Study of the Cloud Microphysical Process and Mechanisms of Movement and Propagation of a Dry Downburst

In this study, we conducted an analysis of the occurrence and evolution characteristics of a dry downburst event under weak synoptic forcing, hereinafter referred to as “5.14,” in the semiarid region of Lanzhou on May 14, 2020. We used observational data from wind profile radar, runway automatic observation, and Doppler weather radar. We then employed the mesoscale numerical model WRF (Weather Research and Forecasting) to simulate the formation, movement, and evolution of the hydrocondensates over the divergence outflow area during the “5.14” event. Furthermore, we explored the potential mechanisms behind the outflow propagation of the “5.14” event. Our results revealed that the “5.14” event had a duration of approximately 30 min, with cloud tops reaching heights exceeding 9 km. Several factors were identified as possible causes of the outflow of the downburst, i.e., the sudden intrusion of a dry cold air jet at altitudes of 3–6 km behind the moving cloud body, cloud body fracture, cloud top collapse, momentum downward transport, and the presence of divergence outflow jets at 1–4 km altitudes in the middle and lower atmosphere. During the event, snow crystals collided with supercooled cloud droplets, resulting in the rapid growth of graupel particles in both the sinking and outflow regions. Notably, the simulated mixing ratio of the graupel particles was found to increase by a factor of 10⁵ during the initial outbreak of the downburst. The graupel particles in the sinking zone played a significant role in accelerating the formation of cold pools within the clouds, which contributed to the downdraft intensification. With the cloud body moving, the strong downdraft generated divergence outflow at the surface, causing the convergence and upward movement of airflow. This process resulted in the formation of two updraft areas on both sides of the downdraft, creating vertical circulations within the cloud. These vertical circulations comprised an updraft and a neighboring wet downdraft, which contributed to the spreading of cold pools near the surface, thereby promoting the outbreak of the downburst and stimulating the gust front. The gust front, accompanied by warm, moist updrafts ahead, propagated in the outflow direction from the divergence center of the downburst, which facilitated the merging of new storm cells into the original, strengthening it. However, as the gust front advanced, it severed the warm and moist updrafts, leading to the continuous descent of the heavy cold cloud top. This process caused the upward rush and subsequent collapse of the cloud top, shaping the outflow propagation process of the downburst. Rainwater particles transported by updrafts ahead of the gust front froze near the 0°C layer. The heat released during freezing caused the 0°C layer to rise continuously during the outflow propagation, while the melting layer of graupel particles descending within the clouds also ascended. The melting of graupel particles and the evaporation of rainwater during their descent caused the expansion of the rainwater particle evaporation layer. The heat absorption during these processes, along with the strengthening of cold pools near the surface, contributed to the increased surface wind speed during the outflow propagation, playing a crucial role in the formation of surface gales during the outflow propagation of the downburst. Furthermore, updrafts heated the atmosphere via condensation, thus enhancing the upward motion. The development of the downdraft played a pivotal role in creating and maintaining the convective circulation and cold pools. After the formation of the downburst, a vertical closed circulation continuously developed in the direction of cloud body movement, serving as the moving mechanism of the divergence center of the downburst. However, as the cloud body moved eastward and the circulation of water condensate that maintained the vertical closed circulation structure weakened, the vertical closed circulation structure dissipated, giving rise to the weakening and eventual extinction of the divergence center of the downburst. Compared with previous studies, this dry downburst event showed similar characteristics, including the presence of an inflow jet behind the cloud body, a decrease in radar echo reflectivity factor, momentum downward transport, high water content of graupel particles, and heat absorption and cold pool formation during the melting and evaporation processes of the water condensate. Importantly, this dry downburst event featured a distinct vertical closed circulation in its divergence center, which was crucial to initiating and sustaining the downburst. In addition, the divergence center of the downburst was closely linked to the formation of the gust front, with the gust front process serving as the primary driver of surface gale formation during the outflow propagation of the dry downburst.