Short-term variability and temporary structures of tides and mean wind in the polar summer mesosphere

Data obtained from the mobile SOUSY VHF radar at Andoya/Norway (69 degreesN , 16 degreesE) during the MAC/SINE campaign in summer 1987 have been used t o study the short-term variability of tides and mean wind in the polar meso sphere. It is shown that the spectral peak corresponding to the semidiurnal tide dominates and persists almost uninterruptedly in time. The spectral p eak corresponding to the diurnal tide is usually secondary and interrupted in time. The terdiurnal tide rarely presents a prominent spectral peak. The frequency corresponding to each tidal component often shows a deviation fr om its definition value. This Likely represents a biased or perturbed state of the tide. The frequency fluctuation of the semidiurnal tide is usually smaller than those of the diurnal and terdiurnal tides. At some heights, th e time variation of the semidiurnal tide amplitude is similar to that of th e 36-h mean wind, which may be ascribed to the nonlinear interaction betwee n the tide and planetary waves. But for most heights, there is no clear cor relation between the tide and the mean wind. The time variations in either of the zonal and meridional mean winds for two heights with a distance of 2 .7 km show a considerable similarity, but the time variation trend in the z onal mean wind is distinct from that in the meridional mean wind, indicatin g an anisotropy in the variability of the horizontal mean wind. The time va riations of the semidiurnal tidal amplitude in the zonal wind for two heigh ts with a distance of 2.7 km exhibit a considerable difference, but they pr esent somewhat similarity in the meridional wind. This is a manifestation o f the anisotropy and localization in the tidal variability. Since the frequ ency values of each tidal component in the zonal and meridional winds are u sually different, the hodograph of its wind vector with respect to time is no longer a closed ellipse in general, but still has a clockwise rotation. The semidiurnal tidal wind vector shows a clockwise rotation with increasin g height, indicating a downward phase progression. The diurnal tide wind ve ctor does not display certain rotation with height, implying that it is not a travelling wave in the vertical. (C) 2001 Elsevier Science Ltd. All righ ts reserved.