# Cloud Seeding Evidenced by Coherent Doppler Wind Lidar
## 4 Application in an Artificial Precipitation Operation
During the cloud seeding, the CNR at 4.0-4.5 km altitude was increased from 5:30 p.m. to 6:00 p.m. local time. The corresponding vertical velocity had significant changes in the ice region near 4 km, as shown in Figure 6f. The injection of AgI particles into a supercooled cloud layer released latent heat, forming buoyancy in the ice-affected region [4,13]. Thus, a small-scale and persistent updraft airflow was accompanied by downdrafts on the outside edges [55]. There was a vacancy region in the ice-affected region near 6:00 p.m. local time. It might be that rising and descending airflow were neutralized, causing the cloud water to erode by evaporation, and leading to formation holes in clouds [56]. After the seeding began, the CNR at the 0-1.5 km altitude was increased from 5:30 p.m. to 7:00 p.m. local time (Figure 6a). There was a large amount of silver iodide-type aerosols in the boundary layer via wet deposition over large areas [57], which were not completely injected into the cloud environment. These silver iodide-type aerosols were washed out near the atmospheric boundary layer (ABL), with variation of wind speed and direction, which served as an inadvertent transporter of the seeding agent that sometimes happens in weather modification practices [58]. The lidar using VAD scanning can only detect the seeding particles in the inverted cone. Low altitude particles are detected at first, and then higher altitude particles can be detected with particle transport. Silver iodide-type aerosols might serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud condensation. Figure 6: A cloud seeding operation process detected by CDWL during 12–13 September 2019. **(a)** CNR, **(b)** spectrum width, **(c)** skewness, **(d)** horizontal wind speed, **(e)** horizontal wind direction, **(f)** vertical wind velocity, **(g)** turbulent kinetic energy dissipation rate (TKEDR), **(h)** shear intensity. The TKEDR is used to estimate turbulence intensity [35]. The vertical red dash-dotted line in **(a)** represents the moment (5:30 p.m.) when seeding begins. The vertical black dash-dotted line represents the moments half an hour before and after seeding. The vertical white dash-dotted line stands for the moment (6:40 p.m.) when precipitation begins. properties and the initiation of precipitation [59; 60; 61]. After a while, the precipitation began (6:40 p.m.), which can be seen from the broadened spectrum width and increased skewness in Figure 6b,c, indicating the positive effectiveness of the cloud seeding. The airport was experiencing strong winds, with ground-level wind speed exceeding 15 m/s frequently, as shown in Figure 6d. It can easily produce wind shear and turbulence, which may threaten the safety of aircraft operation. For instance, strong low-level jets were detected during the experiments, as presented in [51]. Therefore, an important role of lidar is to provide windshear and turbulence alerts, as shown in Figure 6g,h. Phase transformation and precipitation formation in seeded clouds are associated with the Wegener-Bergeron-Findeisen (WBF) process. The equilibrium vapor pressure of water vapor with respect to ice is less than that with respect to liquid water at the same subfreezing temperature [62]. Before cloud seeding, supercooled liquid water and ice crystals coexist in mixed phase clouds, but the concentration of natural ice nuclei is very small. There is a competitive relationship between the rate of generation and scavenging of supercooled liquid water in the target region [63]. After the seeding flight, Agl particles are injected into supercooled water supplying a large number of tiny artificial ice nucleus, and the process of supercooled liquid water scavenging by evaporation and vapor condensing into ice crystals is greatly strengthened. Thus, the ice crystals would gain mass by vapor deposition, at the expense of the supercooled water that would lose mass. Upon attaining sufficient weight, the ice crystals would fall.