Development of first ice hydrometeors and secondary ice in a tropical oceanic deep convective cloud system near Africa
A cloud system typical of convective clouds over tropical oceans was profiled by the NASA DC-8 aircraft on 20 Aug. 2006 during NAMMA. An analysis of the development of ice hydrometeors in this cloud was conducted. Of central importance was the deep layer of warm rain drop distributions formed and then lofted to the 00C level and colder. From 2DS probe data and vertical velocities measured at multiple levels from five transects within 23 min., nominally at +110C, -20C to -30C, -110C, -210C, and -340C, we interpreted the ice formation process and the potential role of the warm rain lofted to subfreezing temperatures. The initial ice development consisted of frozen drops, and fragments of frozen drops, and virtually all of the large hydrometeors wre frozen by the -110C level. The measured updraft distributions (+w) in the cloud have values that bracket the terminal velocities (Vt) of the raindrop sizes arriving at the -20C level. Since the initial terminal velocities of the frozen drops are very close to those of the parent raindrops, where +w = Vt, these freezing/frozen drops have very long residence times at any given temperature level in the cloud, as well as the critical -3°C to -8°C layer. These frozen drops rime at rate determined solely by Vt, regardless of the w value. So, these frozen drops become instantaneous rimers in a Hallett-Mossop ice multiplication scenario. Immersion freezing is a likely candidate for the initial freezing of the supercooled rain drops, as the total aerosol probably becomes immersed in the -2°C arrival raindrops. This conceptual model of ice development through the near-suspension of drops which are formed in the warm rain zone is a major factor in the development of ice in this important category of storm.