A multisensor investigation of rime splintering in tropical maritime cumuli

Three flights from the Ice in Clouds Experiment-Tropical (ICE-T) field campaign examined the onset of ice near the ascending cloud tops of tropical maritime cumuli as they cooled from 0° to -14°C. Careful quantitative analysis of ice number concentrations included manual scrutiny of particle images and corrections for possible particle-shattering artifacts. The novel use of the Wyoming Cloud Radar documented the stage of cloud development and tops relative to the aircraft sampling, complemented the manual estimates of graupel concentrations, and provided new clear evidence of graupel movement through the rime-splintering zone. Measurements of ice-nucleating particles (INPs) provided an estimate of primary initiated ice. The data portray a dynamically complex picture of hydrometeor transport contributing to, and likely resulting from, the rime-splintering process. Hundreds per liter of supercooled raindrops ascended within the updrafts as the cloud tops reached 0°C and contributed in part to the 0.1 L^-1 graupel detected soon after the cloud tops cooled to -5°C. Rime splintering could thus be initiated upon first ascent of the cloud top through that zone and arguably contributed to the 1 L^-1 or more graupel observed above it. Graupel ascending/descending into, or balanced within, the rime-splintering zone were found. In wider, less isolated clouds with dying updrafts and tops near -14°C, ice particle concentrations sometimes reached 100 L^-1. Future 3D numerical modeling will be required to evaluate if rime splintering alone can explain the difference of three to four orders of magnitude in the observed INPs and the graupel observed at -5°C and colder.