Characterising the spatial overlap between liquid and ice in mixed-phase clouds

Mixed-phase clouds are important for simulating precipitation formation and cloud radiative effects in numerical weather prediction (NWP) and climate models. One challenge to reduce model uncertainties is how best to represent the subgrid distribution of liquid and ice within a model grid box. This is poorly constrained by observations, yet is key for representing microphysical process rates that grow ice crystals at the expense of liquid droplets. This study uses in situ airborne observations from stratiform, shallow cumulus, deep convective, and frontal clouds to investigate the horizontal spatial overlap of liquid and ice phases on length-scales ~1km, which are appropriate for current regional NWP models. We place observational constraints on a simple parametrisation that describes the mixed-phase fraction as a function of subgrid liquid and ice cloud fractions and demonstrate that most of the observations show that, when ice and liquid are present, they are close to fully overlapped.