Is There a Potential for Precipitation Enhancement by Inducing Cumulus Invigoration by Seeding with Pollution-Sized Hygroscopic Aerosol?

Over the last decade or so, modeling studies suggests that anthropogenic hygroscopic pollution aerosol can lead to convective invigoration of the updrafts of cumuli. In many of those studies, the invigorated convection resulted in enhanced rainfall. However, there are some noted exceptions, particularly when the simulated convection reaches the tropopause where the convection has reached its maximum potential. There are two schools of thought regarding how high concentrations of pollution-sized aerosol can invigorate convection. One I shall call “mixed-phase convective invigoration” and the is called “condensational invigoration”.

The conceptual model for “mixed-phase invigoration” is as follows. High concentrations of particulate pollutants lead to high concentrations of cloud droplets, they suppress warm rain formation, transport greater quantities of cloud droplets to supercooled levels, which freezes thereby releasing greater amounts of latent heat, and the added buoyancy invigorates the updrafts of cumuli, and the invigorated updrafts process more water thereby enhancing rainfall. Enhanced rainfall can create colder and larger area cold-pools, which can trigger more convection and lead to greater duration of rain events. This process has been simulated in ordinary cumuli, cumuli embedded in tropical cyclones, intense supercell-like cumulonimbi, and cumuli embedded in mesoscale convective systems.

The conceptual model for “condensational invigoration” is 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. Early work on this effect suggested this process saturates at droplet concentrations of a few hundred CCN cm^-3. Recent studies, however, suggest that at heights roughly above 3km above cloud base where droplet collection can be prevalent, and scavenges the droplet concentrations, supersaturations can exceed nominal near-cloud-base values, which can lead to appreciable enhancement of condensation in a polluted cloud relative to a clean cloud. Thus latent heat by condensation of droplets can be enhanced enough to invigorate updrafts, lead to greater amounts of condensed water, produce broader, longer-lived cumuli and thereby enhance rainfall. Only a few studies have definitively isolated the relative contribution of “condensational invigoration” versus “mixed-phase invigoration” to rainfall enhance of deep supercooled clouds. In those clouds, these two processes are intertwined.

We therefore propose that seeding cumuli with anthopogenic pollution-sized hygroscopic aerosol will enhance rainfall. In a sense, this methodology is counter-intuitive. Normal hygroscopic seeding is done to enhance the efficiency of rainfall without a direct intent to alter the dynamics of clouds. The intent with normal hygroscopic seeding is to introduce aerosol larger in size (typically 0.5 micrometers in radius) and more active chemically (more hygroscopic) than background aerosols, and in concentrations of a few hundred/cc. By contrast seeding to induce convective invigoration is done with smaller-sized particles (typically 0.2 micrometers or less) and in concentrations of the order of 1000/cc. With these high concentrations, droplet collision and coalescence is suppressed. But there is the potential that invigorated convection can process more water through the cloud system leading to rainfall enhancement of much greater magnitudes than can be achieved with normal hygroscopic seeding. However, there appears to be some environmental regimes in which convective invigoration leads a reduction of rainfall. This has to be more clearly identified.

In this talk we will attempt to further clarify the relative roles of condensational vs mixed phase cumulus invigoration and examine the potential for rainfall enhancement by seeding cumuli with high concentrations of small hygroscopic aerosol.