Tidal structure and variability in the mesosphere and lower thermosphere over Yamagawa and Wakkanai

Observations of winds in the mesosphere and lower thermosphere (MLT) measured at Yamagawa (31.2degreesN, 130.6degreesE) and Wakkanai (45.4degreesN, 141.7degreesE) are used to study the structure/variability of solar semidiurnal and diurnal tides. MF radar observations over these sites were recently started by the Communications Research Laboratory, Tokyo, Japan. A database of 3-4 years has been used in the present analysis. The tidal features are discussed, and these are compared with the results from other similar mid-latitude sites. The semidiurnal and diurnal tides are also compared with a recent tidal model. The analysis on semidiurnal tides indicates that the variations over these sites seem to have reasonable similarities. Generally, at both locations the semidiurnal tidal amplitudes have the values in the range 5-15 m/s. Although seasonal variations in the semidiurnal tide amplitude are not clearly seen, the multi-year average indicates that the maximum amplitude is observed in August/September at both sites. The summer season is characterized by a large vertical wavelength, while the winter season has a comparatively smaller vertical wavelength. Comparison of the observed semidiurnal tide amplitude and phase parameters, and the Global Scale Wave Model (GSWM-98) outputs is generally not satisfactory and confirms the need for further effort in the development of a more realistic model. Diurnal tide amplitude at Yamagawa and Wakkanai shows significant differences in their strengths. The amplitude is consistently larger in Yamagawa. Phase structure shows a better agreement between the sites. Again, comparison of the observed tidal parameters and the GSWM-98 values is generally not good. However, the overall assessment is that the comparison is better in the case of diurnal tides than that of semidiurnal tides. The best comparison result is observed for the diurnal tidal phase. The interannual variability observed in both the semidiurnal and diurnal tides is larger in the Yamagawa data compared to the Wakkanai data. The semidiurnal and diurnal tidal phases around 90 km generally exhibit a bimodal state with the phase being almost constant near the summer (June) and winter (December) solstices, and making rapid transitions in the equinoxes. The duration of the constant phase is longer in the summer solstice than in the winter solstice. (C) 2002 Elsevier Science Ltd. All rights reserved.