A Numerical Study Of Aerosol Effects On Thunderstorm Electrical Activities

The effects of aerosol on electrical activities based on WRF v3.4.1 model coupled electrification and discharge processes were discussed. Firstly, a squall line was simulated by using WRF coupled electrical processes for verifying the model ability of simulating the lightning processes. We discussed the effects of aerosol as CCN on the intensity of electrical activities and charge structure and analyzed the role of the thermal condition in the effects of aerosol on electrical activities for perceiving the mechanism of the effects of aerosol on lightning deeply through sensitivity studies. And we investigated the effects of aerosol on intensity and electrification of an idealized tropic cyclone. The research contents and conclusions showed as the following:(1) The processes of Non-inductive charging and simple discharging are coupled to Morrison microphysical scheme. A squall line occurred in North China on June 16th 2009 is simulated by suing WRF model coupled electrical processes, the model recreates the macroscopical and electrical feature of this squall line. By contrasting the LPI and simulated lightning density with observed lightning density, one can see that the simulated lightning density is close to the actual situation. So, the WRF model coupled electrical processes has the simulated ability of the electrical activities in thunderstorm.(2) The effects of aerosol on electrification of thunderstorm are investigated using the Weather Research and Forecasting model coupled with electrification and discharge parameterizations and an explicit treatment of aerosol activation. It is found that the microphysical and electric processes of the thunderstorm are distinctly different under different aerosol background. Enhancing aerosol loading increases growth rate of snow and graupel particles, and leads to higher concentration of ice particles. Increasing aerosol concentration also results in enhancement in electrification process, due to more ice particles participating in the electrification process in the polluted case. In the clean case, the charge structure maintained dipolarity throughout the simulation, while in the polluted case the charge structure transformed from dipolarity at the initial stage of charging separation to the structure of a negative charge region above the main positive and the main negative charge centers at the later stage. A detailed analysis of the microphysical processes shows that increasing aerosol loading led to more liquid water content and higher rime accretion rate above the freezing level, which was in favor of graupel charge positively and ice crystal and snow charge negatively in this region. In a word, increasing aerosol loading leads to increased cloud water content, resulting in a new negative charge region developed above the main positive charge center.(3) In order to further understand the effects of aerosol on microphysical and electric processes in different atmospheric thermal condition, the role of vapor condition in the effects of aerosol on electrical activities is discussed by changing vapor mixing ratio and initial CCN concentration simultaneously. The increase of cloud droplet concentration caused by increasing aerosol concentration in the thunderstorm with lower vapor content becomes less significant. Lower vapor content suppresses the effect of aerosol on ice particle, and then suppresses the effect of aerosol on electrical activities of thunderstorm. Lower vapor content results in unconspicuous increase of liquid water content in the charging region with increasing aerosol loading, and then suppresses the effect of aerosol on charge structure.(4) The effects of aerosol on intensity and electrification of an idealized tropic cyclone are investigated by using the Weather Research and Forecasting model coupled with an explicit treatment of aerosol activation and electrification parameterizations. It is found that the effects of aerosol on the intensity and electrification of tropic cyclone at different stages of development are distinct. At the developmental stage of tropic cyclone, increasing aerosol concentration leads to forming more but smaller cloud droplets suppressing warm rain formation by collision-coalescence process. The smaller cloud droplets are difficult to grow into rain drops and are transported to the mixed-phase region with updraft that may result in forming more ice particles. The freezing process releases more latent heat, inspiring the development of tropic cyclone eyewall and enhancing the intensity and electrification of tropic cyclone. At the mature stage, more ice particles melt into rain water and the rainfall particles drag the updraft in the process of falling. The rainfall particles evaporating at the sea surface absorb more latent heat and decrease the temperature in this area. The low temperature area blocks the transportation of warm and wet air into tropic cyclone, and suppresses the intensity of tropic cyclone. The electrification process is weaker at mature stage because of less ice particles in the cloud.