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Abstract

Ice formation in clouds has long been studied through field measurements and in laboratories under controlled conditions in cloud chambers. It has been observed that the concentration of ice particles exceeds the concentration of suitable ice nuclei particles by several orders of magnitude. This apparent contradiction may be explained by secondary ice production (SIP) via different mechanisms. Up to date, in situ observations and laboratory experiments have been inconclusive in identifying the importance of each of these mechanisms. Therefore, ice multiplication is only very crudely included in cloud models. Another key characteristic of clouds is their turbulent nature. It is already known that turbulence plays a major role in the droplets growth (Grabowski, 2013) but it could also be important for SIP-mechanisms as it affects the hydrometeors dynamics. In this work, direct numerical simulations (DNS) of homogeneous-isotropic turbulence at low Reynolds numbers with point-particle tracking are used to deepen our understanding on SIP. In DNS, the Navier-Stokes equations are solved numerically without any sub-grid turbulence model which means the whole range of spatial and temporal scales of the turbulence is resolved. We focus on SIP mechanisms that involve collisions between particles. Therefore particles characteristics, such as their diameter and concentration, as well as the turbulence intensity, are crucial in describing their movement, their collision rate and whether clusters can form. These information are decisive to estimate the feasibility of the studied mechanism.Reference : Grabowski and Wang (2013). Growth of cloud droplets in a turbulent environment. Annual review of fluid mechanics