Thunderstorms are rather frequent in the warm, very humid tropical atmosphere of the Amazon rainforest. When thunderstorms mature, they produce precipitation that falls to the ground and cools the surrounding air. The cool, rainy air from the thunderstorm is denser than the surrounding air so it descends to the ground, producing a downdraft. As the cool air reaches the ground, it spreads outward in all directions. The leading edge of the spreading cool air is a line of gusty winds known as a gust front.
Luciane Reis and her colleagues now performed a new study at the ATTO site that shows that gust fronts can also cause large jumps in carbon dioxide concentrations. But what are the sources of this carbon-dioxide-rich air?
The scientists analyzed high-frequency measurements of a rare southwesterly gust front event that impacted ATTO in December 2021. They wanted to assess the evolution of the lower atmospheric flow during the passage of the gust front. As expected, their data show that the gust front caused large drops in temperature and moisture above and within the forest. However, more remarkably, the data also show that the gust front was responsible for transporting large carbon dioxide concentrations toward the observation site. The team identified two potential sources of the carbon-dioxide-rich air:
- Vertical transports from upper levels (the downdraft mentioned above): The concentration of carbon dioxide in higher levels of the boundary layer is larger than in the lower atmosphere. This case study suggests that the storm can partially mix the carbon dioxide of the above layer with the cold air of the thunderstorm and transport it with the gust front.
- Biomass burning upstream from the ATTO site: Satellite imagery on the day of the gust front and the day before shows evidence of forest fires upstream from ATTO, where the gust front traveled through. In addition, aerosol data show that the amount of large aerosols in the atmosphere increases during the gust front event. This can indicate particles produced by the fires. Thus, gust fronts can redistribute carbon dioxide hundreds of kilometers across the Amazon basin. This process may be particularly prominent during the dry season when forest fires occur frequently.
© Figure from Reis et al. (2024)
This case study is especially important given the challenges of numerically predicting deep convection in the Amazon. If the two potential sources of the carbon-dioxide-rich air mentioned above are characteristics of gust fronts, new studies are necessary to better understand how gust fronts (or, in general, thunderstorms) can interact with regions with forest fires and the processes by which deep convection may redistribute carbon dioxide in the Amazon region.
Reis et al. published the study “Tall tower observations of a northward surging gust front in central Amazon and its role in the mesoscale transport of carbon dioxide” Open Access in the journal Meteorological Applications.
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