LATITUDE AND SEASONAL DEPENDENCE OF THE SEMIDIURNAL TIDE OBSERVED BY THE HIGH-RESOLUTION DOPPLER IMAGER

The high resolution Doppler imager (HRDI) on the upper atmosphere rese arch satellite (UARS) has provided measurements of the horizontal wind field in the mesosphere and lower thermosphere (MLT) since November 1 991. The observed dynamical patterns are dominated by the (1,1) diurna l tide, but semidiurnal perturbations are also very prominent in the H RDI data particularly at latitudes greater than 40 degrees. This paper presents a preliminary study of the semidiurnal tide and discusses la titudinal and seasonal variations in the phenomenon. Regular seasonal changes are clearly detected, and significant interhemispheric asymmet ries have been identified. In general, the observed features are consi stent with previous measurements and model results, but some discrepan cies have been found. This new technique, which brings a global perspe ctive to the observation of MLT dynamics, is complementary to the very detailed but localized radar data sets currently available and should provide important constraints for numerical models.