New Non-LTE Model of OH and CO2 Emission in the Mesosphere-Lower Thermosphere and its Application to Retrieving Nighttime Parameter

The hydroxyl, OH, and carbon dioxide, CO2, molecules and oxygen atoms, O(3P), are important parameters that characterize the chemistry, energetics, and dynamics of the nighttime mesosphere and lower thermosphere (MLT) region. Hence, there is much interest in obtaining high quality observations of these parameters in order to study the short-term variability as well as the long-term trends in characteristics of the MLT region. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) satellite has been taking global, simultaneous measurements of limb infrared radiance in 10 spectral channels, including the OH 2.0 and 1.6-micron and CO2 4.3-micron emissions channels, continuously since late January 2002. These measurements can be interpreted using sophisticated non-Local Thermodynamic Equilibrium (non-LTE) models of OH and CO2 infrared emissions which can then be applied to obtain densities of these parameters (2.0 and 1.6-micron channel for O(3P)/OH and 4.3-micron channel for CO2). The latest non-LTE models of these molecules, however, do not fully represent all the dominant energy transfer mechanisms which influence their vibrational level distributions and infrared emissions. In particular, non-LTE models of CO2 4.3-micron emissions currently under-predict SABER measurements by up to 80%, and its application for the retrieval of CO2 will result in unrealistic densities. Additionally, current O(3P) retrievals from SABER OH emissions have been reported to be at least 30% higher compared to studies using other instruments. Methods to obtain OH total densities from SABER measurements have yet to be developed. Recent studies, however, have discovered a new energy transfer mechanism which influences both OH and CO2 infrared emissions, OH(v) → O(1D) → N2( v) → CO2(v3). This study focuses on the impact of this new mechanism on OH and CO2 infrared emissions as well as model applications for the retrieval of nighttime O( 3P), OH, and CO2 densities.;We first study in detail the impact of the new mechanism on OH( v) vibrational level populations and emissions. We compared our calculations with the SABER/TIMED OH 1.6 and 2.0-micron limb radiances of the MLT and with ground and space observations of OH(v) densities in the nighttime mesosphere. We find that the new mechanism produces OH(v) density distributions which are in good agreement with both SABER limb OH emission observations and ground and space measurements.;We then couple our OH non-LTE model with CO2 to study the impact of the new mechanism on CO2(v3) vibrational level populations and emissions. We compare our calculations with the SABER/TIMED 4.3-micron CO2 limb radiances and find that the new mechanism provides a strong enhancement of the 4.3-micron CO2 emissions, agreeing to within a 10-30% range.;Further, a two-channel retrieval algorithm is developed to self-consistently invert the SABER measured radiances in the OH 2.0 and 1.6-micron channel to obtain vertical profiles of OH and O(3P) Volume Mixing Ratio (VMR). Studies of the inversion algorithm made with synthetic radiances indicate that a stable solution of the inverse problem can be obtained that is nearly independent of the starting conditions.;The results presented from the two-channel algorithm to the SABER v2.0 data include comparisons of retrieved O(3P) with current SABER O(3P), in addition to O(3P) retrievals measured by the SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) instrument, as well as those calculated by the WACCM (Whole Atmosphere Community Climate Model) model for four different days. The O( 3P) density retrieved between 90-95 km are, on average, lower than current SABER O(3P) by 10-50%. OH retrievals are performed over the same days and are compared with OH WACCM calculations as well as other studies.;Finally, a similar self-consistent algorithm used for the retrieval of daytime CO2 densities is adopted for nighttime. The situation, however, is more complex for nighttime CO2, where lack of solar irradiation excitation greatly reduce 4.3-micron emission sensitivity to CO 2 density and, therefore, produces unrealistic retrievals. Alternative retrieval methods will be required to overcome these obstacles. For daytime, retrieval of temperature and CO2 are performed simultaneously due to strong coupling between these two parameters. Consideration of this effect will be crucial to obtain accurate nighttime CO2 densities.