On homogeneous ice formation in liquid clouds

Primary ice formation by homogeneous freezing of supercooled cloud droplets in moist adiabatic air parcels is investigated theoretically and by means of numerical simulations. Dependencies of nucleated ice crystal number and size and associated freezing temperatures are systematically studied as functions of updraught speed and cloud droplet number and size. Droplet freezing temperatures range between 235 and 239 K, depending on dynamical forcing and liquid water content. Vertical resolutions on the order of 10 m are required in numerical cloud models to resolve the thin layers in which homogeneous ice formation processes unfold. Only a fraction of droplets freeze in weakly forced clouds. Small-scale variability in updraught speeds encountered by air parcels containing supercooled cloud droplets results in broad ice crystal number distributions when sampled across a parcel ensemble. The strong dynamical control of ice initiation in tropospheric clouds initiates homogeneous freezing in the presence of heterogeneous ice nuclei even at moderate mean updraught speeds, suggesting that such particles play a limited role in affecting droplet freezing in cold convective cloud outflow regions. These findings have important implications for microphysical and optical properties of condensed-phase convective detrainment, setting the stage for the evolution of anvil cirrus.