High ice concentration observed in tropical maritime stratiform mixed-phase clouds with top temperatures warmer than −8 °C

In this study, airborne in situ measurements of ice concentrations in mixed-phase clouds over the tropical ocean sampled during the Ice in Clouds Experiment (ICE-T) project are analyzed. High concentrations of ice larger than 250 μm in diameter (0.05–10 L⁻¹) were observed in three shallow stratiform clouds whose top temperatures were warmer than −8 °C. The observed ice particles were mostly needles and columns, which are preferred ice crystal shapes between −3 °C and − 8 °C. Drizzle size drops (>100 μm in diameter) were also observed. Biological particles are a potential source of ice nucleating particles (INPs), however, any direct connection to the high ice concentrations observed in the stratiform clouds at temperatures warmer than −8 °C could not be shown, because the observed INP concentrations were 3 orders of magnitude lower than the ice concentrations. Simulations using a parcel model suggest that secondary ice mechanisms, such as the Hallett-Mossop process and fragmentation of freezing drops, which were often assumed previously to be the explanation for observed high ice concentration, cannot fully explain the observed high ice concentration in these shallow stratiform clouds either. The Hallett-Mossop process and fragmentation of freezing drops are important to the strong ice production only in clouds with relatively high concentration of large drops and rimed graupel observed, such as convective clouds. Therefore, other potential ice generation mechanisms, such as droplet collisional freezing and pre-activated INPs, which are still poorly understood, could play significant roles in tropical stratiform and convective mixed-phase clouds.