Effects of smoke produced by forest fires on warm rain processes in clouds over Mexico and Indonesia

The effects of aerosol particles produced by forest fires on the evolution of warm-rain processes in clouds over Mexico and Indonesia were studied. Data obtained from two field projects were used along with a one-dimensional numerical model that calculates simultaneous growth by condensation and coalescence.

The first project took place in Coahuila, Mexico, and consisted of a three-year scientific program aimed to evaluate the viability of increasing rainfall through hygroscopic seeding. The second field experiment was an intensive, short-term program of cloud physics measurements conducted in potential rainclouds over Indonesia. Both data sets were analyzed in the context of the drought and forest fires that afflicted the regions in 1997 and 1998.

In the Coahuila region, measured cloud droplet spectra were found to be typical of continental conditions, i.e., high concentrations over a narrow range of sizes, with average values around 350 cm^-3. Cloud droplet concentrations in Indonesia ranged from more than 1000 cm^-3 in continental areas, to less than 200 cm^-3 in maritime clouds affected by the smoke plumes. The results show that droplet concentrations in clouds forming in air contaminated by smoke are greatly increased, and that coalescence would be slower. It might be anticipated that the production of rain by coalescence processes would be much retarded and that less rain would result.