Lidar observations of the middle atmospheric thermal tides and comparison with the High Resolution Doppler Imager and Global‐Scale Wave Model: 1. Methodology and winter observations at Table Mountain (34.4°N)

Abstract

The tidal signature in the middle atmospheric thermal structure was investigated using more than 140 hours of nighttime lidar measurements at Table Mountain (34.4°N) during January 1997 and February 1998. The lidar profiles (30–85 km) revealed the presence of persistent mesospheric temperature inversions around 65‐ to 70‐km altitude with a clear local solar time (LST) dependence. Daytime temperature profiles (65–105 km) obtained by the High Resolution Doppler Imager (HRDI) on board the Upper Atmosphere Research Satellite (UARS) in January and February from 1994 to 1997 and zonally averaged at the latitude of the Table Mountain Facility (TMF) were considered together with the lidar results. The daytime HRDI and nighttime lidar temperature differences from their respective daytime and nighttime averages were compared to the equivalent differences predicted by the Global Scale Wave Model (GSWM). A remarkable consistency was observed between the lidar and the HRDI upper mesospheric thermal structure, with a continuous downward propagation of warm temperatures from 100 km at 1000 LST to 75 km at 2000 LST and 65–70 km at 0300–0500 LST, surrounded above and below by colder temperatures. This structure was predicted by GSWM but with a 2‐ to 4‐hour delay and a weaker amplitude. On the lower side of this structure (i.e., 65–70 km) a thin layer, characterized by early night cold temperatures and late night warm temperatures, was identified as the result of the downward propagation of the temperature inversions. Using a new analysis technique, which we have named “constrained wave adjustment” for convenience in future reference, and assuming that the observed temperature variability was entirely driven by tides, some estimations of the diurnal and semidiurnal phases and amplitudes were made from the lidar measurements between 40‐ and 85‐km altitude. Although it does not allow a complete and accurate extraction of the tidal components, this new method appeared to work well for the present TMF study. The estimated diurnal amplitude exhibited a minimum at 63 km with a fast phase transition, characteristic of the transition between the upper stratospheric trapped modes (phase at 1800 LST) and the upward propagating modes. This transition layer was predicted by GSWM to be at 5‐km lower altitude. This altitude shift was present throughout the middle mesosphere. Immediately above the transition layer, the very fast growing diurnal amplitude between 65 and 72 km was followed by a substantial decrease and by the emergence of the semidiurnal component, resulting in the formation of the mesospheric temperature inversion layers. However, the amplitude of the inversions remained large compared to the theoretical tidal predictions and a different formation mechanism should possibly be considered. Recent modeling studies have shown that gravity wave breaking can be significantly affected by the tidal background winds and some preferential wave breaking times could emerge that are dependent on the phase of the diurnal tide and the characteristics of the dissipating waves. This “LST filtering” could result in LST‐dependent temperature inversion layers similar to those observed by lidar. The new analysis technique presented in this paper was also applied to study the temperature tidal oscillations at Mauna Loa Observatory (19.5°N). The results are presented in a companion paper [Leblanc et al., this issue].