Vertical Observation and Numerical Simulation of the Clouds Physical Characteristics of Warm Conveyor Belt within a Winter Mesoscale Snowstorm

This study provides an in-depth analysis of a system snow event influenced by the warm conveyor belt (WCB) on December 14, 2023, using ground-based observations from cloud radar, disdrometers, wind profiler radar, and the mesoscale numerical model WRF. Our results showed that the snow event was primarily influenced by the southwest WCB and the northeast cold air return flow. Two distinct snow events (accumulated snowfall reached 8 mm) were observed in the Huozhou site of southern North China. The snowfall cloud is primarily composed of solid-phase particles and exhibits a well-defined layered structure. During the early stage of the snowfall, there was almost no upward motion within the clouds. In the WCB region (2-3 km), water vapor primarily condensed into supercooled cloud water(~0.4g kg-1), while the upper levels were dominated by ice and snow deposition processes. As latent heat was continuously released through the condensation process, significant upward motion developed within the cloud, facilitating the vertical mixing of ice and snow particles. These particles underwent deposition and aggregation, with larger particles descending into the WCB region. Through accretion and Bergeron processes, liquid water was rapidly consumed, leading to the formation of numerous graupel particles, which eventually fell to the ground with a peak diameter of 1.4 mm. As the snowfall cloud continues to develop, the water vapor transport by WCB weakens. In the mid-to-lower troposphere (2–6 km), the ambient water vapor pressure lies between the saturation vapor pressures over ice and water. The maximum water vapor depletion rate exceeds 4.5 × 10-4 g·kg-1·s-1. Under supersaturation with respect to ice, ice and snow particles grow continuously through deposition. During their descent, they collide and aggregate with graupel particles in the lower layers, eventually falling to the surface together. At this stage, surface precipitation primarily consists of a mixture of small snowflakes and snow graupel, with peak particle diameters ranging from 0.6 to 0.8 mm. Despite the weakening of the water vapor transport, the substantial latent heat released during the deposition process continued to promote upward motion and cloud development, sustaining the snowfall at the surface. These results highlight that the significant upward motion triggered by the release of latent heat during water vapor deposition was a key factor in leading to the observed heavy surface snowfall.