The nighttime mesopause, marking the boundary of mesosphere and thermosphere and having the coldest atmospheric temperature, is formed mainly by the combination of radiative cooling by CO₂, and the adiabatic flow in the Mesosphere and Lower Thermosphere.

WACCM-X (2.0) simulated summer temperature profiles of year 1974 (black solid-line) and year 2003 (black dashed-line); winter temperature profiles of year 1974 (red solid-line) and year 2003 (red dashed-line).

An advanced Na Doppler lidar nocturnal temperature data base, obtained at Fort Collins, CO (41N, 105W) and at Logan, UT (42N, 112W) between 1990 and 2018, provides an opportunity to study the long-term variations of this important atmospheric boundary. In this study, we categorize the lidar observed mesopause into “high mesopause” above 97 km altitude and “low mesopause” below 92 km. The “high mesopause” mostly occurs between late fall and early spring and, thus, is mainly formed through the radiative cooling mechanism, while the “low mesopause” occurs in late spring and summer and, thus, is generated mostly by the adiabatic cooling. This long-term trend study finds a cooling trend of more than 2 K/decade in absolute mesopause temperature for both kinds of mesopause, and a decrease of the mesopause height. The peak of the winter “high mesopause” moved downward at a rate of ~ 440 ± 90 m/decade, and the summer “low mesopause” peak altitude reduced at a rate of ~ 230 ± 160 m/decade. A recent long-term study using WACCM-X indicated a reduction of mesopause temperature, but at a lower rate than the lidar results. The model results also show a decrease of meospause height, mostly caused by stratosphere and lower mesosphere cooling and contraction.

Publication Name: Journal of Geophysical Research—Space Physics

First HAO Author's Name: Stanley C. Solomon