The development of precipitation in the top of an isolated maritime cumulus tower is traced by four rapid penetrations with an instrumented aircraft between 400 and 1000 m below the visible top of the growing tower. The hydrometeor distribution evolves from the first appearance of a few large supercooled drops [0.45 l−1, D > 0.5 mm] to well developed precipitation (largely ice) in 500 s. This development results from accretion and coalescence growth in the cloud top volume, not from advection by the updraft of large drops from below. Large supercooled drops precede the appearance of ice at −9°C near the cloud top. The cloud- and precipitation-water budgets are computed and compared with observed values, which indicate that, once precipitation is well developed, the convective tower cannot maintain itself as a steady state entity. The budget computations demonstrate a sensitivity of cloud evolution to the ice particle density.
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Willis et al. (1991) studied this question.