Precipitation formation processes: A global perspective

The large- and meso-scale dynamics determining the characteristics of the cloud systems down to the small-scale microphysics determining the nucleation and growth characteristics of water droplets and ice particles all form part of the chain of events of precipitation development. Although our knowledge of the individual aspects in the chain has significantly increased in the past twenty years there still exist major gaps about certain physical processes. Precipitation initiation and development can proceed via several physical paths, involving various microphysical processes which proceed simultaneously but at different rates, with one path becoming dominant because of its greater efficiency under given atmospheric conditions. The efficiency with which clouds produce rain at the surface varies greatly. Precipitation efficiency, defined as the ratio of the rate of rain reaching the ground to the flux of water vapor passing through cloud base, can range from zero in non-precipitating clouds to greater than unity for short times, in very intense, time-dependent, convective systems. Some of the earliest studies showed that ordinary thunderstorms transform less than 20% of the in-flux of water vapor into rain on the ground. The principles of most, if not all, precipitation enhancement hypotheses are rooted in these efficiency factors which, in general, seek to improve the effectiveness of the precipitation evolution path.

Aerosol affects can lead to changes in the hydrological cycle, which connects directly to the availability of fresh water that could result in a major environmental issue. The effects of aerosols on cloud microphysical and dynamical processes and precipitation were also highlighted by the results from experiments to enhance rainfall by hygroscopic seeding. Evidence exists that: 1) the particles produced by hygroscopic flares do indeed broaden the droplet spectrum and 2) seem to increase the lifetime of precipitating convective clouds possibly by changing the downdraft/updraft structures. The aerosol effects on precipitation development, however, can have different effects on the precipitation development in clouds depending on the thermodynamic structure of the atmosphere and the cloud base temperatures. Some intriguing similarities and differences of the effects of aerosols on precipitation development have recently been observed in different experiments around the world that have direct impacts on cloud seeding experiments. The talk will focus on primarily on convective clouds growing in different meteorological regimes around the world.