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METEOROLOGY

The hidden seeds of rain

Researchers find the missing particles that explain how clouds form in the Amazon

Rain cloud in a clean-atmosphere region over the forest: aerosols concentrated near the top

FABIO COLOMBINIRain cloud in a clean-atmosphere region over the forest: aerosols concentrated near the topFABIO COLOMBINI

The answer is now becoming clearer for a question that has intrigued scientists studying climate and precipitation patterns in the Amazon for two decades: the place of origin of the microscopic particles that contribute to the formation of rain clouds in the world’s largest tropical forest. Years ago, a piece of the answer came to light. The seeds of clouds in the Amazon are suspended particles, or aerosols, of organic origin—formed in particular through chemical conversion of the isoprene gas emitted by plants—around which water vapor condenses and cloud droplets form (see Pesquisa FAPESP issue nº 97). But measurements taken by aircraft from four kilometers (km) above the ground have detected only a fraction of the aerosols released by the forest.

Now the rest of the answer has been found. The aerosol particles are present in the clouds, but not at their bases, as scientists had thought. In August and September 2014, a German jet aircraft measured the chemical composition and physical characteristics of clouds in 14 flights over the Amazon. The measurements are being used in a joint campaign for two projects: Green Ocean Amazon (GoAmazon) and Acridicon-Chuva, funded by FAPESP, the Amazonas Research Foundation, the Max Planck Institute and the U.S. government (see Pesquisa FAPESP issue nº 217).

The jet, able to fly at an altitude of 15 km, confirmed that most of the new aerosol particles are found above 8 km—concentrated at the tops of the clouds, which in the Amazon can reach 16 km high—in regions where there is preserved forest. “We have looked for the mechanisms that form these particles for 20 years,” says physicist Paulo Artaxo of the University of São Paulo (USP). “Now we’ve seen that most of them are formed in clouds and brought to the forest surface by descending air currents,” explains the researcher, who heads a project linked to GoAmazon.

The mechanisms by which these aerosols are generated in clouds are still being studied, and the extent to which they explain the rains in the Amazon is not yet known. Scientists conjecture that the organic gases emitted by the forest enter into deep clouds and, as they get pushed upwards by ascending air currents, they freeze at -20 or -30 degrees Celsius and form the aerosols. “This strong interaction of the clouds with forest emissions feeds back into the most intense water cycle on the planet,” Artaxo says. “In Rondônia, a state that has cut down 60% of its forests, the composition and properties of the aerosols change and rain is produced through other mechanisms.”

On some of the flights, the German aircraft was accompanied by an American plane that collected data at lower altitudes, having already flown over the forest months earlier during the rainy season. Aggregated with the data from radar, satellites and radiosondes, this information revealed two cloud patterns in the Amazon. Over the areas of forest that had undergone little change and were nearly pollution-free, clouds have fewer aerosols. “Their droplets are concentrated at the base of the clouds, are larger and grow more rapidly, so they generate abundant rainfall,” says meteorologist Luiz Augusto Machado, a researcher at the National Institute for Space Research (INPE) and the coordinator of Projeto Chuva [the Rain Project], who investigated the types and distribution of rain clouds in Brazil.

056-058_Chuvas na Amazônia_240-BThis was the profile found in the area of Boa Vista, in the state of Roraima. The clean-atmosphere region at the base of clouds contained nearly 200 droplets per cubic centimeter (cm3), each measuring 100 to 1,000 micrometers in diameter. In the area of forest that receives pollutants from Manaus or from fires, the clouds have more aerosols. With more nuclei around which to condense, the water is distributed in more droplets (400 per cm3) of smaller size (60 micrometers). The droplets take longer to grow in volume, and they can evaporate instead of raining. These clouds are higher, have more ice crystals and generate frequent lightning.

Meteorologist Rachel Albrecht of the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) at USP also observed that in polluted regions, storm clouds generate unusual positive lightning, in which the discharge is more intense and occurs only once. These lightning strikes are more frequent at the end of the dry season, when there are more fires.

“Knowing these mechanisms is essential for feeding into models with high spatial resolution and better capability to reproduce local rainfall and prevent disasters,” says Machado, who will discuss the data from Acridicon-Chuva this month at a workshop in Ilhabela, on the coast of São Paulo State.

“What we are observing in the region affected by the Manaus pollution plume could signal the future of an urbanized tropical forest,” Artaxo says. “And it is representative of the present-day mechanism of rain formation in Africa or Indonesia, where there are many cities surrounded by forest, or in a more urban Amazon.” Maria Assunção da Silva Dias, a meteorologist at IAG and researcher with GoAmazon, cautions that what is happening in the Amazon could have global impact. “Changes in the clouds in Amazonia,” she says, “affect climate throughout the planet.”

Projects
1. GoAmazon: interactions of the urban plume of Manaus with biogenic forest emissions in Amazonia (nº 2013/05014-0); Grant Mechanism Research Program on Global Climate Change; Principal Investigators Paulo Artaxo Netto (IF-USP) and Maria Assunção da Silva Dias (IAG-USP); Investment R$ 3,246,351.45.
2. Cloud processes of the main precipitation systems in Brazil: a contribution to cloud-resolving modeling and to the GPM (Global Precipitation Measurement) (nº 2009/15235-8); Grant Mechanism Thematic Project; Principal Investigator Luiz Augusto Toledo Machado (INPE); Investment R$ 2,362,708.53.

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