Research-Environment modeling

 

Interactions between aerosols, clouds and precipitation

 
 

    Autoconversion of cloud droplets to embryonic raindrops is one of the most important microphysical processes in warm clouds. From first principles, a three-moment theoretical expression is analytically derived for the autoconversion rates of the number concentration, mass content, and radar reflectivity based on the generalized gamma distribution function for cloud droplet size distributions. 

    It is found that the autoconversion rate increases significantly with decreasing shape parameter μ of the gamma distribution, no matter how high or low the liquid water content is, but the tail parameter q only plays an important role at low liquid water content. These results may have many potential applications, especially to studies of the indirect aerosol effect and the influence of μ and q on cloud and precipitation.

Xie, X., and X. Liu (2009), Analytical three-moment autoconversion parameterization based on generalized gamma distribution, J. Geophys. Res., 114, D17201, doi:10.1029/2008JD011633.

 

    From first principles, we find that the radar threshold reflectivity between nonprecipitating clouds and precipitating clouds is strongly related to not only the cloud droplet number concentration but also the spectral dispersion of cloud droplet size distributions. Further investigation indicates that the threshold value is an increasing function of spectral dispersion and cloud droplet number concentration. These results may improve our understanding of the cloud-precipitation interaction and the aerosol indirect effects.

Xiaoning Xie and Xiaodong Liu (2010), Spectral dispersion of cloud droplet size distributions and radar threshold reflectivity for drizzle, Chin. Phys. B, 19, 109201.

 

    The effects of spectral dispersion on clouds and precipitation in mesoscale convective systems have been studied by conducting 10 numerical simulations with different values of spectral dispersion (epsilon = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0) in the clean, semi-polluted, and polluted backgrounds. It is found that the effects of spectral dispersion on simulated precipitation differ in these three aerosol backgrounds and for various relative humidity levels.

   In the clean background and at relatively lower humidity, the average accumulated precipitation is reduced significantly with an increase in spectral dispersion. Precipitation varies nonmonotonically in the semipolluted background, increasing with spectral dispersion at smaller values, while decreasing at larger values. In the mean time, precipitation is continuously enhanced with increasing spectral dispersion in the polluted background. 

Xiaoning Xie and Xiaodong Liu (2011), Effects of spectral dispersion on clouds and precipitation in mesoscale convective systems, J. Geophys. Res.,116, D06202, doi:10.1029/2010JD014598.