University of São Paulo: Scientists Reveal How Fires Affect Rain Cloud Formation in the Amazon

Research with the participation of USP’s Physics Institute (IF) reveals how fires interfere with the development of rain clouds in the Amazon. The researchers used satellite images and measurements of the amount of particles formed by the fires and found that they make the atmosphere more stable and make it difficult for the air masses to move vertically. This prevents clouds from gaining height and limits the cooling that causes drops of water to freeze, possibly reducing rainfall and increasing the incidence of sunlight on the ground. The results of the work are described in an article published in the journal Communications Earth & Environment , by the Nature group .

The researchers studied the temperature at which convective cloud droplets in the Amazon freeze, trying to understand which elements are most important that control this phenomenon. “In this case, the freezing temperature is not zero degrees Celsius (°C). Depending on different conditions, this temperature can vary from a few degrees below 0 °C to a minimum of -38 °C which, in nature, is the freezing limit for water”, reports Professor Alexandre Correia to Jornal da USP , of the IF, one of those responsible for the research. “Thus, the objective of the work was precisely to study the main factors that control the temperature at which the cloud drops freeze.”

Convective clouds form when water vapor is transported vertically and condenses on aerosols, which are microscopic particles always present in the atmosphere. “Convective clouds develop a lot vertically, reaching high altitudes, with tops above 10 kilometers”, describes the professor. “During the path between the base and the top of the clouds, the water droplets cool and freeze at a certain temperature, since, in general, the higher the altitude in the atmosphere, the lower the temperature, until they reach the stratosphere.”

The researchers evaluated 15 years of satellite imagery, between 2000 and 2014, to obtain the temperature of each pixel identified as a cloud over the Amazon region. “We also use data from NASA [ National Aeronautics and Space Administration ] on the amount of aerosol present in the atmosphere. These aerosol measurements were carried out in soil using solar photometers in five locations: Alta Floresta and Cuiabá, in Mato Grosso; Rio Branco, in Acre; and Ji-Paraná and Ouro Preto Do Oeste, called in the work ‘Abracos Hill’, in Rondônia”, describes the professor. The database also included atmospheric data from the European Center for Medium-Range Weather Forecasts (ECMWF), which allowed the assessment of the state of the atmosphere in each situation analyzed.

The survey results show that the average temperature of glaciation or cloud freezing in the Amazon depends on three main factors: atmospheric humidification, aerosol particles and solar radiation. “These factors act in association, what is called the ‘coupled’ mode”, highlights Correia. “Outside the burning season in the Amazon, the combined action of humidifying the atmosphere and the presence of aerosols causes the glaciation temperature to decrease the more aerosol and humidity are present.”

For example, the average glaciation temperature can vary between -10 °C and -18 °C as the amount of aerosol increases, but still under clean conditions, with little aerosol, explains the professor. “This is because in an unpolluted natural environment, the more aerosol, in general, the smaller the cloud droplets that form, and the less efficient the freezing process.”

In conditions of intense pollution due to burning in the Amazon, the effect on freezing depends on the humidity in the atmosphere, notes Correia. “For a humid atmosphere, the average glaciation temperature decreases the more aerosol is present, and it can approach the limit of -38 °C. If the atmosphere is relatively dry, it must first be considered that the formation of clouds is difficult”, he says. “This is partly due to the shading that the aerosol causes on the surface, which counteracts the convective movement of air masses. The clouds that manage to form do not develop much vertically, and the glaciation temperature remains at levels such as -15 °C to -16 °C on average, a result that has not yet been described in the scientific literature.”

According to Correia, there is a period of the year in the Amazon when the atmosphere is extremely clean, in which the amount of aerosols is minimal, and these particles are of natural origin. But there is also a period every year when the atmosphere is highly polluted due to fires, which occur between August and October. “The smoke from fires contains a huge amount of aerosol particles that, also being present in the atmosphere, can influence how cloud droplets are formed and the subsequent freezing process”, he points out.

The presence of ice in clouds influences their development, such as their average “lifespan”, that is, the average time interval in which they are present in the atmosphere. “The longer the average duration of clouds, the more solar radiation is reflected back to space, contributing to the planet’s cooling”, observes the professor. Ice in clouds also affects precipitation formation, a crucial element of the climate system. Physical properties of clouds and their role in climate are current research topics.

The researcher emphasizes that by bringing knowledge about cloud physics and its development dynamics, the results of the work can be used in the future in models and other studies that seek to improve the quantification of the climate impact of clouds. “By better understanding the dimension of these effects, the need to preserve natural environments, particularly in the Amazon, becomes clearly urgent”, he emphasizes. The research included the participation of professors Maria Assunção Silva Dias, from the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) at USP, Elisa Sena, from the Federal University of São Paulo (Unifesp), as well as Ilan Koren, from the Weizmann Institute of Science (Israel).