An idealized modeling study to examine the potential impacts of seeding ordinary convective clouds with pollution-sized hygroscopic particles

Here we are not talking about conventional hygroscopic seeding convective clouds with relatively large hygroscopic particles, such as NaCl. Instead, we are talking about seeding with high concentrations of ultra-fine particles that can be marginally hygroscopic. The intent is to invigorate convective clouds so that deeper, longer lasting, more vigorous clouds are produced that enhance rainfall.

Two schools of thought have emerged to explain previous simulated responses of deep convective cells to high concentrations of hygroscopic pollution aerosol. The first, we call “mixed-phase invigoration” holds that high concentrations of particulate pollutants lead to high concentrations of cloud droplets, suppress warm rain formation, transport greater quantities of cloud droplets to supercooled levels, which freeze thereby releasing greater amounts of latent heat, and the added buoyancy invigorates the updrafts of cumuli which, in turn, increases rainfall. The second school of thought called “condensational invigoration” holds that high concentrations of cloud droplets formed on numerous pollution-sized aerosol exhibit greater net surface areas upon which condensation occurs, thereby enhancing net vapor deposition rates which leads to enhanced latent heat release by condensation in cumuli. At heights roughly above 3km above cloud base where droplet collection can be prevalent, droplet concentrations can be depleted resulting in supersaturations that exceed nominal near-cloud-base values. This can lead to appreciable enhancement of condensation in a polluted cloud relative to a clean cloud, thus enhancing the latent heat by condensation of droplets and invigorating updrafts, and leading to enhanced rainfall.

Here, we present results from idealized large eddy simulations(LES) of convective clouds over south Florida. We use the Ocean-Land-Atmospheric Model(OLAM) to perform the LESs. OLAM is initialized with a Miami 15Z 28 July 2002 sounding. For aerosol we use output from the GEOS-Chem global atmospheric chemistry model run with and without anthropogenic sources. OLAM is run for a period of 8 hours and we analyze simulated updraft velocities, amounts of supercooled liquid water content, and rainfall. The results clearly show that for this case updrafts are invigorated, and rainfall is enhanced.

The implication is that seeding with high concentrations of ultra-fine hygroscopic particles is an effective strategy for enhancing rainfall in ordinary cumulus, and cumulus congestus clouds. The regimes of clouds in which this strategy might be most affective has yet to be examined. But it is suggested that warm-based maritime clouds where there is a natural warm-rain process, are the most likely candidates for this procedure to be effective.