Optimal retrieval method to estimate ozone vertical profile in the mesosphere and lower thermosphere (MLT) region from submillimeter-wave limb emission spectra

Abstract

Spectrum width and intensity of ozone (O3) observed in the MLT region behaves quite differently than in the stratosphere for submillimeter-wave limb emission spectroscopic observation. For example, O3 spectra in the stratosphere are stronger during the day than at night. Conversely, spectra in the MLT region at night are stronger than those occurring during the day due to diurnal variations in O3 behavior. These opposing behaviors cause problems, including oscillations and inaccuracies particularly for O3 vertical profiles in the MLT region retrieved with an application of one retrieval procedure for the entire vertical range (stratosphere to thermosphere). Recently, we developed an optimal retrieval method for O3 in the MLT region for spectra, observed by the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) instrument on the International Space Station. Optimizations were performed for frequency window range, retrieval vertical range, vertical grids, and a priori information for O3 and temperature. Precision and accuracy were evaluated by error analysis and comparisons with previous products. The random error was estimated to be about 5% and 35% in the mesosphere and lower thermosphere, respectively, for nighttime O3 profiles in the MLT region. The total systematic error was about 6% in the MLT region. Certain improvements for both random noise (from 50% to 35%) and systematic error (from 10% to 6%) were obtained. We succeeded in revealing the positive correlation between O3 and ClO at nighttime in the upper mesosphere using the optimized O3 profiles; furthermore, its chemical mechanism was explained quantitatively.