Semidiurnal Tidal Motions in the Friction Layer

The obscrved semidiurnal wind in the troposphere at five niid-latmit,ude stations shows a, phase lead over the wind dcduccd from frictionlcss theory, and a smaller amplitude, espccially near the earth’s surface, than the theoretical value. By making use of the obscrved fact that the tidal pressure-dependent (frictionless) wind changes much less rapidly with height than does the observed wiiid in the friction layer, it is possible to extend the Ekman theory t o explain approximately the observcd solar semidiuriial wind distribution. It is concluded that the existcncc of surfacc friction causes the timc of maximum of the semidiurnal pressure wave t o be advanced relative to the timc tha t frictionless theory should predict; or, in agreement with Gold’s conclusion tha t friction produces a phase lag of the tidc with iiicreaue in elevation. The theoretical argument would appear t o be valid for the lunar atmospheric tide also, but dircct evidence tha t similar coiiclusioiis apply t o the lunar tide is lacking.