Study On The Effect And Mechanism Of Mitigating Or Suppressing Precipitation By Cloud Seeding On Opening Ceremony Of Nanjing Youth Olympic Games

As the opposite effect of artificial rainfall enhancement,artifical rainfall reduction has great significance in weakening heavy rain disaster,guaranteeing large-scale outdoor activities,and more accurately performing artificial rainfall.However,as a frontier subject of Weather modification,the feasibility and methods of artificially reducing rainfall and the mechanism of rain reduction are not yet clear.This paper implemented the catalytic function in mesoscale mode by combining the silver iodide and cloud interaction in the Thompson microphysics scheme of the mesoscale WRF model and considering three nucleation mechanisms of AgI.In this way,the feasibility,methods,and mechanisms for reducing rainfall are studied.To simulate the precipitation process during the opening ceremony of the Nanjing Youth Olympic Games on August 16,2014,at the same time to analyze the characteristics of cloud physics and the source and sink of precipitation particles,the results are as follows:1)To compare the simulation results with the accumulated precipitation on the ground,the circulation situation at 500 hPa,and the comparison between the satellite imagery and Radar reflectivity,we can see that the WRF model simulation results are satisfactory.It can do well in simulating the precipitation circulation patterns,cloud distribution,and ground precipitation.The results can be used for subsequent studies.2)The cloud water content of the precipitation is low,the content of snow,graupel and ice crystals is small,and the vertical movement does not develop vigorously.Therefore,it is a typical convective-stratiform mixed clouds precipitation process.3)Outputing the source and sink items of each precipitation particle can find:the main sink of raindrop content is the evaporation of raindrops,and the largest source of raindrops is the melting of snow,followed by rain drops collecting cloud droplets,there is still a small amount of melting graupel in the period of large rainfall such as 10 to 13 o'clock.By simulating the actual silver iodide seeding operation scheme using mesoscale WRF model with Thompson microphysical scheme coupled Agl,we can find:1)Since the cloud system studied in this paper is a typical stratiform cloud system,the nucleation mechanism of AgI is contact freezing nucleation.The nucleation rate is about 3.5%.2)For the entire simulation area,the addition of AgI will cause the redistribution of water vapor in the simulated area,while the total amount of water vapor will remain unchanged,there is a slight change in cloud droplet concentration.The main change is that AgI nucleation will increase the cloud ice content and ice number concentration.3)After precipitation of silver iodide,precipitation on the ground decreased.The reason for the reduction are as follows.For one thing,after the excessively seeding of AgI the artificial ice nucleus are involved in the nucleation to form a large amount of ice crystals,resulting in much autoconversion of ice crystal to snow;For another thing,the consumption of water vapor leading to less condensation of snow,so that producing a large number of smaller snow crystal,smaller snow crystal melt into smaller raindrop,the smaller raindrops quickly evaporate,leading to reduction of ground precipitation.At the same time,the small raindrops has a slower falling speed then the big one,which results in a decrease in the precipitation rate on the ground.Through the sensitivity tests on the seeding rate,seeding height,seeding start time,and seeding duration,it can be found that the excessing of silver iodide can actually reduce rain.Within a certain range,as the amount of silver iodide seeding increases,the rate of rain reduction will increase,but rainfall reduction efficiency does not increase with the unlimited amount of silver iodide.Study found that seeding silver iodide below minus five degrees Celsius has better effect of reducing rain,taking into account the water vapor conditions and then find suitable sites for seeding.The seeding duration is similar to the seeding rate.Increasing the duration in a certain period of time will increase the rate of rain reduction,but it is also not an unlimited increase.