NSFGEO-NERC: Wave-Induced Transport of Chemically Active Species in the Mesosphere and Lower Thermosphere (WAVECHASM)

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". Wave-breaking 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, 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 significantly underestimated in global chemistry-climate models. The vertical fluxes of Na and Fe atoms, produced from ablating meteors, have recently been measured by the ground-based lidar technique and are 5 to 10 times larger than in a state-of-the-art climate model. The higher fluxes are supported by astronomical models of dust evolution in the solar system. There is also a significant deficit in the modelled concentrations of O atoms and O3 in the MLT. The most likely reason for these apparent model deficiencies 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 - especially incorporating chemistry - is currently prohibitive.

The aim of the WAVECHASM 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 four stages. First, we will run a global model with the facility to increase the horizontal resolution regionally down to ~ 14 km, 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 vertical fluxes of Na, Fe (in some cases) and heat at 6 lidar stations, the Na density at 16 stations, and satellite measurements of Na and other MLT constituents (e.g. O, O3, NOx, CO2). 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, we will obtain an accurate estimate of the amount of interplanetary dust entering the atmosphere, and thus constrain astronomical models of dust evolution in the solar system and improve our understanding the impacts of this dust throughout the atmosphere.

The proposed research will be of societal and economic benefit in a number of ways:

1. Educational benefits to the wider public on the question of anthropogenic versus natural climate change. The mesosphere/lower thermosphere (MLT) is a particularly sensitive region, both to anthropogenic influences (increased greenhouse gases, stratospheric ozone depletion) and solar influences (strength and frequency of the solar cycle and longer-term changes in solar activity). It is one region of the atmosphere where there is an unambiguous climate change signal which is easy to observe - noctilucent clouds. Auroral emissions in the lower thermosphere are the most visual manifestation of space weather. The WAVECHASM project also involves understanding the cometary sources of interplanetary dust and the impacts of meteors in the atmosphere, and there is a great deal of public interest in meteor showers and cometary missions (e.g. Rosetta). Linking these different phenomena together provides a natural forum for public lectures and debate on the relative significance of solar versus anthropogenic influence on climate.

2. Satellite operators are already able to fly satellites for longer periods in lower orbits as a result of the reduced drag caused by thermal contraction in the middle atmosphere. A validated chemistry-climate model of the MLT can be used to predict future trends. The satellite re-entry region is 90-140 km, and a precise knowledge of the local characteristics of the MLT is needed to determine the position and required change in velocity to de-orbit a satellite. Similarly, satellite launch operators need to know the small-scale, local fluctuations in density above 80 km to calculate accurately the aerodynamic forces acting on the launcher.

3. Several aerospace companies in Europe and North America (e.g. EADS Astrium, Northrop Grumman, Virgin Galactic and XCOR) are currently building sub-orbital "space planes" which are designed to fly through the MLT to around 110 km. The environmental impact of the huge quantities of water vapour, carbon soot particles and metals which these re-usable vehicles will inject into the mesosphere will need to be assessed through a properly validated whole atmosphere chemistry-climate model which, crucially, correctly describes the vertical transport of constituents.

4. NASA is planning to mount a Na lidar on the International Space Station in the next 4 years. The instrument will be used to study gravity waves and chemistry in the MLT. The new WACCM-SE WT model that will be produced in WAVECHASM will be a useful tool for interpreting these very challenging space-based observations.

5. The metallic ions produced by meteoric ablation are the major constituents of sporadic E layers. These layers have a significant effect on ground-to-satellite and over-the-horizon radio transmissions. Understanding the processes which control the distribution of metallic ions in the lower thermosphere, and hence predicting sporadic E occurrence, is important to many industrial and governmental organizations.

6. Numerical weather forecasting organizations (e.g. the UK Met Office) are extending their operational forecast and climate models into the thermosphere. One reason is that a well-constrained mesosphere is now considered to be an important element of climate modelling due to the impact of middle atmospheric chemistry. There is also increasing evidence for an improved accuracy of weather forecasting, particularly if mesospheric data from satellites such as Aura-MLS and SABER is assimilated. Another reason for developing high-top models is for space weather prediction, since there is a clear impact in the thermosphere of upward-propagating waves from the lower atmosphere. The WAVECHASM project will provide calibration/validation data both for satellite remote sensing and high-top model development.