Clusters of storms have been decreasing in the Amazon – 05/16/2023 – Environment

The formation of thunderstorms depends on the moisture and energy available in the atmosphere. But there are conditions that cause these heavy rains to clump together, forming the so-called mesoscale convective systems (MCSs) — roughly speaking, large storms that feed back, extending for kilometers and can last for hours.

According to a new article published in the journal Climate Dynamics, they are responsible for 40% of the precipitation in the Amazon and have been impacted by climate change: their occurrence has been reduced. This is the first study linking the occurrence of mesoscale convective systems with climate change in the Amazon, according to the authors.

“We had indications that precipitation in the region was being affected during the months of September, October and November, with the rainy season being reduced and the dry season increasing. So, we asked ourselves if mesoscale convective systems could be related to this phenomenon. In the Amazon, there was no study on convective systems and climate change,” says Amanda Rehbein, a postdoctoral fellow at the Department of Atmospheric Sciences at the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP) and first author of the work.

According to the researcher, comparing the past period (1950 to 1960) to the present, there was a reduction of almost 3% in the occurrence of MCSs. “Of course, when we separate by seasons, we get a more punctual view of what happens in each period of the year. But, in an overview of the past for the near future, the SCMs tend to decrease. On the other hand, their intensity, in the sense of precipitation, it is increasing. And this increased precipitation is also projected into the future, between 2040 and 2050, which was the period we modeled”, she reveals.

For Tércio Ambrizzi, professor at IAG-USP and co-author of the article, understanding these systems and how they will behave in the future will give an idea of ​​the variability of rainfall in the Amazon and may also indicate, seasonally, how it will be impacted. “We found that precipitation is most affected between September and December and between June and August, but less so from March to May.”

The work was funded by FAPESP through four projects (16/10557-0, 17/09659-6, 14/50848-9 and 18/17134-3).

Data

The duo used observational data from remote sensing, mainly from satellites, and also data from measurement stations used by GOAmazon (a program integrated into the Large Scale Experiment in the Biosphere-Atmosphere in the Amazon [LBA] and supported by FAPESP), in addition to climate models. GOAmazon, focused on the central region of the Amazon, resulted in the first cloud census in Brazil (read more here).

“To study these systems we need data with high temporal and spatial resolution, and these satellite data only exist from the 2000s onwards for the Amazon. So, to study the past and future climate, we had to use models. But these models we that take into account the input of climate change generally have a very low resolution, are very generic, simulate more general circulations and, with them, we are unable to represent these storms”, explains Rehbein.

According to the scientist, in the early 2000s, a group of Japanese scientists developed a model called Nicam (Nonhydrostatic ICosahedral Atmospheric Model), which takes into account these general circulations of the atmosphere, but represented with a higher resolution.

“We were using another tool when we came across this model and we thought it would be more useful to us. I ended up doing an internship at the University of Tokyo, Japan, with one of the people who developed this model. I learned how to run and use Nicam and , so we did some simulations for the study”, he reveals.

Rehbein explains that the models are divided into squares. “If the points are too far away, the cloud in the middle cannot be seen and then the model has to ‘guess’. As the resolution increases, these points are approximated and the chance of detecting the clouds is greater. resolution, the greater the chances of detecting convective systems.”

According to her, the final simulations used in the study, which take 30 years of data, were generated by the Japanese. “They require very large computational processing power and storage.”

Ambrizzi explains that the model used performs simulations by interpolating data: it uses satellite and meteorological station data, when available. “Using this information, the analysis of the SCMs in the past and in the present is carried out. We made a comparison of the past with the present and its extrapolation to a warmer climate in the future.”

He notes, however, that the deforestation factor was not taken into account in the study. “Deforestation, which alters the thermodynamic structure of the forest, can lead not only to a decrease in SCMs, but to a reduction in rainfall — which is what the projections, in general, indicate. The forest, as it is, has conditions even to generate more humidity and, this humidity, to generate clouds and precipitation. That is why, in some months, we find a tendency of increase of MCSs. Deforestation changes this balance and, probably, if we consider it, we will have less rains, as indicate some models that work with deforestation.”

According to Ambrizzi, it is now possible not only to better understand these systems, but also to look into the future, in the scenario of global warming, and try to project how this influences the SCMs. “Amanda has been studying and describing these systems: how long they live, when they are born and when they die, the percentage of precipitation they generate in the Amazon, overall. This is all new, few studies had been done. GOAmazon’s resolution is good and she used that data. After that, she scaled up to the whole basin and used all kinds of data available.”

downward trend

What the scientists discovered differs from a few studies that exist on the subject in other places, such as the central region of the United States and the Sahel (a strip of territory in Africa located between the Atlantic Ocean and the Red Sea), in which it is observed an increase in the occurrence of systems and also in their intensity.

“In the United States, there are long-term data that point to this trend of increasing occurrence and intensity. So, the first important result actually leads to another question: why in the Amazon is this trend different from other places on the globe? A second insight along these lines is that our models only consider climate change, that is, the increase in temperature; we are not considering other variables. What if we did? And a third contribution is the discovery of the great potential of Nicam to study storms in the Amazon “, underlines Rehbein.

Basically, what the scientists concluded is that the SCMs already have this variability between seasons (in winter they tend to produce more precipitation than in summer) and, in the future, the tendency to produce more precipitation is reaffirmed, regardless of the station.

They explain that mesoscale convective systems form when there are special conditions in the atmosphere that cause storms to clump together and feed off each other. “It starts to rain, a cold pool is formed in the lower atmosphere, cold air that helps to lift more warm air and feeds back the storm, which grows, grows, takes the dimension of several kilometers and lasts for many hours. And these clusters form in the Amazon as well. Now we know better how they work. We know that the increase in temperature causes more humidity in the atmosphere and this contributes to these phenomena being more intense. We are investigating several hypotheses: for example, why in June, July and in August an increase in SCMs was observed, when in all other periods they are decreasing? It could be some impact on the dynamics of the atmosphere, a change in the general dynamics of the atmosphere, but these are still investigations and hypotheses”, adds Ambrizzi.

The article “Mesoscale convective systems over the Amazon basin in a changing climate under global warming” can be accessed here.