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A Preliminary Numerical Study into the Effects of Coal Development on Cloud and Precipitation Processes in the Northern Great Plains

Mark R. HjelmfeltInstitute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City 57701

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Richard D. FarleyInstitute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City 57701

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Philip C. S. ChenInstitute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City 57701

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Abstract

Numerical simulations are conducted to determine the possible effects of air pollution from coal-fired power plants on cloud and precipitation processes in the northern Great Plains. This study can only be considered as preliminary in nature since a complete cloud simulation is not employed and the ice phase is not considered.

Natural and polluted particulate distributions are developed based on observations in the northern Great Plains and of coal-fired power plant effluent. Cloud droplet growth on these distributions is simulated in a condensation model. Results of this model indicate that the number concentration and dispersion coefficient (breadth) of the cloud droplet size distributions are increased by the addition of pollutant particles, especially if these are more hygroscopic than the background nuclei.

Coalescence calculations using the results of the condensation studies as input are also reported. These results indicate that the rate of production of large drops, while being slowed by an increase in the number concentration, is hastened by an increase in the dispersion coefficient. These two effects nearly cancel each other out so that the time required for precipitation development is very nearly the same for cloud droplet distributions initialized on background and polluted particulate distributions. If, however, both distributions have the same dispersion, the polluted case requires a considerably longer time to develop precipitation.

Abstract

Numerical simulations are conducted to determine the possible effects of air pollution from coal-fired power plants on cloud and precipitation processes in the northern Great Plains. This study can only be considered as preliminary in nature since a complete cloud simulation is not employed and the ice phase is not considered.

Natural and polluted particulate distributions are developed based on observations in the northern Great Plains and of coal-fired power plant effluent. Cloud droplet growth on these distributions is simulated in a condensation model. Results of this model indicate that the number concentration and dispersion coefficient (breadth) of the cloud droplet size distributions are increased by the addition of pollutant particles, especially if these are more hygroscopic than the background nuclei.

Coalescence calculations using the results of the condensation studies as input are also reported. These results indicate that the rate of production of large drops, while being slowed by an increase in the number concentration, is hastened by an increase in the dispersion coefficient. These two effects nearly cancel each other out so that the time required for precipitation development is very nearly the same for cloud droplet distributions initialized on background and polluted particulate distributions. If, however, both distributions have the same dispersion, the polluted case requires a considerably longer time to develop precipitation.

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