Climatology and variability of the semidiurnal tide in the Lower thermosphere over Millstone Hill

Data obtained by the Millstone Hill incoherent scatter radar during Lower Thermosphere Coupling Study campaigns in 1987-1997 were analyzed in order to determine the characteristics of the semidiurnal variations in horizontal neutral winds and temperatures at midlatitudes in the altitude range of 95-130 km. We present altitude profiles of amplitude and phase of the semidiurnal variations for climatological mean data, seasonally averaged data, as well as individual multiday campaigns. The climatologically mean semidiurnal amplitude attains similar to 50m/s for eastward wind, similar to 35m/s for the northward wind, and similar to 27 K for the temperature. At most altitudes, the standard deviation is 50-70% of the magnitude of the climatological mean value, and a large part of the observed variability is due to seasonal differences. The strongest semidiurnal oscillations are found for equinox and summer winds, reaching 80-90 m/s, while winter amplitudes are generally smaller. Summer winds are characterized by a consistent pattern in amplitude and phase; however, winter winds are more variable from campaign to campaign. Similarly, temperatures reveal low variability in phase for the summer season and high variability in winter, but the amplitudes of the 12-hour oscillations in temperature are variable for all seasons. Comparison with other experimental data suggests good agreement in general but larger variability. We discuss the possible sources of the observed variability and link it to processes in the mesosphere.