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- Author or Editor: MILES F. HARRIS x
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Abstract
The observed semidiurnal wind in the troposphere at five mid-latitude stations shows a phase lead over the wind deduced from frictionless theory, and a smaller amplitude, especially near the earth's surface, than the theoretical value. By making use of the observed fact that the tidal pressure-dependent (frictionless) wind changes much less rapidly with height than does the observed wind in the friction layer, it is possible to extend the Ekman theory to explain approximately the observed solar semidiurnal wind distribution. It is concluded that the existence of surface friction causes the time of maximum of the semidiurnal pressure wave to be advanced relative to the time that frictionless theory should predict; or, in agreement with Gold's conclusion that friction produces a phase lag of the tide with increase in elevation. The theoretical argument would appear to be valid for the lunar atmospheric tide also, but direct evidence that similar conclusions apply to the lunar tide is lacking.
Abstract
The observed semidiurnal wind in the troposphere at five mid-latitude stations shows a phase lead over the wind deduced from frictionless theory, and a smaller amplitude, especially near the earth's surface, than the theoretical value. By making use of the observed fact that the tidal pressure-dependent (frictionless) wind changes much less rapidly with height than does the observed wind in the friction layer, it is possible to extend the Ekman theory to explain approximately the observed solar semidiurnal wind distribution. It is concluded that the existence of surface friction causes the time of maximum of the semidiurnal pressure wave to be advanced relative to the time that frictionless theory should predict; or, in agreement with Gold's conclusion that friction produces a phase lag of the tide with increase in elevation. The theoretical argument would appear to be valid for the lunar atmospheric tide also, but direct evidence that similar conclusions apply to the lunar tide is lacking.
Abstract
The frictional and thermal contributions to S 2 2,2 (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.
Abstract
The frictional and thermal contributions to S 2 2,2 (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.
