Lidar observations of the middle atmospheric thermal tides and comparison with the High Resolution Doppler Imager and Global Scale Wave Model 2. October observations at Mauna Loa (19.5 degrees N)

Using more than 145 hours of nighttime lidar measurements obtained during O ctober 3-16, 1996, and October 2-11, 1997, the tidal signature in the middl e atmospheric thermal structure (15-95 km) at Mauna Loa, Hawaii (19.5 degre es N), was investigated. The daytime High Resolution Doppler Imager (HRDI) temperatures taken in September and October 1993-1997 and zonally averaged at the same latitude were also used. The daytime HRDI and nighttime lidar t emperature differences from their respective daytime and nighttime averages were compared to the equivalent differences predicted by the Global Scale Wave Model (GSWM) at the same latitude. Some consistent local solar time (L ST)-related structures were observed in both HRDI and lidar data, suggestin g the presence of important migrating tidal components. In particular, a wa rm period was clearly identified, propagating downward from 105 km at 0800 LST to 65 km at 0000 LST and surrounded by two colder periods above and bel ow. These warm/cold periods were predicted to occur 2 to 3 hours later by G SWM compared to the HRDI observations. Other LST-related structures were ob served by lidar between 30- and 80-km altitude, in particular, a colder ear ly night, warmer midnight, and colder late night around similar to 70 km, s uggesting a significant semidiurnal component at this altitude. As previous ly observed, the amplitudes predicted by GSWM were much smaller than those observed by lidar and HRDI. A new analysis "constrained wave adjustment" me thod described in a companion paper [Leblanc ct nl., this issue] was used t o estimate the diurnal and semidiurnal components from the nighttime-only l idar data. The main point of disagreement between the lidar observations an d GSWM predictions occurred between 60 and 85 km. A large semidiurnal compo nent was observed by lidar, leading to early and late cold night and warm m idnight, while no such large semidiurnal component was predicted by GSWM, l eading to an apparent warm early night at 60 km and an apparent cold midnig ht at 80 km and above. It appears that the tidal structure observed by lida r is more representative of that predicted by GSWM at 24 degrees N, suggest ing a latitudinal shift between theory and observation. It is not clear whe ther this shift is related to an indetermination of the tidal source and/or propagation or if the observed differences are simply due to oscillations related to local/regional local solar time obscuring the global tidal signa ture.