Global distributions of CO<sub>2</sub> volume mixing ratio in the middle and upper atmosphere from daytime MIPAS high-resolution spectra

Abstract

Abstract. Global distributions of the CO2 vmr (volume mixing ratio) in the mesosphere and lower thermosphere (from 70 up to ∼ 140 km) have been derived from high-resolution limb emission daytime MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) spectra in the 4.3 µm region. This is the first time that the CO2 vmr has been retrieved in the 120–140 km range. The data set spans from January 2005 to March 2012. The retrieval of CO2 has been performed jointly with the elevation pointing of the line of sight (LOS) by using a non-local thermodynamic equilibrium (non-LTE) retrieval scheme. The non-LTE model incorporates the new vibrational–vibrational and vibrational–translational collisional rates recently derived from the MIPAS spectra by [Jurado-Navarro et al.(2015)]. It also takes advantage of simultaneous MIPAS measurements of other atmospheric parameters (retrieved in previous steps), such as the kinetic temperature (derived up to ∼ 100 km from the CO2 15 µm region of MIPAS spectra and from 100 up to 170 km from the NO 5.3 µm emission of the same MIPAS spectra) and the O3 measurements (up to ∼ 100 km). The latter is very important for calculations of the non-LTE populations because it strongly constrains the O(3P) and O(1D) concentrations below ∼ 100 km. The estimated precision of the retrieved CO2 vmr profiles varies with altitude ranging from ∼ 1 % below 90 km to 5 % around 120 km and larger than 10 % above 130 km. There are some latitudinal and seasonal variations of the precision, which are mainly driven by the solar illumination conditions. The retrieved CO2 profiles have a vertical resolution of about 5–7 km below 120 km and between 10 and 20 km at 120–140 km. We have shown that the inclusion of the LOS as joint fit parameter improves the retrieval of CO2, allowing for a clear discrimination between the information on CO2 concentration and the LOS and also leading to significantly smaller systematic errors. The retrieved CO2 has an improved accuracy because of the new rate coefficients recently derived from MIPAS and the simultaneous MIPAS measurements of other key atmospheric parameters (retrieved in previous steps) needed for non-LTE modelling like kinetic temperature and O3 concentration. The major systematic error source is the uncertainty of the pressure/temperature profiles, inducing errors at midlatitude conditions of up to 15 % above 100 km (20 % for polar summer) and of ∼ 5 % around 80 km. The errors due to uncertainties in the O(1D) and O(3P) profiles are within 3–4 % in the 100–120 km region, and those due to uncertainties in the gain calibration and in the near-infrared solar flux are within ∼ 2 % at all altitudes. The retrieved CO2 shows the major features expected and predicted by general circulation models. In particular, its abrupt decline above 80–90 km and the seasonal change of the latitudinal distribution, with higher CO2 abundances in polar summer from 70 up to ∼ 95 km and lower CO2 vmr in the polar winter. Above ∼ 95 km, CO2 is more abundant in the polar winter than at the midlatitudes and polar summer regions, caused by the reversal of the mean circulation in that altitude region. Also, the solstice seasonal distribution, with a significant pole-to-pole CO2 gradient, lasts about 2.5 months in each hemisphere, while the seasonal transition occurs quickly.

Modeling of Nonlocal Thermodynamic Equilibrium Effects in the Classical and Principal Component‐Based Version of the RTTOV Fast Radiative Transfer Model

“…Note that compared to the published version of the algorithm, we have applied modifications that include the use of collisional rates recently derived from Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) spectra (Jurado‐Navarro et al, ) and the update of two of the above collisional rates (see pp. 6084–6085 of Jurado‐Navarro et al, , for details).…”

Validation of the MIPAS CO2 volume mixing ratio in the mesosphere and lower thermosphere and comparison with WACCM simulations

“…Here we use the global distributions of the CO 2 vmr in the mesosphere and lower thermosphere (from 75 km up to 140 km) (version v5r_CO 2 _622) that have been retrieved from the daytime MIPAS spectra in the 4.3 μm region taken in the UA observation mode [ Jurado‐Navarro et al , ]. The data set spans from January 2005 to March 2012.…”

“…The non‐LTE vibrational populations of CO 2 are modeled with the Generic RAdiative traNsfer AnD non‐LTE population Algorithm [ Funke et al , ] within each iteration of the retrieval but incorporating the new collisional rates recently derived from the MIPAS spectra by Jurado‐Navarro et al []. A more detailed description of the MIPAS CO 2 retrieval can be found in Jurado‐Navarro et al [].…”

“…The inclusion of those parameters in the retrieval, together with the new rate coefficients recently derived from MIPAS, leads to a substantial improvement in the accuracy of the retrieved CO 2 . Furthermore, the inclusion of the LOS as a joint fit parameter also improves the retrieval of CO 2 and reduces significantly the systematic errors [ Jurado‐Navarro et al , ].…”

“…The third set of satellite measurements has been made available recently from the inversion of Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) high‐resolution limb emission spectra in the 4.3 μ m region [ Jurado‐Navarro et al , ]. MIPAS is able to discriminate the contributions of the many CO 2 bands that give rise to the 4.3 μm atmospheric emission and thus has allowed us to improve our knowledge of the CO 2 non‐LTE processes that control the population of the emitting levels near 4.3 μm [ Jurado‐Navarro et al , ].…”