Deep convective clouds with sustained supercooled liquid water down to -37.5 °C

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Abstract
In cirrus1 and orographic wave clouds2, highly supercooled water has been observed in small quantities (less than 0.15 g m-3). This high degree of supercooling was attributed to the small droplet size and the lack of ice nuclei at the heights of these clouds1,2. For deep convective clouds, which have much larger droplets near their tops and which take in aerosols from near the ground, no such measurements have hitherto been reported. However, satellite data suggest that highly supercooled water (down to -38 °C) frequently occurs in vigorous continental convective storms3. Here we report in situ measurements in deep convective clouds from an aircraft, showing that most of the condensed water remains liquid down to -37.5 °C. The droplets reach a median volume diameter of 17 µm and amount to 1.8 g m-3, one order of magnitude more than previously reported2. At slightly colder temperatures only ice was found, suggesting homogeneous freezing. Because of the poor knowledge of mixed-phase cloud processes4, the simulation of clouds using numerical models is difficult at present. Our observations will help to understand these cloud processes, such as rainfall, hail, and cloud electrification, together with their implications for the climate system.