Observations of cloud microphysics and ice formation during COPE

Intense rainfall generated by convective clouds causes flash flooding in many parts of the world. Understanding the microphysical processes leading to the formation of precipitation is one of the main challenges to improving our capability to make quantitative precipitation forecasts. Here, we present microphysics observations of cumulus clouds measured over the Southwest Peninsula of the UK during the COnvective Precipitation Experiment (COPE) in August 2013, which are framed into a wider context using ground-based and airborne radar measurements. Two lines of cumulus clouds formed in the early afternoon along convergence lines aligned with the peninsula. The lines became longer and broader during the afternoon as a result of new cell formation and stratiform regions forming downwind of the convective cells. Aircraft penetrations at −5 °C showed that all the required conditions of the Hallett–Mossop (H–M) ice multiplication process were met in developing regions, and ice concentrations up to 350 L−1 were measured in the mature stratiform regions, indicating that secondary ice production was active. Detailed sampling focused on an isolated liquid cloud that glaciated as it matured to merge with a band of cloud downwind. In the initial cell, a few drizzle drops were measured, some of which froze to form graupel; the ice images are most consistent with freezing drizzle, rather than smaller cloud drops forming the first ice. As new cells developed in and around the cloud, ice concentrations up to two orders of magnitude higher than the predicted ice nuclei concentrations began to be observed and the cloud glaciated over a period of 12–15 min. Ice splinters were captured by supercooled drizzle drops causing them to freeze to form instant-rimers. Graupel and columns were observed in cloud penetrations up to the −12 °C level, though many ice particles were mixed-habit due to riming and growth by vapour diffusion at multiple temperatures. Frozen drizzle/raindrops initially made up the majority of precipitation-sized particles in the H–M zone, while ice splinters required time to grow by vapour diffusion. It is therefore clear that the freezing of supercooled drizzle drops not only provides a pathway to advance the onset of the H–M process, it also accelerates glaciation and the formation of precipitation once it has begun. Accurate representation of both the warm rain and H–M processes, including their interactions with each other and cloud dynamics, appears key to determining the timing and location of precipitation.