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.
Only when the air inside of the forest canopy mixes with the air above can there be exchange. The physical movement of the air, its turbulence, determine how well these two layers of air, the one inside the forest canopy and the one above, mix. Daniela Cava, Luca Mortarini, Cleo Quaresma and their colleagues set out to address some of these questions with two new studies that they conducted at ATTO. They wanted to define the different regimes of atmospheric turbulence or stability (Part 1) and describe the spatial and temporal scales of turbulent structures (Part 2).
In a new study, Marco A. Franco and his colleagues analyzed when and under what conditions aerosols grow to a size relevant for cloud formation. Such growth events are relatively rare in the Amazon rainforest and follow and pronounced diurnal and seasonal cycles. The majority take place during the daytime, and during the wet season. But the team also discovered a few remarkable exceptions.
It is long known that aerosols, directly and indirectly, affect clouds and precipitation. But very few studies have focused on the opposite: the question of how clouds modify aerosol properties. Therefore, Luiz Machado and his colleagues looked into this process at ATTO. Specifically, they studied how weather events influenced the size distribution of aerosol particles.
Biogenic volatile organic compounds remove OH from the atmosphere through chemical reactions, which affects processes such as cloud formation. In a new study, Pfannerstill et al. reveal the important contributions of previously not-considered BVOCs species and underestimated OVOCs to the total OH reactivity.
Although located in the tropics, the Amazon sporadically experiences incursions of cold waves called friagem events. They significantly impact the weather patterns during the time they occur, causing for example a temperature drop and increased cloudiness. Guilherme Camarinha-Neto and his colleagues now found that they also affect atmospheric chemistry.
The majority of global precipitation is formed through the pathway of ice nucleation, but we’re facing large knowledge gaps that include the distribution, seasonal variations and sources of ice-nucleating particles. To fill some of those knowledge gaps, Jann Schrod and his co-authors produced a record of long-term measurements of INPs. They collected data for nearly two years at four different locations. One of those sites was ATTO.
Convective storms often occur in the tropics and have the potential to disturb the lower part of the atmosphere. They might even improve the venting of trace gases out of the forest canopy into the atmosphere above. To better understand these processes, Maurício Oliveira and co-authors used the infrastructure at ATTO to study storm outflows during nighttime. They published the results in a new paper in the Open Access Journal Atmospheric Chemistry and Physics.
Christopher Pöhlker and co-authors published an extensive new paper, characterizing the footprint region of ATTO. They hope that fellow researchers in the Amazon region can use this publication as resource and reference work to embed ATTO observations into a larger context of Amazonian deforestation and land-use change. Pöhlker et al. published the paper Open Access in Atmospheric Chemistry and Physics Volume 19.
The Amazon rainforest interacts with the atmosphere by exchanging many substances. Many of these, such as carbon dioxide, methane, ozone, and organic compounds, are produced by the vegetation. They are very influential in both the regional and global climates. Until now, the estimates of their emission and absorption rates are based on classical theories. But those were developed over relatively short vegetation and are valid for the so-called “inertial sublayer.”