SEMIDIURNAL TIDAL MOTIONS IN THE FRICTION LAYER

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.