Vertical Tilts of Tropospheric Waves: Observations and Theory

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  • 1 Department of Geology and Geophysics, Yale University, New Haven, Connecticut
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Abstract

The vertical tilts of planetary waves as functions of zonal wavenumber and frequency were examined by two methods. First, the vertical tilts were computed by a cross-spectral analysis of the geopotential heights at different pressures. This generally used technique was not as sensitive as a cross-spectral analysis of height and temperature at a single level. The two methods yield similar vertical tilts; however, the latter method had a smaller error that allowed us to find statistically significant tilts in the troposphere that the former method did not find.

In the midlatitude troposphere, the eastward-moving waves had a westward tilt with height, as expected. However, the westward-moving waves with frequencies higher than 0.2 day−1 showed statistically significant eastward vertical tilts. For a free Rossby wave, this implies that the Eliassen-Palm flux is downward along with its energy propagation. A downward energy propagation suggests an upper-level source of these waves.

It was proposed that the eastward-tilting waves were forced by the nonlinear interaction of stationary waves and baroclinically unstable cyclone-scale waves. The predicted vertical tilt and phase speed were consistent with the observations. In addition, simulations of a general circulation model were analyzed. In the control run, eastward-tilting waves disappeared when the sources of stationary waves were removed. This is consistent with our theory.

Abstract

The vertical tilts of planetary waves as functions of zonal wavenumber and frequency were examined by two methods. First, the vertical tilts were computed by a cross-spectral analysis of the geopotential heights at different pressures. This generally used technique was not as sensitive as a cross-spectral analysis of height and temperature at a single level. The two methods yield similar vertical tilts; however, the latter method had a smaller error that allowed us to find statistically significant tilts in the troposphere that the former method did not find.

In the midlatitude troposphere, the eastward-moving waves had a westward tilt with height, as expected. However, the westward-moving waves with frequencies higher than 0.2 day−1 showed statistically significant eastward vertical tilts. For a free Rossby wave, this implies that the Eliassen-Palm flux is downward along with its energy propagation. A downward energy propagation suggests an upper-level source of these waves.

It was proposed that the eastward-tilting waves were forced by the nonlinear interaction of stationary waves and baroclinically unstable cyclone-scale waves. The predicted vertical tilt and phase speed were consistent with the observations. In addition, simulations of a general circulation model were analyzed. In the control run, eastward-tilting waves disappeared when the sources of stationary waves were removed. This is consistent with our theory.

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