Rime-Splintering within Cumuli as Modulated by the Cloud Environment

Rime-splintering (also known as the Hallet-Mossop process) is one of a variety of possible ice multiplication processes, but has been studied the most and with a variety of approaches, including the laboratory, in-situ cloud observations, and numerical simulations. This process is dependent upon ice particles falling at speeds of 1-3 m s^-1 while riming supercooled cloud droplets (some of which must exceed 25 mm diameter) within a narrow temperature regime (-3 to -9 °C). When observed ice number concentrations significantly exceed those expected from primary ice nucleation alone, numerous studies have implicated rime-splintering as the cause, while other studies have cast doubt on its effectiveness.

Two field campaigns have provided an opportunity to study the rime-splintering process in developing cumulus clouds in more detail: the Ice in Cumulus-Tropical (ICE-T) field campaign based in the Caribbean in 2011, and the COnvective Precipitation Experiment (COPE) field campaign based in Southwest England in 2013. Combined, these data sets allow an initial examination of its importance in environments that vary thermodynamically, and in the amounts of local aerosol particles and vertical wind shear. Aircraft measurements from in situ probes and the Wyoming Cloud Radar were collected in both field campaigns, where the radar helped establish the stage of cloud development and the distance from the cloud tops.

Numerical modeling with the CM1 model, a 3D, non-hydrostatic cloud model that includes a two-moment representation of microphysical particles, is initialized with these different environments, and used to test and isolate environmental effects upon the rime-splintering process. It is also used to evaluate the effect of rime-splintering upon surface rainfall for the COPE cases.

From the aircraft measurements collected during these field campaigns, an active rime-splintering process is consistent with observations of high ice number concentrations, and this finding is also supported by the numerical simulations. In some cases when the observations suggest this process was not as active, the numerical simulations help to determine possible limiting factors. Both observational analysis and numerical simulations suggest the productivity of rime-splintering can be reduced by stronger environmental vertical wind shear, as well as the strength of the warm rain process, as hypothesized in some past studies. Interestingly, in the deeper Southwest England convection, even when weaker vertical wind shear and a stronger warm rain process enhanced rime-splintering, numerical simulations suggest it does not lead to an increase in the surface rainfall.