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The rapid glaciation of slightly supercooled cumulus clouds
Author(s) -
Mason B. J.
Publication year - 1996
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712253003
Subject(s) - graupel , supercooling , pellets , atmospheric sciences , glacial period , environmental science , geology , meteorology , ice crystals , physics , geomorphology , oceanography
It is demonstrated that, contrary to current opinion, the rapid glaciation of shallow supercooled cumulus clouds with tops no colder than −8°C can be accounted for by the growth of a very low concentration (<10 m −3 ) of graupel pellets in a cloud of modest liquid‐water content of ∼1 g m −3 and a modest updraught of ∼3 m s −1 . In an illustrative calculation, a graupel pellet, in ascending from the −3°C to the −7°C level and returning to the −5°C level, attains a radius of 3.6 mm and sheds > 10 4 ice splinters by the Hallett—Mossop mechanism in 750 s, one‐third of them in only 150 s. Ice splinters shed in the early stages of graupel growth develop into small graupel pellets of diameter ∼1 mm and account for these being observed in concentrations of ∼1 per litre. The whole process, from activation of the ice nuclei at the −8°C level to the final stage of explosive glaciation, takes about one hour and therefore occurs only in aged cumulus with weak updraughts and not in rapidly ascending towers.