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3. Tidal effects

Lidar data are restricted to night time and clear weather. Individual lidar profiles are derived by integrating measurements taken during the night over several hours. The integration period depends on factors such as local cloud cover,  measurement protocol, and availability of the operator. The integration time for lidar measurement at OHP should have induced residual temperature changes due to tidal fluctuations. In the summer, stratospheric temperature tides contain a diurnal cycle with an amplitude around the stratopause of ±4 K (from minimum to maximum) peaking at 18:00 solar local time (Keckhut et al., 1996). Temperature anomalies have been estimated according to these tidal characteristics and the exact period of the lidar sounding at OHP.  The simulation shows tidal effects associated with inter-annual changes smaller than 1K, with most of the structure between 1986 to 1991. The overall residual, non atmospheric trend for this period is smaller than +0.02 K per decade.
A diurnal oscillation with a maximum at 14-15 hours (local time) with an amplitude up to ±10 K around the stratopause has been observed in the past with successive rocket measurements. This effect is probably mainly due to sensor heating induced by solar radiation, which causes amplitudes larger than natural tides. Trend results appear to differ quite a bit when time of the measurements is taken into account. For example, Ascension Island data base (8°S) exhibits the largest cooling in the stratosphere with a maximum around 4 K/decade at 40 km in disagreement with other sites. The re-analysis of this data set, in selecting data minimising the time of day variation of the measurement, reveals a very different signature, smaller by a factor of two at 45 km in better agreement with other tropical and subtropical rocket sites (Keckhut et al.; 1998).
Rayleigh lidar operating in the south of France during the same periods were compared with the interpolated adjusted NCEP data (Keckhut et al., 2000). Abrupt changes appear coincident sometime with dates of satellite replacements, mainly at 2 and 1 hPa levels. The NCEP stratospheric analyses are derived from one of two operational, polar orbiting, sun synchronous satellites. One satellite has its ascending equatorial crossing near 13:30 LT (afternoon satellite) and the other its descending crossing near 7:30 LT . Adjustments, based on rockets launched from US sites do not differentiate temperature changes of instrumental origin versus deviations induced by the portion of the diurnal cycle observed. As  satellite orbits do switch several times every 2-3 years, providing shifts of the local time of the TOVS measurements over a single location.  So abrupt changes may appear for each switch of the orbit of the satellite. Those bias prevent any inter-annual studies with the upper stratospheric NCEP analyses.