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

Clouds between −39° and 0°C can be topped by a liquid or ice layer, which affects their radiative forcing and precipitation. The cloud-top ice-to-total frequency (ITF) quantifies the occurrence of clouds with an ice top relative to total cloud occurrence, but the factors controlling ITF are poorly understood. Using 35 years of satellite data, we show that in the Northern Hemisphere, between −15° and −30°C, dust aerosol is strongly correlated with ITF in both time and space. Furthermore, we found that the sensitivities of ITF to temperature and dust are in a ratio that agrees with laboratory measurements of droplet freezing, showing that ITF can be attributed to dust aerosol.