Validation of the global distribution of CO₂ volume mixing ratio in the mesosphere and lower thermosphere from SABER

Abstract

The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite has been measuring the limb radiance in 10 broadband infrared channels over the altitude range from ~ 400 km to the Earth's surface since 2002. The kinetic temperatures and CO2 volume mixing ratios (VMRs) in the mesosphere and lower thermosphere have been simultaneously retrieved using SABER limb radiances at 15 and 4.3 µm under nonlocal thermodynamic equilibrium (non‐LTE) conditions. This paper presents results of a validation study of the SABER CO2 VMRs obtained with a two‐channel, self‐consistent temperature/CO2 retrieval algorithm. Results are based on comparisons with coincident CO2 measurements made by the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE‐FTS) and simulations using the Specified Dynamics version of the Whole Atmosphere Community Climate Model (SD‐WACCM). The SABER CO2 VMRs are in agreement with ACE‐FTS observations within reported systematic uncertainties from 65 to 110 km. The annual average SABER CO2 VMR falls off from a well‐mixed value above ~80 km. Latitudinal and seasonal variations of CO2 VMRs are substantial. SABER observations and the SD‐WACCM simulations are in overall agreement for CO2 seasonal variations, as well as global distributions in the mesosphere and lower thermosphere. Not surprisingly, the CO2 seasonal variation is shown to be driven by the general circulation, converging in the summer polar mesopause region and diverging in the winter polar mesopause region.

Impact of the lower thermospheric winter‐to‐summer residual circulation on thermospheric composition

“…We examined the residual circulations presented in Liu [2007], Smith et al [2011, andRezac et al [2015]. These residual circulations have upwelling at midlatitude and mid-high latitude in the winter hemisphere and downwelling at high latitudes in the summer hemisphere, in the altitude region~95 km to 110 km.…”

“…This residual circulation is caused by the gravity waves with the opposite velocity that dissipate above the mesopause. Unlike the well-studied residual circulation in the mesosphere, this lower thermospheric residual circulation was only brought to attention recently due to its impact on the trace species distributions, such as CO 2 , in the mesosphere and lower thermosphere regions [Smith et al, 2011;Rezac et al, 2015]. This secondary wind reversal induces a winter-to-summer meridional circulation through the Coriolis force.…”

Simultaneous retrieval of T(p) and CO2 VMR from two-channel non-LTE limb radiances and application to daytime SABER/TIMED measurements

“…Then at 85 km the decrease is more rapid again up to altitudes around 100 km above which the decrease becomes again more gradual. Detailed comparison and discussion of the retrieved CO 2 VMR and comparisons with the WACCM model and ACE-FTS profiles is presented in Rezac et al (2014).…”

“…During equinoxes a clear transition phase is seen in the latitudinal distribution of the CO 2 VMR between the hemispheres. A detailed validation and comparison of the self-consistently retrieved T k /CO 2 from SABER to model results as well as other instruments is presented in the paper Rezac et al (2014).…”

“…This symmetry is routinely observed in the SABER CO 2 VMR. More detailed comparisons are provided in Rezac et al (2014).…”