In recent years, the drought situation in Beijing area is very serious, especially in the winter.How to effectively utilize the water resources in the air and carry out rain enhancement operation is important.Rain enhancement operation in winter is also likely to be used to remove fog and haze, and furthermore to improve air quality and visibility.Cloud models have been used in weather modification to formulate cloud-seeding hypotheses, assessments of the cloud-seeding potential or "seed ability".In this study, the numerical simulations of a snowfall case was carried out by using the two-moment explicit cloud scheme (nssl) of WRF model.The nssl scheme can predict the particle number concentration and mixing ratio of cloud water, rain, ice, snow, graupel and hail.For the snowfall case in Beijing on 19 March 2013, several seeding tests were designed to study the effects of different seeding time and seeding rate on cloud processes and precipitation amounts.The experimental results showed that all seeding tests could increase rainfall amount.When seeding with an amount of 1×107 kg-1 in cloud developing period, the rain enhancement effect was the best.The water vapor and supercooled cloud water mixing ratio in seeding area decreased after seeding, meanwhile the mixing ratios of ice and snow increased.The updraft and temperature of this area also increased.In 40 min after seeding, snow mixing ratios mainly grew through microphysical processes of deposition, the automatic conversion of ice to snow, snow accretion of cloud droplets, the collision between ice and snow.In 200 min after seeding, microphysical conversion processes of snow sources in the catalytic clouds were all higher than the natural clouds.Because of the consumption of supercooled cloud water in seeding clouds, the accretion process between cloud droplets and snow was very small.The snow mixing ratio mainly increased through terms of snow deposition and the interaction between snow and ice.The research results are helpful to understand the seeding effects on the macro and micro-processes of winter stratiform clouds, which will provide the basis for the rainfall enhancement field operations.
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