Vertical Tilts of Tropospheric Waves: Observations and 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.