Gravity Waves

Atmospheric gravity waves are generated by atmospheric disturbances such as storm fronts, strong wind shears, and flow over mountains and play a key role in coupling the lower and upper atmosphere, causing a redistribution of momentum and energy from the troposphere and lower stratosphere into the upper atmospheric regions of the middle stratosphere, mesosphere, and lower thermosphere. Gravity waves trigger convection and induce mixing and transport of atmospheric chemicals such as ozone. Gravity waves typically form within or near the back edge of a precipitation shield. The strongest upward motions of gravity waves occur just following the surface pressure trough and lead to maximum precipitation rates just ahead of the ridge.

Atmospheric Waves

Atmospheric gravity waves can occur at all altitudes in the atmosphere and are important for the transport of energy and momentum from one region of the atmosphere to another, to initiate and modulate convection and subsequent hydrological processes, and to inject energy and momentum into the flow. When the gravity wave breaks, the resulting turbulence mixes atmospheric chemicals. These wave-breaking processes occur globally and affect climate of the mesosphere and stratosphere.

These mesoscale–regional scale processes have global significance because of their accumulative effects from the global distribution of various wave sources. The primary challenges to observational,

numerical, and analytical studies are how to better quantify gravity wave excitation as it is related to various tropospheric processes, the global distribution of the wave sources, their propagation and breaking, and the multiscale interactions involving gravity waves.

The difficulty in producing the observed Arctic climate change in models may be a result of not including gravity waves in the models. The most likely energy source mechanisms are latent heat release in deep convection and shear instability, in which waves can extract energy from the jet stream when vertical wind shear is sufficiently strong to reduce the Richardson number below 0.25. Alternatively, wave energy loss can be prevented by an efficient wave duct, which appears to be the most prevalent of the three mechanisms described.

Gravity waves are maintained by wave-ducting processes requiring a layer of static stability (the duct depth near the surface), no critical levels (wind moving in the direction of the wave at the same speed) in the lower stable layer that would absorb the wave’s energy, and a reflecting layer above the stable layer to keep the wave from losing its energy.

Gravity waves can affect an existing cloud pattern in several ways as they propagate: Through modulating the cloud pattern, with the development of wave cloud formations, the wave and cloud can propagate in tandem with little effect on the overall cloud pattern. Convection can generate a broad spectrum of waves, ranging from short-period waves excited by the development of convective cells along a thunderstorm gust front to large wavelength disturbances from the release of latent heat in a thunderstorm complex.

The challenge of including gravity waves in global climate models stems from the resolution ability of computers; with increasing computer power, more complex equations over smaller distances can be resolved to examine gravity waves. Current models often use one of the available gravity wave drag parameters and assume a fixed gravity wave source for proper representation of turbulence on the small scale.

The vast spatial and temporal extent of gravity waves has important implications for the atmosphere, from the mesoscale to the global scale, and poses a stiff challenge to improving weather and climate predictions at all ranges. An important part of the National Center for Atmospheric Research mission is to understand the coupling of the lower and upper atmosphere through dynamical, chemical, and radiative processes. 

Sudden stratospheric warming involves dynamical changes on vastly different scales from the troposphere to the lower thermosphere, and thus provides an opportunity to understand the coupling process. Further wave source sensitivity studies and observations will help to define gravity wave sources and behavior.