How much water can pollution aerosols hold in deep tropical clouds by suppressing warm rain?

The development of warm rain depends on cloud drop size and cloud water content. In the upper yet unfrozen parts of deep tropical clouds warm rain is expected to form quickly because both cloud drops would grow diffusionally to large sizes and the cloud water content would be very high if not depleted by rain forming processes. Therefore, the documentation of deep tropical clouds with suppressed warm rain at depth>5 km above base is particularly challenging. Respectively, documenting the cloud depth for onset of warm rain is of fundamental importance and a powerful quantity for validating rain forming processes in cloud models. Vertical profiles of aerosols, cloud drop size distribution and hydrometeors were measured by cloud physics airplanes in a wide range of locations in the world and a wide range of aerosol properties. This includes tropical clouds in the Amazon and Indian Monsoon, severe convective clouds in Argentina and the USA, and winter clouds in California and Israel. The aerosol conditions span from pristine maritime to heavy air pollution and smoke from forest fires, and includes large aerosols such as desert dust and sea spray. It is found that heavy air pollution that is dominated by small cloud condensation nuclei (CCN) can suppress the warm rain processes up to heights of more than 5 km above cloud base of deep tropical clouds with cloud base temperature of 25C, which is well above the freezing level. At these heights precipitation is initiated as supercooled rain that freezes and gets rimed into graupel. Such clouds have intense electrical activity. In microphysically highly continental vigorous convective clouds with cold base of 12C warm rain is also delayed to more than 5 km above base. The onset of warm rain in these clouds occur at temperatures close to the homogeneous ice nucleation isotherm of -35C and colder. The resultant drizzle forms the embryos of ice hydrometeors. Giant CCN from dust and sea salt initiate the rain faster than would have been expected based on the concentration of small CCN. A relation between CCN concentration and cloud depth for onset of warm rain can be formulated and applied to cloud parameterization in models that do not calculate explicitly the rain forming processes in clouds.