Does topography affects atmospheric turbulence?

Atmospheric turbulence

Whirling wind motions called turbulence commonly occur in the lowest 100 m of the atmosphere. Here, they play an important role in transporting energy, gases, and particles away from and toward the land surface. Accordingly, these exchanges are crucial for the interaction between the atmosphere and the biosphere. This includes processes such as:

  • transport of latent heat (energy)
  • transport of local methane emissions (gases)
  • rise of forest-produced pollen into the atmosphere, where they might function as condensation nuclei for cloud formation (particles)

Quantifying and predicting the energy available to promote this mixing is therefore critical to better understand how strong these interactions are.

The role of topography

And this is exactly what Marcelo Chamecki and his co-authors are working on. Marcelo Chamecki, Cleo Quaresma Dias‐Júnior, and several researchers of the ATTO project published two papers on the structure of atmospheric turbulence back in 2018 and 2019 (we wrote about this). They found several unexpected results. So now, they and their team followed up on those first results with more details. Specifically, they looked at how gentle topography covered by dense forests affects turbulence.

“Gentle topography” in this case means about 50-70 meters height difference between the highest and lowest points in the area. This terrain is covered by dense rainforest with a canopy height of 35 meters on average.

To approach this, they used daytime observations from two field campaigns in central Amazonia, as well as computer turbulence simulations. The field studies are the GoAmazon campaign and data from ATTO.

And indeed, the scientists found that even the gentle topography underneath the Amazon forest strongly impacts the turbulence in those lowest 100 meters. This is really valuable information. Thus far, studies in the region have interpreted their observations based on the simplified assumption of wind flow over flat topography. If future studies take this effect on turbulence into consideration, they will likely be able to improve existing estimates of energy, gas and pollen fluxes mentioned in the opening.

The shadow and the light of a low sun highlight the gentle topography under the Amazon forest, which affects atmospheric turbulence. © Paulo Brando
The shadow and the light of a low sun highlight the gentle topography under the Amazon forest, which affects atmospheric turbulence. © Paulo Brando

Chamecki et al. recently published the study “Effects of Vegetation and Topography on the Boundary Layer Structure above the Amazon Forest” Open Access in the Journal of the Atmospheric Science.

Similar articles

Air inlet tube at the top of the ATTO tall tower. © Eva Pfannerstill / MPI-C

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.

Os eventos da Friagem trazem muitas nuvens e muita chuva com eles. © Steffen Schmidt / MPI-BGC

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 storm over the forest

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.

The footprint region of ATTO shifts from pristine northeast to more human influenced southeast

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.”

Aquino et al. published a new study in Agricultural and Forest Meteorology about the characteristics of turbulence within the forest canopy at two Amazonian sites. They found that the air layer close to ground is largly decouples from the air layer in the upper canopy and above.