Observational evidence of high ice concentration in a shallow convective cloud embedded in stratiform cloud over North China

In this study we observed the microphysical properties, including the vertical and horizontal distributions of ice particles, liquid water content and ice habit, in different regions of a slightly supercooled stratiform cloud. Using aircraft instrument and radar data, the cloud top temperature was recorded as higher than −15°C, behind a cold front, on 9 September 2015 in North China. During the flight sampling, the high ice number concentration area was located in the supercooled part of a shallow convective cloud embedded in a stratiform cloud, where the ambient temperature was around −3°C. In this area, the maximum number concentrations of particles with diameter greater than 100 μm and 500 μm (N₁₀₀ and N₅₀₀) exceeded 300 L⁻¹ and 30 L⁻¹, respectively, and were related to large supercooled water droplets with diameter greater than 24 μm derived from cloud–aerosol spectrometer probe measurements. The ice particles types in this region were predominantly columnar, needle, graupel, and some freezing drops, suggesting that the occurrence of high ice number concentrations was likely related to the Hallett–Mossop mechanism, although many other ice multiplication processes cannot be totally ruled out. The maximum ice number concentration obtained during the first penetration was around two to three orders of magnitude larger than that predicted by the Demott and Fletcher schemes when assuming the cloud top temperature was around −15°C. During the second penetration conducted within the stratiform cloud, N₁₀₀ and N₅₀₀ decreased by a factor of five to ten, and the presence of columnar and needle-like crystals became very rare.