Fig. 1: The partLEM model framework.

An air parcel is subject to an updraught speed (w) and rises along the dry adiabatic lapse rate to an altitude (h), eventually covering a full homogeneous freezing event (HFE). The vertical parcel dimension is equal to the depth of a homogeneous freezing layer (L_f), corresponding to a narrow range of ice supersaturation (s). The high-resolution computational domain of the Linear Eddy Model (LEM) is placed vertically within the air parcel, where molecular diffusion and turbulent mixing act on temperature (T) and water vapour mass mixing ratio (q_v) fields. The full partLEM model encompasses a wide range of microphysical processes involving liquid solution droplets and ice crystals derived from them via stochastic homogeneous freezing without assuming thermodynamic equilibrium. Mixing causes stochastic turbulent temperature fluctuations and similarly affects water vapour and a large number of simulation particles present at different Lagrangian LEM altitudes (z). In addition, all particles are subject to random Brownian motion and settle due to gravity. Initially, T and q_v and particles are uniformly distributed across the vertical grid levels. We refer to the combination of turbulent mixing and dissipation scale processes (vapour and heat diffusion and Brownian particle motion) as ‘LEM physics’. This model set-up replicates conditions of ice formation at the top of cirrus clouds.