Evidence is presented of 10 cm radar echoes observed from Brisbane, Queensland, which occurred as extensive layers downstream of thunderstorm cells, but mainly below the normal base of anvil cloud. These echoes are attributed to ice crystals, the measured reflectivity factors (103 - 104 mm6 m-3) being comparable with those calculated from ice-crystal distributions sampled by others at similar heights and temperatures. Particle trajectory and trail pattern models indicate that the source of the crystals is situated across the summit of the nearest upstream storm cell. This is consistent with a source in the anvil canopy of mature storms prior to it being sheared or "blown off". It is thus suggested that ice-crystal layers of this type may be more common and extensive than is usually supposed and may play a part in natural cloud seeding.
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FA BERSON (1969) studied this question.
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