When you picture a clear, starry night in the tropics, you will likely think of calm air, disturbed by nothing than perhaps a light breeze. And while that may not be entirely wrong, it is only part of the story. The atmosphere is never still. There is always movement and what you perceive as a gentle breeze may in fact be a quite turbulent flow of air.
In a new study, Polari Corrêa, Cléo Quaresma, Luca Mortarini and their co-authors analyzed the atmospheric dynamics in and above the forest canopy during one particular night at ATTO as a case study. Getting a better understanding of these dynamics is important because it helps to understand how gases, particles and energy are transported and exchanged between the forest and the overlying atmosphere. For example, a highly stratified atmosphere will prohibit exchange processes, while turbulent structures on the other hand will facilitate such exchange. Therefore, understanding atmospheric dynamics also means better understanding the exchange between biosphere and atmosphere.
A night in the Amazon
The night the team studied was one in the transition from wet season to dry season in November 2015. The team collected data in a 12-hour window between 7 pm, just after sunset, and 7 am, just after sunrise.
The first part of the night until 11 pm was windy with intense atmospheric turbulence above the forest. It was generated by the rough surface of the uneven canopy. Inside the canopy, however, the trees acted to drastically reduce the wind speeds, making conditions much calmer on the ground.
Towards midnight, the wind direction oscillated for a while, while the wind speed decreased until the air became almost still at all heights. This second part of the night is characterized by a so-called gravity wave. When the atmosphere is stably stratified and the wind blows orthogonally to a hill ridge, the interaction between the flow and the orography generates vertical oscillations that propagates in the atmosphere (orographic gravity waves).
But it didn’t stay calm for long. After 1 am, the winds picked up again in a shallow layer above the canopy with low-wind speeds in the upper layers. This marks the inception of a low level jet, a fast-moving ribbon of air in the low levels of the atmosphere. It came from the southwest, the direction of the Uatumã River. Higher up in the atmosphere the winds came from other directions, at 150m from the South-East, and at 325m from the East. But throughout the night the winds all shifted to align with the lower-lost layer. The low level jet had a great impact on the scalar dispersion above and inside the forest.
The scientists hypothesize that the Uatumã River and the hilly terrain are the reason for the jet formation. The gravity waves earlier in the night might have also trigged it.
3D simulation of the topography of the area around ATTO, with the Uatuma river and the plateau on which the station sits. © Luca Mortarini
The observations of Polari Corrêa, Cléo Quaresma, Luca Mortarini and their co-authors during this one night highlight the complex dynamics and mechanisms in the atmosphere above a dense forest. They also show that we need more and longer datasets to understand these processes even better, as these dynamics affect the transport of gases and particles within and above the forest.
They published the study “A case study of a gravity wave induced by Amazon forest orography and low level jet generation” in the journal Agricultural and Forest Meteorology.
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