FRICTIONAL AND THERMAL INFLUENCES IN THE SOLAR SEMIDIURNAL TIDE

Abstract

The frictional and thermal contributions to S22,2 (p), the dominant wave type in the progressive solar semidiurnal pressure wave, are evaluated from upper air observations at nine rawinsonde stations. The theoretical basis for the investigation follows from the approximation of friction as a potential force in the tidal equations. The model parameters and boundary conditions are those adopted by Siebert. Surface friction is evaluated semi-empirically, by the use of a friction model which is essentially an adaptation, to the semidiurnal motions, of the Ekman theory of the boundary layer. The assumption of a constant coefficient of the vertical transfer of momentum leads to uncertainties in the magnitude of the frictional contribution to the wave. Further uncertainties arise from a systematic error in the observed temperatures, caused by radiation effects on the radiosonde instrument. The latter error, however, is believed to be negligible in the lower troposphere, where an unexpectedly large temperature variation is apparently caused by eddy transfer of heat from the earth's surface. The results of the study must be considered in the light of the probable errors arising from data sampling, from the diurnal bias in the radiosonde observations, and from the restrictive assumptions of the theory. Considered in this light, the results suggest that the semidiurnal oscillation may be explained by three processes, of approximately equal importance: (1) eddy transfer of heat from the earth's surface; (2) direct absorption of solar radiation by water vapor and ozone, as computed by Siebert; and (3) surface friction, or eddy transfer of momentum. Surface friction apparently delays the surface pressure oscillation by about one hour.