Dust-driven droplet freezing explains cloud-top phase in the northern extratropics.

Clouds with temperatures between −39° and 0 °C can be capped by either a liquid or an ice layer, strongly influencing their radiative forcing and precipitation. The cloud-top ice-to-total frequency (ITF) quantifies the occurrence of clouds with ice tops relative to all clouds, yet the processes controlling ITF remain poorly understood. Using 35 years of satellite observations (Cloud_cci v3) and dust reanalysis (MERRA2), we show that in the Northern Hemisphere, at temperatures between −15° and −30 °C, ITF is strongly correlated with dust aerosol variability in both time and space. Moreover, we find that the sensitivities of ITF to temperature and dust occur in a ratio consistent with laboratory measurements of immersion droplet freezing, indicating that dust aerosols impose a logarithmic control on cloud-top phase.