Aircraft Observations and Numerical Simulations of Precipitation Processes in a Case of Summer Stratiform Clouds with Embedded Convection over North China

In this paper, aircraft observation data combined with WRF (Weather Research and Forecasting) model coupled with the ISHMAEL (Ice-Spheroids Habit Model with Aspect-Ratio Evolution) microphysical scheme are used to analyze the microphysical characteristics and vertical structure of summer stratiform clouds with embedded convection precipitation process in the North China area that occurred on 27 August 2017. The analysis of aircraft observations reveals that the supercooled water content in the stratus cloud region was not considerable, whereas it is locally considerable in the convective cloud region, with a peak value of 0.566 g m⁻³. The maximum total water content in the upper layer can reach 2.267 g m⁻³. In the upper and middle layers of stratiform clouds, a sudden increase is observed in the concentration of cloud droplets, ice, and snow crystals. The shape of ice crystals is mainly in the form of large aggregates, and the growth process of ice crystals mainly involves aggregation and condensation. Near the 0℃ layer in the middle of the layered clouds, the liquid water content considerably increases compared with that in the upper layer, mainly consisting of raindrops and partially melted precipitation particles. In the warm cloud layer, located in the middle and lower parts of the layered clouds, the number and concentration of cloud droplets substantially increase compared to those in the middle and upper layers. The cloud droplet spectrum is considerably narrower. The simulation results indicated that the contents and distributions of different types of ice crystals vary substantially. The content of ICE1 was the highest, while that of ICE2 was the lowest. The distributions of ICE1 and ICE3 demonstrated a high degree of consistency, with a predominant concentration at temperatures exceeding −10℃. Conversely, the distribution height of ICE2 was lower, primarily confined to the ice-water mixing layer. The results implied that during the process of stratiform cloud precipitation, a “seeding-supply” mechanism operates during the stage when precipitation is enhanced. At this phase, layers of snow and ice crystals primarily grow through deposition and coagulation, exerting a seeding effect on the lower layers. As these layers descend into the warm layer, the subsequent melting considerably contributes to the augmentation of raindrops. By contrast, during the weak precipitation stage, the role of seeding clouds was observed to be less pronounced, with precipitation formation mainly occurring through the conversion of clouds to rainwater in the warm region.