A Quicker Start to the Hallet-Mossop Process in Maritime Clouds

Past observations of large numbers of ice crystals with cloud top temperatures greater than -12 °C in tropical maritime cumuli have remained mysterious. Some studies have implicated a Hallet-Mossop (rime-splintering) mechanism, but others have suggested this mechanism is unlikely to have been of importance or even suggested that the observations themselves were in error.

Here we present new 3D, high-resolution simulations of a tropical maritime cumulus congestus to study the Hallet-Mossop process in detail. The microphysical parameterizations in the model have been designed to match conditions observed during the ICE-T field campaign: the parameters in the warm rain parameterization are based on observed cloud- and raindrop-size distributions, the creation of ice splinters by the Hallet-Mossop process has been added, and new particle species to track the formation of splinters during the riming of graupel and frozen raindrops (treated separately) are included. Implementation of a two-moment microphysical parameterization was also necessary to see the effects of the splinters on the overall microphysical development.

The results show that the quick production of warm raindrops in these maritime clouds allows the Hallet-Mossop process to be initiated on first ascent of the cloud top through the -3 to -8 °C temperature range. Observations from the Wyoming Cloud Radar during ICE-T support the presence of these early rimers. Sensitivity studies in which the warm rain process is slowed show that the Hallet-Mossop process is delayed until cloud top has ascended past this temperature range, and thus must await the graupel or frozen raindrops to fall through it from above.

Additional simulations investigate the number of ice nuclei that must be active at higher temperatures to initiate the rimers, and eventually explain the high 2D-C number concentrations (exceeding 100/L) observed in some cases. The effects of changing the number of ice nuclei active within various temperature intervals are also shown, and consistency with ice nuclei observations is evaluated.