Aerosol effects on the microphysics of precipitation development in tropical and sub-tropical convective clouds

In-situ aerosol and cloud microphysical measurements in convective clouds in Indonesia, India, the United Arab Emirates, Australia and Saudi Arabia over the past ten years have shown large spatial and temporal variations in microphysical characteristics. These variations can be linked to temporal and spatial variations atmospheric aerosol characteristics. Higher concentrations of aerosols and Cloud Condensation Nuclei (CCN) resulting in high droplet concentrations can have widely different effects on precipitation formation processes depending on if clouds remain “warm” (cloud body at >0oC) or are in the mixed phase stage. While higher levels of pollution could decrease precipitation formation in the one instance it may increase it in the other. Airborne and radar observations collected will be utilized to show these differences. These differences are due to strong interactions between the microphysics and dynamics of the clouds.

Large nuclei (aerosol/CCN) are found to be essential for initiating coalescence in polluted environments. Once coalescence has been initiated, the rate at which cloud liquid water is transferred to precipitation-size drops depends on the amount of drizzle which forms in the cloud. Drizzle production is retarded when accumulation mode aerosol concentration is high, but is promoted when coarse mode aerosol concentration is high. Ultra-giant CCN (diameter >10 micron) accelerate the onset of coalescence but suppress drizzle production, and result in a slower production of raindrops. Larger nuclei slow the rate of drizzle formation but condensate is converted to precipitation-size drops, and may fall out of the cloud before most of the available water has been harvested. In addition, recent dual polarization radar data have shown that the evolution of rain drop spectra could also change as a function of aerosols and microphysical processes which will have impacts on other dynamical processes in the clouds.

These results in turn determine the ice processes in clouds once they grow to temperatures colder than 0oC and affect droplet freezing and secondary ice processes, the overall precipitation formation and possibly efficiency and amount of precipitation that will reach the surface. These results have implications for both advertent and inadvertent weather modification.