What can satellites tell us about cloud glaciation? A time-resolved view.

Below 0 °C, cloud droplets can freeze, altering a cloud’s optical and radiative properties and thereby affecting Earth’s energy balance. The microphysical mechanisms that govern this process, known as glaciation, are expected to act on minute timescales. Nevertheless, stratiform clouds can persist in the mixed-phase temperature range (0 °C to -38 °C) for hours, thus glaciation events remain poorly characterised.

We analysed satellite observations of individual cloud tops to track their temporal phase evolution and to quantify the extent of glaciation. We find that most glaciation events do not result in complete freezing. Rather, they induce a sustained shift in cloud properties while the clouds remain in the mixed‐phase regime. While the precise glaciation initiation mechanism remains unknown, higher hemispheric and seasonal ice-nucleating particle concentrations are shown to correlate with glaciation occurrence rate.

A cloud that retains supercooled liquid water after glaciation will have higher shortwave reflectance than a fully glaciated cloud. Inaccurate representations of glaciation can therefore bias radiative fluxes and, ultimately, climate projections. We are currently using our dataset to evaluate how accurately ICON represents mixed-phased cloud evolution. Simulations with progressively higher resolution are expected to yield higher phase heterogeneity in the cloud tops, thereby improving the representation of glaciation. This should help provide insights into the physical mechanisms that limit the extent of glaciation.