Atmospheric Gravity Waves and their Effects on the Global Circulation

A fundamental difficulty in modeling global atmospheric circulation is the strong interaction among motions on a wide range of space and time scales. Explicit numerical representation of all the spatial scales that determine the global atmospheric circulation is impractical. An example of such difficulties is the effect of small-scale inertia-gravity waves on the circulation. Inertia-gravity waves have relatively high frequency (typically periods between a few minutes to a few hours) and usually represent only a minor component of the tropospheric circulation. The waves propagate vertically , however, and the associated wind and temperature perturbations grow rapidly with height as the mean density drops. Thus the higher one looks in the atmosphere, the more significant inertia-gravity waves become. Gravity waves act to exchange mean momentum between the surface and the atmosphere and among different layers of the atmosphere and, as such, are crucial in forcing the global-scale circulation in the stratosphere. Since the distribution of many trace constituents in ozone chemistry is strongly affected by the atmospheric circulation, an understanding of gravity-wave effects in the middle atmosphere has become a central issue for practical modeling of stratospheric ozone, and hence for comprehensive climate-chemistry modeling of the global atmosphere. It is likely that waves with horizontal wavelengths as short as about 25 km are significant for mean momentum transport. The development and application of para-meterization schemes to adequately account for the effects of gravity waves that cannot be explicitly resolved is now an important task for scientists in global climate-chemistry modeling.