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Radiative Forcing of Stationary Planetary Waves

Leo J. DonnerNational Center for Atmospheric Research, Boulder, CO 80307

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Hsiao-Lan KuoDepartment of Geophysical Sciences, The University of Chicago, IL 60637

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Abstract

The stationary wave components of the planetary-scale circulation are maintained by topographic forcing and by latent and sensible heat transfers and radiation. These waves have a potential vorticity balance mainly due to vertically differential thermal advection, advection of planetary vorticity, heating, and topographic convergence or divergence. To elucidate the role of solar and longwave transfers in maintaining stationary planetary waves, the potential vorticity equation appropriate to these disturbances in the middle latitudes of the Northern Hemisphere during winter is solved with realistic radiation physics included in the heating term.

A series of experiments with the model isolates the roles of the various optically active constituents in maintaining the stationary planetary waves. Clouds and the net radiative heating tend to amplify stationary planetary wavenumber 1 by increasing the forcing asymmetry, but destructive interference between radiative beating and other diabatic processes and topography causes the net radiative heating to dampen stationary planetary wavenumber 2. In the presence of clouds, water vapor and, to a lesser extent, carbon dioxide, damp the waves by reducing cloud-generated forcing asymmetries, particularly at zonal wavenumber 1.Ozone asymmetries have a minor role because they produce a heating asymmetry in the stratosphere where the zonally-averaged state limits the forcing. Radiative forcing comprises a significant source of diabatic forcing at wavenumber 1, while surface exchange processes are very important at wavenumber 2. The topographic component of the waves was smaller than the thermal component in these calculations.

Abstract

The stationary wave components of the planetary-scale circulation are maintained by topographic forcing and by latent and sensible heat transfers and radiation. These waves have a potential vorticity balance mainly due to vertically differential thermal advection, advection of planetary vorticity, heating, and topographic convergence or divergence. To elucidate the role of solar and longwave transfers in maintaining stationary planetary waves, the potential vorticity equation appropriate to these disturbances in the middle latitudes of the Northern Hemisphere during winter is solved with realistic radiation physics included in the heating term.

A series of experiments with the model isolates the roles of the various optically active constituents in maintaining the stationary planetary waves. Clouds and the net radiative heating tend to amplify stationary planetary wavenumber 1 by increasing the forcing asymmetry, but destructive interference between radiative beating and other diabatic processes and topography causes the net radiative heating to dampen stationary planetary wavenumber 2. In the presence of clouds, water vapor and, to a lesser extent, carbon dioxide, damp the waves by reducing cloud-generated forcing asymmetries, particularly at zonal wavenumber 1.Ozone asymmetries have a minor role because they produce a heating asymmetry in the stratosphere where the zonally-averaged state limits the forcing. Radiative forcing comprises a significant source of diabatic forcing at wavenumber 1, while surface exchange processes are very important at wavenumber 2. The topographic component of the waves was smaller than the thermal component in these calculations.

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