Temperature Tides and Gravity Waves

The tidal signature in the middle atmospheric thermal structure was investigated using more than 140 hours of nighttime lidar measurements at Table Mountain (34.4ºN) during January 1997 and February 1998 and 145 hours of nighttime lidar measurements obtained during October 3-16, 1996 and October 2-11, 1997 at Mauna Loa, Hawaii, (19.5ºN)

Tides at TMF

The lidar profiles (30-95 km) at TMF revealed the presence of persistent mesospheric temperature inversions around 65-70 km altitude with a clear Local-Solar-Time (LST) dependence. Daytime temperature profiles (65-105 km) obtained by the High Resolution Doppler Imager (HRDI) onboard the Upper Atmosphere Research Satellite (UARS) in January and February from 1994 to 1997 and zonally averaged at the latitude of 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).

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Figure 1
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Figure 2


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 10:00 LST to 75 km at 20:00 LST and 65-70 km at 3:00-5:00 LST surrounded above and below by colder temperatures (Figure 1).

This structure was predicted by GSWM but with a 2-4 hour delay and a weaker amplitude (Figure 2). 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", 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 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 18:00 LST) and the upward propagating modes (Figure 3). 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.
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Figure 3

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.

Tides at MLO

Consistent LST-related structures were also observed at MLO in both HRDI and lidar data suggesting the presence of important migrating tidal components. In particular, a warm period was clearly identified, propagating downward from 105 km at 8:00 LST to 65 km at 00:00 LST and surrounded by two colder periods above and below (Figure 4).

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Figure 4
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Figure 5

These warm/cold periods were predicted to occur two to three hours later by GSWM compared to the HRDI observations. Other LST-related structures were observed by lidar between 30 and 80 km altitude, in particular a colder early night, warmer midnight, and colder late night around ~70 km suggesting a significant semidiurnal component at this altitude.

As previously observed the amplitudes predicted by GSWM were much smaller than those observed by lidar and HRDI. The main point of disagreement between the lidar observations and GSWM predictions occured between 60 and 85 km (Figure 5). Using the "constrained-wave adjustment" method a large semidiurnal component was observed by lidar leading to early and late cold night and warm midnight (Figure 6) while no such large semidiurnal component was predicted by GSWM, leading to an apparent warm early night at 60 km, and an apparent cold midnight at 80 km and above.

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Figure 6
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Figure 7

It appears that the tidal structure observed by the MLO lidar is more representative of that predicted by GSWM at 24ºN (Figure 7), suggesting a latitudinal shift between theory and observation. It is not clear whether this shift is related to an indetermination of the tidal source and/or propagation or if the observed differences are simply due to local/regional Local-Solar-Time-related oscillations obscuring the global tidal signature.

References

Leblanc, T., I. S. McDermid, and D. A. Ortland, Lidar observations of the middle atmospheric thermal tides and comparison with HRDI and GSWM. Part I: Methodology and winter observations at Table Mountain (34.4ºN)., J. Geophys. Res., 104, 11,917-11,930, 1999

Leblanc, T., I. S. McDermid, and D. A. Ortland, Lidar observations of the middle atmospheric thermal tides and comparison with HRDI and GSWM. Part II: October observations at Mauna Loa (19.5ºN)., J. Geophys. Res., 104, 11,931-11,938, 1999

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