How topography effects the formation of gravity waves

Gravity waves

The movement of air is affected by a number of factors, including differences in air pressure and temperature. But topography may also be a factor. Eiky Moraes, Cléo Dias-Júnior, and their colleagues now wanted to find out if the local topography at the ATTO influenced the atmospheric movements. In particular, they were interested in the effect that topography has on the formation of gravity waves.

Gravity waves (not to be confused with gravitational waves in astrophysics) are motions that form at the interface of two different layers or mediums. A well-known example of gravity waves are ripples generated by wind moving over a water surface. Something quite similar might also occur within the air itself at the interface of two different layers of air. For example, they might have different densities, because one is colder and thus denser, and one is warmer and thus less dense.

Polari Correa and his colleagues had already detected the occurrence of gravity waves at ATTO and described it in a study published last year. They already speculated that the formation of the gravity wave was linked with the topography. However, they needed additional data to prove this connection.

© Sebastian Brill / MPI-C

The effect of topography

The team around Eiky Moraes now wanted to learn more about these connections with two model simulations. They designed the first to represent the situation at ATTO with all its topography. They developed a simulation that could reproduce the data measured in the night of November 12 to November 13, 2015, the night studied by Polari Correa. The second simulation kept all parameters the same but removed the landscape topography.

Comparing the two simulations revealed some important differences in the dynamics and chemistry of the atmosphere. The simulation with topography showed a pronounced undulating pattern of parameters like air temperature and wind speed, which is characteristic of a gravity wave. This pattern is not entirely absent in the simulation without topography but is much more subdued and not nearly as well defined. This shows that the topography was an important factor in the formation of the gravity wave.

Beyond its formation, the scientists could show that the gravity wave was also relevant for the transport of gases. They found that the atmospheric movement associated with the gravity wave transported ozone and carbon monoxide both horizontally and vertically. Concentrations of both gases undulate as a result of the gravity wave moving across the ATTO site. In the simulation without topography, the occurrence of the gravity wave is much less pronounced. Hence, the movement of gases is also subdued in comparison.

Moraes et al. published the paper “Simulation of an orographic gravity wave above the Amazon rainforest and its influence on gases transport near the surface” in Atmospheric Research. It is closed access, but available freely for 50 days post publication with this link.

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