Numerical Simulation of the Macro and Micro Physical Responses of Stratiform Cloud Seeding

Stratiform cloud systems are important for the exploitation and utilization of cloud water resources. Precipitation enhancement requires scientific and feasible operational technical indicators to guide the implementation of actual operations, and a reasonable and accurate assessment of operation effects is an important issue that needs to be solved. One of the necessary ways to establishing and improving operation technologies is to simulate the seeding operation process reasonably through a numerical model and study the changes and mechanisms of a series of macro and micro characteristics of cloud and precipitation after seeding operations. Evaluating the effect of realistic precipitation enhancement is also an effective method through a seeding model simulation. An aircraft seeding operation during the stratiform cloud precipitation in Hebei province on April 15, 2014 was simulated in accordance with a real operation process by a 3D mesoscale cold cloud seeding model. The actual operation process was reasonably simulated by the numerical model. The diffusion and transmission characteristics of AgI particles seeded by aircraft in the atmosphere were studied, the seeding influence on the macro and micro characteristics of clouds and precipitation was analyzed, and the precipitation enhancement effect of the aircraft seeding operation was evaluated. Results show that the horizontal scale of AgI plume can extend to more than tens of kilometers, and most AgI particles in the vertical direction are concentrated within the range of approximately 1 km above and below the seeding layer. Moreover, the upward transport of AgI particles is significantly stronger than the downward transport. The outstanding increase in ice crystals and snow particles in clouds after seeding leads to the inhibition of graupel growth in the early simulation stage. However, the enhancement of the graupel collection snow process and ice phase particle riming processes near the zero layer gradually increases the total mass of graupel after some time. After the aircraft seeding operation, radar reflectivity is enhanced; in addition, it shows different structural characteristics with time. Precipitation decreases first and then increases with time due to seeding. Three hours after seeding, the operation influence area extends to more than 100 km to the downstream of the operation area, showing the distribution characteristics of rainfall-reducing area first and then rainfall-increasing area in general. The model evaluation indicates that the net rainfall increases by 3.6×10⁷ kg in the entire evaluation area, with an average rainfall-increasing rate of 1.1%; furthermore, the increase in the concentration and size of graupel particles in the warm layer is the main reason for rainfall increase. Given the weak seeding operation conditions in the target cloud area, the AgI seeding amount of this operation is relatively high, resulting in a low effect of precipitation enhancement.