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Wave-induced transport of chemically active species in the mesosphere and lower thermosphere

Prof. John Plane (SoC), Prof Daniel Marsh (SoC/SPA), Dr Wuhu Feng (SEE)

Project partner(s): NCAR, Boulder, CO, USA (CASE)

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Tides, planetary waves and gravity waves play major roles in establishing the thermal structure and general circulation of the mesosphere/lower thermosphere (MLT) region of the atmosphere (70 - 120 km). For example, the summer mesopause region is the coldest place in the atmosphere due to the meridional circulation induced by gravity wave dissipation. Less well known and understood are the equally important roles that waves play in vertical constituent transport, which is a fundamental atmospheric process that has profound effects on the chemistry and composition of the atmosphere below the turbopause at around 105 km.

Atmospheric gravity waves are generated by a variety of mechanisms (e.g. orographic forcing, convection, wind shears) in the troposphere and stratosphere. As the waves propagate upwards their amplitudes grow because of the exponentially falling air pressure, causing a fraction of the waves to become superadiabatic and “break”, which is the main source of turbulence in the MLT. A final fraction of the wave spectrum can survive and penetrate into the thermosphere.

Waves, and the turbulence they generate, contribute to vertical constituent transport by inducing large-scale advection (upward in summer, downward in winter), eddy transport through turbulent mixing, dynamical transport associated with dissipating, non-breaking waves and chemical transport associated with perturbed chemistry. Recently compelling evidence has emerged that dynamical and chemical transport is very significantly underestimated in global chemistry-climate models. Vertical fluxes of Na atoms (produced from ablating meteors) have been measured by the ground-based lidar technique and are 5 to 10 times larger than in a state-of-the-art climate model. There is also a significant deficit in the modelled concentrations of O atoms and O3 in the MLT. The most likely reason for this is that a fraction of the gravity wave spectrum is not explicitly captured in models because the wavelengths are smaller than the model horizontal grid-scale (typically > 100 km), and these small waves make a major contribution to vertical transport. The computational cost of increasing the horizontal resolution to include small-scale wave transport effects directly in global models – particularly for chemistry - is prohibitive.

The aim of this project is to produce a parameterization which can be used to calculate all components of vertical transport in a global model. The project will proceed in several stages. First, a global model (produced by the National Center for Atmospheric Research in Boulder, Colorado - who are a remote partner in the project) will be run with the facility to increase the horizontal resolution over a regional area, in order to demonstrate the importance of short wavelength waves. In the second step, we will parameterise a recent mathematical treatment of dynamical and chemical transport, which shows that these transport terms can be computed in a relatively straightforward way from the wave spectrum in each model grid box. For the third stage we will assemble a data-base of measurements of the Na vertical flux and satellite measurements of Na and other MLT constituents (O, O3 etc.). In the final stage, the new global model with wave transport will be run for 20 years (covering the period of these observations), to study the impact of wave transport on the global distribution and seasonal variations of the important, chemically active species. Once the vertical flux of Na atoms can be reconciled with the abundance of Na in the layer around 90 km, an accurate estimate of the amount of interplanetary dust entering the atmosphere can be obtained, thus constraining astronomical models of dust evolution in the solar system and improving our understanding the impacts of this dust throughout the atmosphere.

The project will involve partners at the US National Center for Atmospheric Research, the University of Illinois at Urbana-Champaign, the University of Colorado at Boulder, and the UK National Centre for Atmospheric Science.

Click here for a full project description.

Related undergraduate subjects:

  • Applied mathematics
  • Astronomy
  • Atmospheric science
  • Chemistry
  • Computing
  • Earth system science
  • Geoscience