Factors controlling secondary ice production in cumulus clouds

Aircraft measurements of two cumulus clouds were made during the Ice and Precipitation Initiation in Cumulus campaign over the British Isles. The 18 May 2006 cloud had high concentrations of ice particles and conditions were conducive for the Hallett–Mossop (HM) process of secondary ice production, but the 13 July 2005 cloud had low concentrations. A bin-resolved cloud model was used to investigate several factors that are known to control the HM process using the observations of the two clouds. For the 2006 cloud, the model results show that the fast production of graupel by directly freezing of supercooled raindrops through collisional collection with ice particles was crucial to the activation of the HM process. Switching off raindrop freezing led to much delayed and suppressed formation of graupel particles, and hence a negligible HM process. Sensitivity studies were performed on the concentration of primary ice particles required to kick-start the HM process. It was found that a concentration of the first ice as low as 0.01 L⁻¹ could be sufficient, as long as there was a large enough concentration of cloud droplets (small and large) available when a significant number of graupel particles developed in the HM temperature zone. For the modelled 2005 cloud, the HM process did not operate effectively mainly because of the low concentration of supercooled raindrops and hence graupel. The HM process was also hindered by the relatively greater number of aerosols, and higher temperatures at cloud base and top.