Influence of Mean Zonal Motion and Meridional Temperature Gradients on the Solar Semidiurnal Atmospheric Tide: A Revised Spectral Study with Improved Heating Rates

Abstract

Calculations of the semidiurnal atmospheric tide at solstice using improved heating rates are presented. The heating rates for solar absorption by water vapor are based on a global water vapor distribution (Jenne, 1969, 1975; Jenne et al., 1974), the data of McClatchey et al. (1972), and an absorptivity parameterization of Lacis and Hansen (1974); rates for solar absorption by ozone are based on the midlatitude ozone distribution of the U.S. Standard Atmosphere (COESA, 1976) and detailed radiative calculations using the solar fluxes and absorption cross sections of Ackerman (1971) with the Schumann-Runge band cross sections of Kockarts (1971). The heating rates for solstice are quite similar to those reported by Forbes and Garrett (1978) and significantly different from those of Lindzen and Hong (1974), which were used in the previous study of Walterscheid and Venkateswaran (1979b) investigating the influence of mean zonal motion and meridional temperature gradients on the solar semidiurnal tide. The basic conclusions of this study based on the present heating rates are as follows: 1) Although effects of mean-wind related (nonclassical) generation are clearly discernible, the improved agreement with observations of the present results over those with the earlier rates with respect to wave-lengths of the semidiurnal tidal oscillations in the lower thermosphere (100–115 km) and with respect to both their wavelengths and amplitudes in the region (80–100 km) is primarily attributable to the improved heating rates; 2) without nonclassical generation the calculations exhibit a phase shift below 30 km in disagreement with observations; and 3) without mean-wind related effects the amplitude of the surface pressure oscillation is significantly underpredicted.