Microphysical view of the development and ice production of mid-latitude stratiform clouds with embedded convection during an extratropical cyclone

Abstract

The microphysical properties associated with ice production importantly determine precipitation rates. In this study, the microphysical properties of stratiform clouds with embedded convection during an extratropical cyclone over northern China were characterized in situ. Stages of clouds, including young cells rich in liquid water and developing and mature stages with high number concentrations of ice particles (NIce), were investigated. NIce could reach 300 L−1 in the mature stage, approximately 2 orders of magnitude higher than the primary NIce. The secondary ice production (SIP) rate was 0.005–1.8 L−1 s−1, which was derived from the measured NIce. The SIP rate could be produced using a simplified collision–coalescence model by considering the collection of large droplets by graupel. The collection efficiency between the graupel and the droplet was found to increase when the size of the droplet approached that of the graupel, which may improve the agreement between the measurements and the model. Importantly, the overall NIce was found to be highly related to the distance to the cloud top (DCT). The level with a larger DCT had more rimed graupel falling from the upper levels, which promoted coalescence processes between the graupel and the droplets, producing a greater fraction of smaller ice. This seeder–feeder process extended the avalanche SIP process at lower temperatures to −14 °C, beyond the temperature region of the Hallett–Mossop process. The results illustrate the microphysical properties of clouds with convective cells at different stages, which will improve the understanding of the key processes in controlling the cloud glaciation and precipitation processes.