Homogeneous ice formation in convective cloud outflow regions

Homogeneous droplet freezing in the warm cirrus regime (230–240 K) is investigated along idealized convective cloud trajectories using a spectral parcel model developed to track droplet freezing events accurately. The novel model is described and used to study ice formation from rapidly ascending (vertical velocity 0.6–6 m s−1) air parcels containing cloud condensation nuclei (CCN) and liquid water droplets. Homogeneous freezing events in warm cirrus are affected by latent heat exchange and produce a mode of small ice crystals with maximum dimensions 10–100 μm after initial supersaturation quenching. During the formation stage, ice‐crystal number concentrations formed homogeneously in convective cloud outflow are hardly affected by ice‐crystal settling and depend sensitively on vertical velocity. In the case of CCN activation into cloud water droplets prior to or along with freezing, relative humidity variations also result in widely varying ice numbers that are insensitive to CCN solubility. These results offer pointers on how further progress can be achieved in simulating and better understanding the formation of upper tropospheric ice clouds originating from convective detrainment zones. Homogeneous droplet freezing in the warm cirrus regime (230–240 K) is investigated along idealized convective cloud trajectories using a spectral microphysical parcel model. In cold convective detrainment zones, homogeneously nucleated ice‐crystal number concentrations are hardly affected by ice‐crystal settling and depend sensitively on vertical velocity and CCN load. Homogeneous freezing temperatures depend on whether CCN activation into cloud water droplets occurs prior to or along with cloud‐droplet freezing.