Cloud-scale ice-supersaturated regions spatially correlate with high water vapor heterogeneities

Cirrus clouds have large yet uncertain impacts on Earth's climate. Ice supersaturation (ISS) -- where the relative humidity with respect to ice (RHi) is greater than 100% -- is the prerequisite condition of ice nucleation. Here we use 1 Hz (~230 m) in situ, aircraft-based observations from 87° N to 67° S to analyze the spatial characteristics of ice-supersaturated regions (ISSRs). The median length of 1-D horizontal ISSR segments is found to be very small (~1 km), which is 2 orders of magnitude smaller than previously reported. To understand the conditions of these small-scale ISSRs, we compare individual ISSRs with their horizontally adjacent subsaturated surroundings and show that 99% and 73% of the ISSRs are moister and colder, respectively. When quantifying the contributions of water vapor (H2O) and temperature (T) individually, the magnitudes of the differences between the maximum RHi values inside ISSRs (RHimax) and the RHi in subsaturated surroundings are largely derived from the H2O spatial variabilities (by 88%) than from those of T (by 9%). These features hold for both ISSRs with and without ice crystals present. Similar analyses for all RHi horizontal variabilities (including ISS and non-ISS) show strong contributions from H2O variabilities at various T, H2O, pressure (P) and various horizontal scales (~1-100 km). Our results provide a new observational constraint on ISSRs on the microscale (~100 m) and point to the importance of understanding how these fine-scale features originate and impact cirrus cloud formation and the RHi field in the upper troposphere (UT).