer between an individual falling ice sphere and a riming target noted that the sign of the charge transfer was dependent upon temperature and effective liquid water content (EW).  The new work uses a similar experimental technique; however, a cloud of ice crystals is grown in order to study multiple interactions with the riming target.  The results also show charge sign dependence on temperature and EW; positive rimer charging is observed at high temperatures and for low and high values of EW at low temperature, while negative rimer charging is noted at low temperatures for intermediate values of EW.  These results are similar to those obtained by Takahashi (1978) and, as has been reported before, are rather different from those obtained in Manchester by Jayaratne et al. (1983), Saunders et al. (1991), and Saunders and Peck (1998).  Significant differences between the two types of data sets are attributed to the experimental techniques used in the various studies.  In the present work the ice crystal cloud and the cloud of supercooled droplets responsible for riming the target are grown in separate chambers and then mixed shortly before the crystals and droplets encounter the riming target, so that the droplet cloud is not depleted by the growing ice crystals.  In the Manchester experiments, the ice crystals grow in the same supercooled droplet cloud used to rime the target.  It is possible that the mixing process provides an undepleted droplet cloud and a transient enhanced vapor supply that affects both the ice crystal and graupel vapor depositional growth rates, leading to the present results. Comparison of the two laboratory techniques with natural conditions in thunderstorms shows that negative graupel charging, responsible for the negative charge center, will be favored following mixing of cloud particles from regions of high and low liquid water content, possibly at the updraft/downdraft interface, whereas in cloud regions with more steady state conditions, negative charging of graupel will be restricted to lower values of liquid water content.  Further laboratory work is needed to clarify the effect of mixing on charge transfer, while field studies are required to identify the dynamic characteristics of the particle charging regions in thunderstorms.