A Brief History of Secondary Ice

Abstract:

The presence of ice in clouds has been of interest to researchers over the past 80 years after it became established that ice is necessary for so much of the earth’s precipitation, particularly when it appears in supercooled clouds. In the 1960s the focus of many cloud studies shifted to cumuliform clouds whose summit temperatures were never lower than -10°C that contained extraordinarily high concentrations of ice particles (10s to 100s per liter). Since these concentrations could not be explained by ice nucleus concentration measurements, the excess ice particles have been termed, “secondary” ice particles, ones that apparently resulted from a separate process that quickly followed the relatively few ice particles that originated on primary ice nuclei.

Several hypotheses were proposed in the 1960s to account for the occurrences of secondary ice particles. These hypotheses have been winnowed down to one major player based on both further, more sophisticated cloud measurements and modeling efforts.

The leading explanation today is that in those clouds containing a temperature zone of -3° to -8°C, with a broad cloud droplet spectrum that contains appreciable concentrations of droplets >23 µm in diameter, can largely explain ice multiplication. Millimeter-sized graupel formed on primary ice nuclei can be prodigious producers of splinters when falling and being further rimed through such a cloud, particularly near the -4.5°C level where splinter production is maximized. This process has also been shown to be enhanced by the presence of appreciable concentrations of cloud droplets <13 µm in diameter.

But questions remain regarding some aspects of the riming and splintering hypothesis.

Some researchers have claimed that glaciation in slightly supercooled maritime cumuliform clouds has still proceeded too fast to be solely explained by R-S and that some cumuliform clouds lacking in the R-S criteria nevertheless exhibit ice multiplication. This review will try to illuminate some of the remaining questions.