Sudden narrow temperature-inversion-layer formation in ALOHA-93 as a critical-layer-interaction phenomenon

A sudden and dramatic mesospheric heating event was observed over Haleakala, Maui, on October 21, 1993 (day 294), during the ALOHA-93 Campaign. It consisted of a persistent, narrow temperature-inversion layer about 3-4 km wide near 87 km altitude with a peak temperature rise approaching 40 k, lasting for about 3.5 hours. Owing to the large number of ground-based instruments recording the event, it is possible to attempt to seek out a physical explanation for this temperature rise. There is powerful evidence to suggest that the temperature-inversion layer is associated with energy deposition, direct and indirect, resulting from gravity wave/critical layer interaction. Indeed, lidar wind profiles and mesospheric wave structures simultaneously observed with CCD imagers reveal the presence of a critical layer at the appropriate altitude. The data also show a narrow unstable background wind profile over a 2-3 km thickness in the immediate vicinity of the critical layer. We will show quantitatively that there is sufficient energy available from the dissipation of the observed gravity wave and from the observed instability in the background winds to account for the heating. After the critical layer has disappeared, the temperature rise subsides, and the background wind again becomes stable at all height levels of interest.