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Retrieving the total number density [OH], which is used in the chemical balance equations and is the sum of both ground and excited vibrational state populations, is a challenging problem to such a degree there exist no such estimates from recent space observations. We present a novel retrieval approach to simultaneously and self‐consistently derive both [O(3P)] and total [OH] in the nighttime mesosphere and lower thermosphere which operates with the Meinel band volume emission rates and their ratios. Its application to the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) observations shows a good agreement of the retrieved [O(3P)] taking into account the measurement uncertainties and variation of model inputs used. However, retrieved [OH] show a large discrepancy, mainly above 90 km. SCIAMACHY [OH] is generally in good agreement with WACCM [OH] after accounting for uncertainties, while SABER OH is larger than WACCM [OH] by up to a factor of 2. Plain Language Summary: The hydroxyl molecule plays an important role in the physics, chemistry, and dynamics of the middle and upper atmosphere region (60–120 km). Obtaining total hydroxyl density from its molecular emissions is a challenging problem to such a degree there exist no such estimates from the recent space observations. We present a novel approach to obtain both atomic oxygen and total hydroxyl densities in the middle and upper atmosphere from nighttime observations of the hydroxyl molecular emissions. We apply the new approach to two different space‐based instrument observations of hydroxyl emissions at multiple infrared wavelengths. The atomic oxygen densities obtained from the two instruments using our new approach are in good agreement after taking into account uncertainties, while the hydroxyl densities show a large discrepancy. Key Points: A novel approach to the simultaneous retrieval of nighttime O(3P) and OH densities from space‐based satellite observations is presentedTotal OH densities are retrieved for the first time in the mesosphere and lower thermosphere (MLT)Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY retrieved OH densities are comparable to chemical models in the MLT while Sounding of the Atmosphere using Broadband Emission Radiometry retrieved OH densities are higher [ABSTRACT FROM AUTHOR]

Abstract: The recently discovered fast, multiquantum OH(v)+O(3P) vibrational‐to‐electronic relaxation mechanism provided new insight into the OH(v) Meinel band nighttime emission formation. Using a new detailed OH(v) model and novel retrieval algorithm, we obtained O(3P) densities in the nighttime mesosphere and lower thermosphere (MLT) from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) 2.0‐ and 1.6‐μm radiances. We demonstrate how critical the new OH(v) relaxation mechanism is in the estimation of the abundance of O(3P) in the nighttime MLT. Furthermore, the inclusion of this mechanism enables us to reconcile historically large discrepancies with O(3P) results in the MLT obtained with different physical models and retrieval techniques from WIND Imaging Interferometer, Optical Spectrograph and Infrared Imager System, and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography observations of other airglow emissions. Whereas previous SABER O(3P) densities were up to 60% higher compared to other measurements the new retrievals agree with them within the range (±25%) of retrieval uncertainties. We also elaborate on the implications of this outcome for the aeronomy and energy budget of the MLT region. [ABSTRACT FROM AUTHOR]

In the 1970s, the mechanism of vibrational energy transfer from chemically produced OH(ν) in the nighttime mesosphere to the CO2(ν3) vibration, OH(ν)⇒N2(ν)⇒ CO2(ν3), was proposed. In later studies it was shown that this "direct" mechanism for simulated nighttime 4.3 μm emissions of the mesosphere is not sufficient to explain space observations. In order to better simulate these observations, an additional enhancement is needed that would be equivalent to the production of 2.8-3N2(1) molecules instead of one N2(1) molecule in each quenching reaction of OH(ν)+N2(0). Recently a new "indirect" channel of the OH(ν) energy transfer to N2(ν) vibrations, OH(ν)⇒O(1D)⇒N2(ν), was suggested and then confirmed in a laboratory experiment, where its rate for OH(ν = 9)+O(3P) was measured. We studied in detail the impact of the "direct" and "indirect" mechanisms on CO2(ν3) and OH(ν) vibrational level populations and emissions. We also compared our calculations with (a) the SABER/TIMED nighttime 4.3 μm CO2 and OH 1.6 and 2.0 μm limb radiances of the mesosphere-lower thermosphere (MLT) and (b) with ground- and space-based observations of OH(ν) densities in the nighttime mesosphere. We found that the new "indirect" channel provides a strong enhancement of the 4.3 μm CO2 emission, which is comparable to that obtained with the "direct" mechanism alone but assuming an efficiency that is 3 times higher. The model based on the "indirect" channel also produces OH(ν) density distributions which are in good agreement with both SABER limb OH emission observations and ground and space measurements. This is, however, not true for the model which relies on the "direct" mechanism alone. This discrepancy is caused by the lack of an efficient redistribution of the OH(ν) energy from higher vibrational levels emitting at 2.0 μm to lower levels emitting at 1.6 μm. In contrast, the new "indirect" mechanism efficiently removes at least five quanta in each OH(ν ⩾ 5)+O(3P) collision and provides the OH(ν) distributions which agree with both SABER limb OH emission observations and ground- and space-based OH(ν) density measurements. This analysis suggests that the important mechanism of the OH(ν) vibrational energy relaxation in the nighttime MLT, which was missing in the emission models of this atmospheric layer, has been finally identified. [ABSTRACT FROM AUTHOR]

The cloud retrieval scheme developed at the Laboratoire de Météorologie Dynamique (LMD) can now be easily adapted to any Infrared (IR) sounder: the CIRS (Clouds from IR Sounders) retrieval applies improved radiative transfer, as well as an original method accounting for atmospheric spectral transmissivity changes associated with CO2 concentration. The latter is essential when considering long-term time series of cloud properties. For the 13-year and 8-year global climatologies of cloud properties from observations of the Atmospheric IR Sounder (AIRS) and of the IR Atmospheric Interferometer (IASI), respectively, we used the latest ancillary data (atmospheric profiles, surface emissivities and atmospheric spectral transmissivities). The A-Train active instruments, lidar and radar of the CALIPSO and CloudSat missions, provide a unique opportunity to evaluate the retrieved AIRS cloud properties such as cloud amount and height as well as to explore the vertical structure of different cloud types. CIRS cloud detection agreement with CALIPSO-CloudSat is about 84%-85% over ocean, 79%-82% over land and 70%-73% over ice / snow, depending on atmospheric ancillary data. Global cloud amount has been estimated to 67%-70%. CIRS cloud height coincides with the middle between the cloud top and the "apparent" cloud base (real base for optically thin clouds or height at which the cloud reaches opacity) independent of cloud emissivity, which is about 1 km below cloud top for low-level clouds and about 1.5 km to 2.5 km below cloud top for high-level clouds, slightly increasing because the apparent vertical cloud extent is slightly larger for large cloud emissivity. IR sounders are in particular advantageous for the retrieval of upper tropospheric cloud properties, with a reliable cirrus identification down to an IR optical depth of 0.1, day and night. Total cloud amount consists of about 40% high-level clouds and about 40% low-level clouds and 20% mid-level clouds, the latter two only detected when not hidden by upper clouds. Upper tropospheric clouds are most abundant in the tropics, where high opaque clouds make out 7.5%, thick cirrus 27.5% and thin cirrus about 21.5% of all clouds. The asymmetry in upper tropospheric cloud amount between Northern and Southern hemisphere with annual mean of 5% has a pronounced seasonal cycle with a maximum of 25% in boreal summer, which can be linked to the shift of the ITCZ peak latitude. Comparing tropical geographical change patterns of high opaque clouds with that of thin cirrus as a function of changing tropical mean surface temperature indicates that their response to climate change may be quite different, with potential consequences on the atmospheric circulation. [ABSTRACT FROM AUTHOR]

Since January 2004, the planetary Fourier spectrometer (PFS) on board the Mars Express satellite has been recording near-infrared limb spectra of high quality up to the tangent altitude 150 km, with potential information on density and thermal structure of the upper Martian atmosphere. We present first results of our modeling of the PFS short wavelength channel (SWC) daytime limb spectra for the altitude region above 90 km. We applied a ro-vibrational non-LTE model based on the stellar astrophysics technique of accelerated lambda iteration (ALI) to solve the multispecies and multi-level CO2 problem in the Martian atmosphere. We show that the long-standing discrepancy between observed and calculated spectra in the cores and wings of 4.3 μm region is explained by the non-thermal rotational distribution of molecules in the upper vibrational states 10011 and 10012 of the CO2 main isotope second hot (SH) bands above 90 km altitude. The redistribution of SH band intensities from band branch cores into their wings is caused (a) by intensive production of the CO2 molecules in rotational states with j > 30 due to the absorption of solar radiation in optically thin wings of 2.7 μm bands and (b) by a short radiative lifetime of excited molecules, which is insufficient at altitudes above 90 km for collisions to maintain rotation of excited molecules thermalized. Implications for developing operational algorithms for massive processing of PFS and other instrument limb observations are discussed. [ABSTRACT FROM AUTHOR]

Since 2002, SABER (Sounding of the Atmosphere using Broadband Emission Radiometry)/TIMED (Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics) has been continuously measuring the day- and nighttime infrared limb radiances of the mesosphere and lower thermoshere (MLT) in ten broadband channels. Recently, the MLT daytime temperature/pressure and CO2 densities have been obtained self-consistently from SABER 15 μm and 4.3 μm emission observations. However, similar nighttime data remain unprocessed due to a lack of understanding of the 4.3 μm emission generating mechanisms. A previous study suggested the "direct" transfer OH(v)) ↑ N2(v)) ↑ CO2(v3)) ↑ 4.3 μm of vibrational excitation from OH(v) to CO2 in the nighttime mesosphere. However, accounting for this excitation mechanism alone leads to significant under-prediction (by up to 80 %) of observed 4.3 μm limb radiances. Recently, theoretical and laboratory studies have suggested an additional "indirect" nighttime channel OH(v)) ↑ O(¹D)) ↑ N2(v)) ↑ CO2(v3)) ↑ 4.3 μm of this energy transfer. We implemented this new channel in our non-LTE (non-Local Thermodynamic Equilibrium) model and show that, for various latitudinal and seasonal scenarios, including this additional channel brings differences between simulated and measured nighttime SABER 4.3 μm limb radiances to (-20, +30) %. These results confirm the important role of the new mechanism as a source of the nighttime 4.3 μm emission. This finding creates new opportunities for the application of CO2 4.3 μm observations in the study of the energetics and dynamics of the nighttime MLT. [ABSTRACT FROM AUTHOR]

Since January 2004, the planetary Fourier spectrometer (PFS) on board the Mars Express satellite has been recording near infrared limb spectra of high quality up to ≈ 150 km, with potential information on density and thermal structure of the upper atmosphere. We present first results of our modeling of the PFS short wavelength channel (SWC) daytime limb spectra for the altitude region above 90 km. We applied a ro-vibrational non-LTE model based on the stellar astrophysics technique of accelerated lambda 5 iteration (ALI) to solve the multi-species and multi-level CO2 problem in the Martian atmosphere. We show that the long standing discrepancy between observed and calculated spectra in the cores and wings of 4.3 μm region is explained by the non-thermal rotational distribution of molecules in the upper vibrational states 10011 and 10012 of the CO2 main isotope second hot (SH) bands above 90 km altitude. The redistribution of SH band intensities from band branch cores into their wings is caused (a) by intensive production of the CO2 molecules in rotational states with j > 30 due to the absorption 10 of solar radiation in optically thin wings of 2.7 μm bands, and (b) by a short radiative life time of excited molecules, which is insufficient at altitudes above 90 km for collisions to maintain rotation of excited molecules thermalized. Implications for developing operational algorithms for massive processing of PFS and other instrument limb observations are discussed. [ABSTRACT FROM AUTHOR]