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 Hale akala, Maul, on October 21, 1993 (day 294), during the ALOHA-93 Campai gn. It consisted of a persistent, narrow temperature-inversion layer a bout 3-4 km wide near 87 km altitude with a peak temperature rise appr oaching 40 k, lasting for about 3.5 hours. Owing to the large number o f ground-based instruments recording the event, it is possible to atte mpt to seek out a physical explanation for this temperature rise. Ther e is powerful evidence: to suggest that the temperature-inversion laye r is associated with energy deposition, direct and indirect, resulting from gravity wave/critical layer interaction. Indeed, lidar wind prof iles and mesospheric wave structures simultaneously observed with CCD imagers reveal the presence of a critical layer at the appropriate alt itude. The data also show a narrow unstable background wind profile ov er a 2-3 km thickness in the immediate vicinity of the critical layer. , We will show quantitatively that there is sufficient energy availabl e from the dissipation of the observed gravity wave and from the obser ved instability in the background winds to account for the heating. Af ter the critical layer has disappeared, the temperature rise subsides, and the background wind again becomes stable at all height levels of interest.