Μm CO2 Band Research Articles

Seasonal variability of gravity wave activity in Mars's lower atmosphere from MGS–TES nadir observations

Gravity waves (GW) in the lower atmosphere play an important part in Mars' atmospheric dynamics and energy balance, transferring momentum and energy to higher altitudes and affecting cloud formation. Gravity wave activity at the global scale on Mars can be studied from statistical analysis of variances of brightness temperatures observed by orbiting instruments. We use observations collected by the Thermal Emission Spectrometer (TES) aboard Mars Global Surveyor (MGS) during Mars Years 24–27 to explore spatial and seasonal variability of GW activity at horizontal scales ~100 km and on vertical scales >40 km. TES radiances were re-sampled to Mars Reconnaissance Orbiter (MRO) Mars Climate Sounder (MCS) spectral channels A1 (595–615 cm−1), A2 (615–645 cm−1) and A3 (635–665 cm−1) in the wings of the 15 μm CO2 band to sense GWs in the lower atmosphere. Results show that GW activity correlates with topographic roughness and is likely excited by orographic sources. GW activity decreases significantly with height, suggesting critical level filtering. During the global dust storm (GDS) of MY25 GW activity decreased substantially in the southern hemisphere. High levels of instrument noise that varied through the TES mission make it difficult to make a definitive statement about interannual variability in GW activity.

Mars Climate Sounder Observations of Gravity-wave Activity throughout Mars’s Lower Atmosphere

Abstract Gravity waves are one way Mars’s lower atmospheric weather can affect the circulation and even composition of Mars’s middle and upper atmosphere. A recent study showed how on-planet observations near the center of the 15 μm CO2 band by the A3 channel (635–665 cm−1) of the Mars Climate Sounder on board the Mars Reconnaissance Orbiter could sense horizontally short, vertically broad gravity waves at ≈25 km above the surface by looking at small-scale radiance variability in temperature-sensitive channels. This approach is extended here to two additional channels closer to the wings of the 15 μm CO2 band, A1 (595–615 cm−1) and A2 (615–645 cm−1), to sense gravity waves throughout the lower atmosphere. Using information from all three channels demonstrates that gravity-wave activity in Mars’s lowermost atmosphere is dominated by orographic sources, particularly over the extremely rough terrain of Valles Marineris. Much of this orographic population is either trapped or filtered in the lowest two scale heights, such that variations in filtering and nonorographic sources shape the gravity-wave population observed at 25 km above the surface. During global dust storms, however, gravity-wave activity in the first scale height decreases by approximately a factor of 2, yet trapping/filtering of what activity remains in the tropics substantially weakens. Exceptionally high radiance variability at night in the tropics during the less dusty part of the year is the result of observing mesospheric clouds rather than gravity waves.

Absorption coefficient (ABSCO) tables for the Orbiting Carbon Observatories: Version 5.1

The accuracy of atmospheric trace gas retrievals depends directly on the accuracy of the molecular absorption model used within the retrieval algorithm. For remote sensing of well-mixed gases, such as carbon dioxide (CO2), where the atmospheric variability is small compared to the background, the quality of the molecular absorption model is key. Recent updates to oxygen (O2) absorption coefficients (ABSCO) for the 0.76 µm A-band and the water vapor (H2O) continuum model within the 1.6 µm and 2.06 µm CO2 bands used within the Orbiting Carbon Observatory (OCO-2 and OCO-3) algorithm are described here. Updates in the O2 A-band involve the inclusion of new laboratory measurements within multispectrum fits to improve relative consistency between O2 line shapes and collision-induced absorption (CIA). The H2O continuum model has been updated to MT_CKD v3.2, which has benefited from information from a range of laboratory studies relative to the model utilized in the previous ABSCO version. Impacts of these spectroscopy updates have been evaluated against ground-based atmospheric spectra from the Total Carbon Column Observing Network (TCCON) and within the framework of the OCO-2 algorithm, using OCO-2 soundings covering a range of atmospheric and surface conditions. The updated absorption coefficients (ABSCO version 5.1) are found to offer improved fitting residuals and reduced biases in retrieved surface pressure relative to the previous version (ABSCO v5.0) used within B8 and B9 of the OCO-2 retrieval algorithm and have been adopted for the OCO B10 Level 2 algorithm.

Optimization of the OCO-2 Cloud Screening Algorithm and Evaluation against MODIS and TCCON Measurements over Land Surfaces in Europe and Japan

A method to tighten the cloud screening thresholds based on local conditions is used to provide more stringent schemes for Orbiting Carbon Observatory-2 (OCO-2) cloud screening algorithms. Cloud screening strategies are essential to remove scenes with significant cloud and/or aerosol contamination from OCO-2 observations, which helps to save on the data processing cost and ensure high quality retrievals of the column-averaged CO2 dry air mole fraction (XCO2). Based on the radiance measurements in the 0.76 μm O2A band, 1.61 μm (weak), and 2.06 μm (strong) CO2 bands, the current combination of the A-Band Preprocessor (ABP) algorithm and Iterative Maximum A Posteriori (IMAP) Differential Optical Absorption Spectroscopy (DOAS) Preprocessor (IDP) algorithm passes around 20%–25% of all soundings, which means that some contaminated scenes also pass the screening process. In this work, three independent pairs of threshold parameters used in the ABP and IDP algorithms are sufficiently tuned until the overall pass rate is close to the monthly clear-sky fraction from the MODIS cloud mask. The tightened thresholds are applied to observations over land surfaces in Europe and Japan in 2016. The results show improvement of agreement and positive predictive value compared to the collocated MODIS cloud mask, especially in summer and fall. In addition, analysis indicates that XCO2 retrievals with more stringent thresholds are in closer agreement with measurements from collocated Total Carbon Column Observing Network (TCCON) sites.

Assessing reanalysis quality with early sounders Nimbus-4 IRIS (1970) and Nimbus-6 HIRS (1975)

This paper revisits the data collected by early sounders Nimbus-4 IRIS (1970) and Nimbus-6 HIRS (1975), after recovery of ageing tapes by NASA GES DISC. New quality controls are proposed to screen out erroneous or suspicious mission data, based on instrument health status data records and other inspection of the data. Radiative transfer coefficients are derived for the fast computation of clear-sky radiative transfer simulations. Atmospheric profiles from ERA-40 and ERA-20C reanalyses are used in input. These spatio-temporally complete datasets are interpolated to each sounding location, using the closest estimate in time. A modern cloud detection method derived for current hyperspectral sounders is applied to IRIS and yields maps of cloud cover that are in line with current knowledge of cloud climatology. For clear scenes, the standard deviation of brightness temperature differences between IRIS observations and simulations from ERA-20C is around 1 K for the lower-peaking temperature channels of the 15 μm CO2 band, and lower than 1 K for simulations from ERA-40. The IRIS and HIRS instrumental data records are projected in a common sub-space to alleviate issues with different field-of-view resolutions and spectral resolutions. A proxy cloud detection scheme screens out clouds in the same manner in both data records. Considering the month of August, common to both missions, a detailed analysis of the departures from observations suggests that ERA-40 suffers from spurious tropospheric warming, possibly caused by changes in the observation input during the 1970s including a known error in ERA-40 radiance assimilation bias correction. This result, confirmed by considering a climate model integration, demonstrates that it is possible to exploit early sounder data records to derive detailed insight from reanalyses, such as attempting to qualify separately random and systematic errors in reanalyses, even at times when few other independent observation data are available.

Mesospheric CO2 ice clouds on Mars observed by Planetary Fourier Spectrometer onboard Mars Express

We have investigated mesospheric CO2 ice clouds on Mars through analysis of near-infrared spectra acquired by Planetary Fourier Spectrometer (PFS) onboard the Mars Express (MEx) from MY 27 to MY 32. With the highest spectral resolution achieved thus far in the relevant spectral range among remote-sensing experiments orbiting Mars, PFS enables precise identification of the scattering peak of CO2 ice at the bottom of the 4.3 μm CO2 band. A total of 111 occurrences of CO2 ice cloud features have been detected over the period investigated. Data from the OMEGA imaging spectrometer onboard MEx confirm all of PFS detections from times when OMEGA operated simultaneously with PFS. The spatial and seasonal distributions of the CO2 ice clouds detected by PFS are consistent with previous observations by other instruments. We find CO2 ice clouds between Ls = 0° and 140° in distinct longitudinal corridors around the equatorial region (± 20°N). Moreover, CO2 ice clouds were preferentially detected at the observational LT range between 15–16 h in MY 29. However, observational biases prevent from distinguishing local time dependency from inter-annual variation. PFS also enables us to investigate the shape of mesospheric CO2 ice cloud spectral features in detail. In all cases, peaks were found between 4.240 and 4.265 μm. Relatively small secondary peaks were occasionally observed around 4.28 μm (8 occurrences). These spectral features cannot be reproduced using our radiative transfer model, which may be because the available CO2 ice refractive indices are inappropriate for the mesospheric temperatures of Mars, or because of the assumption in our model that the CO2 ice crystals are spherical and composed by pure CO2 ice.

Combining CrIS double CO2 bands for detecting clouds located in different layers of the atmosphere

AbstractDetection of clouds within certain vertical layers of the atmosphere from satellite infrared instruments is challenging, especially of those optically thin clouds due to their small thermal contrasts to the background. This study develops a new method for cloud detection by using the Cross‐track Infrared Sounder (CrIS) hyperspectral radiances at shortwave (~4.3 µm) and longwave (~15 µm) CO2 bands. Specifically, CrIS longwave channels are first paired with shortwave channels based on weighting function altitudes and sensitivity to clouds. A linear relationship of brightness temperatures between each paired channel is then derived for predicting the shortwave channel from the longwave channel in clear‐sky conditions. A cloud emission and scattering index (CESI) can finally be defined as the difference of the paired shortwave channel between the clear‐sky, regression model predicted and the observed brightness temperatures. Spatial distributions of such derived CESI for several paired channels in the troposphere are examined for a winter storm that occurred in the eastern part of the United States during 22–24 January 2016. It is shown that the CESI values over the storm regions with optically thin cirrus, fog, and supercooled water clouds are positively larger than those over optically thick opaque ice and overshooting clouds or in clear‐sky conditions. Of particular interest is that an area of fog and water clouds over Gulf of Mexico, which are indicated by the Visible Infrared Imaging Radiometer Suite day and night band observations, is identified by the CESI. The global distribution of CESIs derived from CrIS double CO2 bands with weighting functions peak around 321 hPa agrees well with the distribution of ice cloud optical thickness from the Atmospheric Infrared Sounder version 6 cloud product data set in both daytime and nighttime.

VENUS CLOUD MORPHOLOGY AND MOTIONS FROM GROUND-BASED IMAGES AT THE TIME OF THE AKATSUKI ORBIT INSERTION∗

ABSTRACT We report Venus image observations around the two maximum elongations of the planet at 2015 June and October. From these images we describe the global atmospheric dynamics and cloud morphology in the planet before the arrival of JAXA’s Akatsuki mission on 2015 December 7. The majority of the images were acquired at ultraviolet wavelengths (380–410 nm) using small telescopes. The Venus dayside was also observed with narrowband filters at other wavelengths (890 nm, 725–950 nm, 1.435 μm CO2 band) using the instrument PlanetCam-UPV/EHU at the 2.2 m telescope in Calar Alto Observatory. In all cases, the lucky imaging methodology was used to improve the spatial resolution of the images over the atmospheric seeing. During the April–June period, the morphology of the upper cloud showed an irregular and chaotic texture with a well-developed equatorial dark belt (afternoon hemisphere), whereas during October–December the dynamical regime was dominated by planetary-scale waves (Y-horizontal, C-reversed, and ψ-horizontal features) formed by long streaks, and banding suggesting more stable conditions. Measurements of the zonal wind velocity with cloud tracking in the latitude range from 50°N to 50°S shows agreement with retrievals from previous works.

Analysis of the radiative budget of the Venusian atmosphere based on infrared Net Exchange Rate formalism

AbstractA detailed one‐dimensional analysis of the energy balance in Venus atmosphere is proposed in this work, based on the Net Exchange Rate formalism that allows the identification in each altitude region of the dominant energy exchanges controlling the temperature. Well‐known parameters that control the temperature profile are the solar flux deposition and the cloud particle distribution. Balance between solar heating and infrared energy exchanges is analyzed for each region: upper atmosphere (from cloud top to 100 km), upper cloud, middle cloud, cloud base, and deep atmosphere (cloud base to surface). The energy accumulated below the clouds is transferred to the cloud base through infrared windows, mostly at 3–4 μm and 5–7 μm. The continuum opacity in these spectral regions is not well known for the hot temperatures and large pressures of Venus's deep atmosphere but strongly affects the temperature profile from cloud base to surface. From cloud base, upward transport of energy goes through convection and short‐range radiative exchanges up to the middle cloud where the atmosphere is thin enough in the 20–30 μm window to cool directly to space. Total opacity in this spectral window between the 15 μm CO2 band and the CO2 collision‐induced absorption has a strong impact on the temperature in the cloud convective layer. Improving our knowledge of the gas opacities in these different windows through new laboratory measurements or ab initio computations, as well as improving the constraints on cloud opacities would help to separate gas and cloud contributions and secure a better understanding of Venus's atmosphere energy balance.

A review of Stratospheric Sounding Unit radiance observations for climate trends and reanalyses

Stratospheric Sounding Units (SSU) on the NOAA polar‐orbiting satellites measured infrared radiances in the 15 μm CO2 band between late 1978 and mid‐2006. From these radiances a time series of layer‐mean stratospheric temperatures has been derived by several groups. Discrepancies in these temperature analyses have been highlighted recently and efforts are now underway to resolve the differences between them. This article is the Met Office response summarising the issues to be resolved in creating a climate data record from the different SSUs, including corrections for radiometric, spectroscopic and tidal differences. Calibration issues identified include the SSU space‐view anomaly and radiometric anomalies in the NOAA‐9 observations. The spectroscopic correction required for changing pressures in the pressure modulator cells is also outlined. The most important correction for the time series is for the solar diurnal and semi‐diurnal tides as the satellite overpass local times change. Comparisons with other stratospheric temperature trend analyses are made and the reasons for the differences discussed. The time series presented here show sustained drops in stratospheric temperatures at all levels after the El Chichon and Pinatubo eruptions but only small trends to lower temperatures between eruptions.

Scattering properties of soot-containing particles and their impact by humidity in 1.6 μm

Short-wave infrared (SWIR) band in wavelength near 1.6μm is one of the key bands used for satellite observation of Carbon Dioxide (CO2). However, one major uncertainty to use this band for the CO2 retrieval is the scattering by cloud and aerosol particles. To better understand the scattering properties of soot-containing particles in this band, this paper studied the scattering properties for three typical types of soot-containing particles in China: (I) internal mixture, (II) pure soot aggregate, and (III) semi-external mixture. Assumed as single non-spherical particle for type I, its scattering property is computed using the T-matrix method combined with the Maxwell–Garnett effective medium theory and the hygroscopic growth theory. For types II and III, a particle-cluster aggregation algorithm is employed to generate fractal-like aggregates, and their scattering properties are computed using the Core–Mantle Generalized Multi-sphere Mie-solution method combined with the hygroscopic growth theory of both monomers and aggregated particles. The simulated results demonstrate that their scattering properties are quite different and strongly impacted by the levels of relative humidity (RH). For type I, the RH plays a much more important role than the morphology in impacting the scattering properties, and the scattering phase functions among different shaped particles have a larger difference for larger particles and higher RH. For type II, both the RH and morphology significantly affect its scattering properties. The single scattering albedo (ω) can be underestimated up to ~50% without considering the effects of RH and morphological changes. For type III, its scattering properties mainly depend on the RH and the size of the large water-soluble particle. Although the enlarged soot aggregate, which is attached to a water-soluble particle, almost does not change the light direction, it can result in a significant reduction in ω (~0.15) at low RH for small particles. By comparing the scattering parameters of wet particles at a certain RH level with the dry ones, the impact by the heterogeneity of aerosols generally becomes larger with the increase of RH, but becomes smaller with the increase of particle size. These results suggest that, although the water vapor absorption itself is small in 1.6μm CO2 band, it can significantly impact the scattering properties of these particles through its effect on the hygroscopic growth of the non-spherical and heterogeneous aerosols. This impact should be taken into account in the retrieval of CO2 using 1.6μm as well as other related remote sensing applications.

Direct observations of Venus upper mesospheric temperatures from ground based spectroscopy of CO2

We report direct observations of temperatures in the Venus upper mesosphere around 110 km altitude (0.15 Pa). Information about temperatures at these altitude regions are sparse especially for the dayside of Venus. Data was acquired during three observing campaigns between March and June 2009 at the McMath‐Pierce Solar Telescope of the National Solar Observatory on Kitt Peak, Arizona, using the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS). The linewidths of fully resolved non‐thermal emission lines in the 10.6 μm band of CO2 were used to derive temperatures on the Venus dayside. Temperatures were measured with high spatial resolution at 115 positions on the Venus dayside between 67°N and 90°S and various offsets from the apparent discs central meridian longitude (CML) providing the first extensive dataset for the dayside mesosphere. The retrieved temperatures show a strong local time and latitude dependence. Values of 160 K are observed close to the terminator and at the South pole while temperaturs reach 250 K at the sub solar point. These high values are in disagreement with the predictions of the Venus International Reference Atmosphere model but are consistent with earlier measurements from 2007.

Real time, non-intrusive measurement of particle emissivity and gas temperature in coal-fired power plants

We present a novel, remote technique for measuring in situ and in real time (every 2 s) the spectral emissivity of particles at λ = 3.95 µm, in optically thick combustion environments. The novelty lies in the use of spectral information in the mid-IR (the blackbody emission profile of the 4.3 µm CO2 band and the gray emission profile of particles between 3.8 and 4.1 µm) to determine the physical and brightness temperatures of the gas–particle medium, from which particle emissivity can be calculated. The retrieved particle emissivity at 3.95 µm is a reasonable average of total particle emissivity between 1 and 15 µm. Thus, CFD researchers who work with radiation sub-models may use this technique to obtain in situ emissivities at different locations, with a portable, rugged and inexpensive device. A small prototype was built with off-the-shelf components: standard light collection optics, a grating spectrometer and a linear-array pyroelectric detector. The particle emissivity is calculated from the asymptotic solution of the radiative transfer equation for optically thick media with isotropic scatterers. Results from a proof-of-concept test at a full-scale, coal-fired boiler 10 m above the top row of burners showed an average particle emissivity of 0.41 and an average gas temperature of 1533 K. Intrinsic and prototype error as well as the impact of temperature gradients in the line of sight of the instrument are discussed.

Space‐based near‐infrared CO2 measurements: Testing the Orbiting Carbon Observatory retrieval algorithm and validation concept using SCIAMACHY observations over Park Falls, Wisconsin

Space‐based measurements of reflected sunlight in the near‐infrared (NIR) region promise to yield accurate and precise observations of the global distribution of atmospheric CO2. The Orbiting Carbon Observatory (OCO) is a future NASA mission, which will use this technique to measure the column‐averaged dry air mole fraction of CO2 with the precision and accuracy needed to quantify CO2 sources and sinks on regional scales (∼1000 × 1000 km2) and to characterize their variability on seasonal timescales. Here, we have used the OCO retrieval algorithm to retrieve and surface pressure from space‐based Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) measurements and from coincident ground‐based Fourier transform spectrometer (FTS) measurements of the O2 A band at 0.76 μm and the 1.58 μm CO2 band for Park Falls, Wisconsin. Even after accounting for a systematic error in our representation of the O2 absorption cross sections, we still obtained a positive bias between SCIAMACHY and FTS retrievals of ∼3.5%. Additionally, the retrieved surface pressures from SCIAMACHY systematically underestimate measurements of a calibrated pressure sensor at the FTS site. These findings lead us to speculate about inadequacies in the forward model of our retrieval algorithm. By assuming a 1% intensity offset in the O2 A band region for the SCIAMACHY retrieval, we significantly improved the spectral fit and achieved better consistency between SCIAMACHY and FTS retrievals. We compared the seasonal cycle of at Park Falls from SCIAMACHY and FTS retrievals with calculations of the Model of Atmospheric Transport and Chemistry/Carnegie‐Ames‐Stanford Approach (MATCH/CASA) and found a good qualitative agreement but with MATCH/CASA underestimating the measured seasonal amplitude. Furthermore, since SCIAMACHY observations are similar in viewing geometry and spectral range to those of OCO, this study represents an important test of the OCO retrieval algorithm and validation concept using NIR spectra measured from space. Finally, we argue that significant improvements in precision and accuracy could be obtained from a dedicated CO2 instrument such as OCO, which has much higher spectral and spatial resolutions than SCIAMACHY. These measurements would then provide critical data for improving our understanding of the carbon cycle and carbon sources and sinks.

Increase in the carbon dioxide infrared emissions during solar proton events

Increase in the nighttime high-latitude nonthermal emissions in the mesosphere and lower thermosphere in the 4.3 and 15 µm CO2 bands during solar proton events has been estimated for the first time. The estimations have been performed for protons with energies not lower than 1 MeV precipitating into the atmosphere. A strong increase in the 4.3 µm emission can be anticipated during the above events; however, a substantial increase in the 15 µm emission is improbable. The 4.3 µm emission can increase only above approximately 80 km regardless of the energy of precipitating protons. The excitation of CO2 vibrational states, transitions from which generate the 4.3 µm emission, is caused by the vibrational excitation of N2 molecules due to collisions with secondary electrons, produced during solar proton events, and the following transfer of this excitation to CO2(0001) molecules during N2-CO2 collisions.

Mapping Martian Atmospheric Pressure with Ground-Based Near Infrared Spectroscopy

AbstractWe present images of Mars obtained in the 2.0 μm CO2 band with the United Kingdom Infrared Telescope (UKIRT). The images reveal topographic features of the Martian surface that are usually invisible in direct albedo images at visible or IR wavelengths, but agree remarkably well with the topography of the planet as mapped by the MOLA instrument on Mars Global Surveyor. The CO2 band depth is a measure of the absorbing column of CO2 and hence determined primarily by the surface atmospheric pressure. The surface pressure variations are detected with a sensitivity of 4–5 Pa. We compare our data with radiative transfer models and find that the measured CO2 variation with pressure is consistent with the models. We discuss the possibility that similar observations might be used to observe atmospheric pressure changes due to Martian weather systems.

Accurate and Simple Computation of Radiative Heating Rate in 15 .MU.m CO2 Band

This paper proposes the empirical correction of the correlated k-distribution method, to calculate the radiative heating rate in the 15 μm CO2 band. The transmittances produced by the correlated k-distribution method in CO2 15 μm band are systematically displaced from the exact transmittances obtained by the line-by-line method. When the empirical corrections are applied to the correlated k-distribution transmittances, the resultant heating rates of clear-sky are significantly improved in accuracy. In addition, this empirical correction is computationally efficient. Thus the method is promising for practical purposes, such as a radiation scheme in a dynamical model.

Using the “blue spike” to characterize biomass‐burning sites during Southern African Regional Science Initiative (SAFARI) 2000

During several flights of the ER‐2 while participating in the Southern African Regional Science Initiative (SAFARI 2000), the University of Wisconsin–Madison's Scanning High Resolution Interferometer Sounder (S‐HIS) obtained spectra containing isolated fires within its field of view (FOV). These fire‐laden FOVs contain a spectral feature caused by rotational hot band transitions of CO2 near 2400 cm−1. Because of its location on the blue side of the 4.3 μm band of CO2, this feature is commonly referred to as the “blue spike.” Using this feature, we detected fires on four flights: 24 and 27 August and 6 and 7 September 2000. Fire locations are further verified by the ER‐2 pilot's flight logs and elevated brightness temperatures in the thermal detectors of the MODIS Airborne Simulator (MAS) also on board the ER‐2. Using line‐by‐line radiative transfer calculations (Genln2) with corrections for a fire's extreme high temperatures (HiTemp), we model S‐HIS spectra for various scenes: background (cool surface and cool atmosphere), smoldering (warm surface and cool atmosphere), hot gas layer (cool surface and warm atmosphere), and fire (hot surface and hot atmosphere) cases. Using the controlled burn in the Timbavati Game Reserve on 7 September 2000 as a test case, we spectrally modeled the blue spike feature seen in the spectra obtained by S‐HIS while the ER‐2 flew over the fire. For this case, we found that ∼4.12 ± 0.05% of the FOV contained the hot gas layer while ∼0.23 ± 0.05% was actively burning. Originally viewed as a straightforward task of using the blue spike to characterize the fire temperature and size (fraction of S‐HIS FOV), our analysis shows that numerous variables, including amount of carbon dioxide, amount of water vapor, and the temperature near the fire, play significant roles in the blue spike's shape and spectral position.

On the temperature structure of the tropical substratosphere

The region of the tropical atmosphere between the main convective outflow level around 200–150 hPa and the cold point, the substratosphere, shares properties of the troposphere and the stratosphere. An analogous region exists in single‐column radiative‐convective models between the top of the convectively adjusted region and the cold point, so a radiative‐convective model has been used to investigate the radiative and dynamical factors that allow a substratosphere to exist and to which it is sensitive. A key result is that localized heating in the 15 μm CO2 band, owing to the sharp curvature in the temperature profile near the convection top, forces apart the cold point and convection top creating the substratosphere. A possible radiation‐convective‐transport feedback involving O3 is identified that could amplify the response of the substratosphere and cold point to changes in forcing such as sea surface temperature or stratospheric meridional circulation. A comparison of timescales suggests that the substratosphere is characterized by (1) a radiative timescale shorter than the convective timescale, so that radiation dominates in setting the temperature structure, but (2) a convective transport timescale shorter than the timescales for other processes affecting O3, so that the import of air from the boundary layer keeps O3 mixing ratios low there.

The CO2 non-LTE problem taking account of the multiquantum transitions on the ν2-mode during CO2–O collisions

The strong anisotopy of the interaction potential during CO2–O collisions can result in transitions in the CO2 state change of two or more ν2-quanta. The CO2 non-LTE problem of the Earth's atmosphere, taking account of these transitions, has been solved. The results have been compared with those obtained for the one-ν2-quantum transitions usually adopted during the CO2–O collisions. An increase of up to tens of Kelvin of vibrational temperatures of the CO2ν1ν2 mode manifold states in the lower thermosphere has been found, since in the case of multiquantum transitions these states can be excited by O from the ground state of the CO2 molecule. Adopting multiquantum pathway results in a considerable increase of the 15 μm band limb radiance above 100 km as compared with the one-ν2-quantum pathway case. The multiquantum transitions can result in an increased cooling rate in the 15 μm CO2 band up to 15–25 K/day at altitudes of 125–130 km. Both these consequences are demonstrated to be caused mainly by increased populations of the 0200, 0220 and 1000 states of the principal isotope of CO2.