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Improving the Scientific Foundation for Atmosphere-Land-Ocean Simulations: Report of A Workshop
observations and which forms the basis for all parameterizations of gravity wave effects in global models.
Gravity wave effects on the global scale include (1) mean-flow forcing effects, which are currently parameterized, and these are considered the most important effect below the stratopause; (2) mixing effects, which are weak in the stratosphere compared to existing model numerical diffusion but are important at higher altitudes; (3) energy dissipation and direct heating, which are only important in the upper atmosphere; and (4) cloud and heterogeneous chemistry interactions, which are important in conditions that are otherwise marginal for ice cloud formation, having impacts on ozone chemistry, stratospheric dehydration, potential radiative feedbacks, and convective cloud initiation, but these effects are not currently parameterized.
There are a variety of global-scale mean-flow forcing effects that are currently treated via gravity wave parameterization. These include (1) providing a drag force on the jets in the upper troposphere and lower stratosphere; (2) alleviation of the winter cold-pole problem common in GCMs; (3) enabling an earlier, more realistic stratospheric springtime transition to summer easterly winds; (4) providing roughly half of the total wave-driven forcing for the quasibiennial oscillation in the equatorial lower stratosphere zonal winds; (5) providing a drag force on the middle atmosphere jets, with accompanied reversal of wind direction and the radiative equilibrium temperature gradient at the mesopause; (6) forcing in the semiannual oscillation in winds near the stratopause and mesopause; and (7) modifying planetary waves and tides in the middle atmosphere with a variety of effects, including both amplification and reduction of amplitudes and vertical wavelengths of the planetary-scale waves.
Arguably the primary way that gravity waves influence tropospheric climate is via their effects on planetary wave propagation. Mechanistic model studies show that planetary wave refraction and vacillation cycles are very sensitive to winter stratosphere wave drag via the mean flow. Monthly-mean latitude-height wind distributions are generally used to assess the fitness of a particular gravity wave parameterization in model tuning, yet mechanistic model studies show that slight variations in gravity wave drag can give very similar mean-wind distributions while giving very different stratosphere warming frequencies because of this wind sensitivity. Without any wave drag, the winter polar jet can get stuck in a perpetual cold phase that excludes planetary waves entirely. The basic components of a gravity wave parameterization are (1) the source definition, (2) the momentum flux spectrum emanating from the source, and (3) the flux dissipation with height. Sources and dissipation are the nonlinear parts of the problem that parameterizations struggle to describe. We expect the dissipation to be controlled by basic instability mechanisms. There is currently some disagreement about how to describe this portion of the problem, but there is hope that small differences in these descriptions will be relatively negligible compared to the high-sensitivity parameterizations that have (and should have)