What role do downdrafts play for turbulence regimes in the Amazon?

Thunderstorms are common in the tropics, including the Amazon rainforest. They often produce downdrafts with extreme wind gusts that can cause widespread tree damage and mortality through defoliation, snapping and/or uprooting. But what role does the structure of the atmosphere play, and how does that change over time? In a new study, Anne Mendonça, Cléo Quaresma, Daniel Marra and their co-authors analyzed different turbulence regimes at the ZF2 site as part of the ATTO-INVENTA project. They also investigated how turbulence is connected to the occurrence of downdrafts and wind gusts.

At first, it is important to consider under what conditions the forest is coupled with the atmosphere. The exchange between the air within the forest and the air in the overlying atmosphere occurs only when turbulence provides continuous mixing. Without turbulence, the atmosphere might separate into distinct layers with hardly any exchange between to two, like oil and water that aren’t stirred to blend.

Thunderstorms in the Central Amazon bring bring strong rain and extreme wind gusts with them. © Antonio Huxley

At night, the latter is often the case in the Amazon. However, convective storms with associated downdrafts occur even during the night. This is why the scientists focused on the nighttime hours in this study. They observed two contrasting regimes: the very stable regime and the weakly stable regime. The very stable regime is characterized by little to no turbulence and thus very little mixing. The weakly stable, on the other hand, leads to a much better coupling between the forest and the atmosphere due to continuous turbulence.

A threshold in wind speed marks the transition between the regimes. However, this threshold value is not always the same. Instead, it changes with height above the forest canopy, and also with season. Specifically, the wind speed threshold is higher in the dry season compared to the wet season. It is also higher at higher altitudes. In other words: During the dry season, higher wind speeds are required to shift to the weakly stable regime.

Additionally, the structure and roughness of the forest canopy play a role. Compared to other studies, the team found that lower wind speeds were required to cross the threshold from one regime to the other. The scientists concluded that this is due to the greater turbulent mixing for a given mean wind speed. This might be due to the rough surface of the Amazon Forest.

Anne Mendonça and her colleagues found that extreme winds-gusts associated with convective downdrafts often initiate the transition in the turbulence regime from very stable to weakly stable. Strong winds are then able to propagate into the forest canopy. There, they have the potential to cause tree damage and mortality.

A small forest clearing created by extreme wind gusts that penetrated into the canopy. © INVENTA/ATTO

Finally, they combined their data with data from another study that estimated critical wind speeds necessary to topple trees with local winching experiments and modeling. Their results indicate that downdrafts in the wet season may cause severe damage. The team identified wind speeds up to 15 m/s in the wet season. In the dry season, wind speeds only reached 7.5 m/s. In the wet season, these winds propagated during downdrafts had a destructive potential approximately four times higher than on nights without downdrafts. However, winds exceeding the critical wind speed do not necessarily result in tree damage and mortality. Further studies are needed to describe the frequency of such gusts and the relationship between speed and disturbance severity.

Mendonça et al. published the study “Turbulence regimes in the nocturnal roughness sublayer: Interaction with deep convection and tree mortality in the Amazon” in Agricultural and Forest Meteorology. It is published Closed Access but available under this link for 50 days upon publication.

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