Miguel Angel Lopez-Valverde

General circulation model simulations of the Mars Pathfinder atmospheric structure investigation/meteorology data

The NASA Ames Mars General Circulation Model is used to interpret selected results from the Mars Pathfinder atmospheric structure instrument/meteorology (ASI/MET) experiment. The present version of the model has an improved soil thermal model, a new boundary layer scheme, and a correction for non-local thermodynamic equilibrium effects at solar wavelengths. We find good agreement with the ASI/MET entry data if the dust observed at the Pathfinder site is assumed to be distributed throughout the lowest five to six scale heights. This implies that the dust is globally distributed as well. In the lower atmosphere the inversion between 10 and 16 km in Pathfinders entry profile is likely due to thermal emission from a water ice cloud in that region. In the upper atmosphere (above 50 km), dynamical processes, tides in particular, appear to have a cooling effect and may play an important role in driving temperatures toward the CO2 condensation temperature near 80 km. Near-surface air temperatures and wind directions are well simulated by the model by assuming a low surface albedo (0.16) and moderately high soil thermal inertia (336 SI). However, modeled tidal surface pressure amplitudes are about a factor of 2 smaller than observed. This may indicate that the model is not properly simulating interference effects between eastward and westward modes.

Density and temperatures of the upper Martian atmosphere measured by stellar occultations with Mars Express SPICAM

[1] We present one Martian year of observations of the density and temperature in the upper atmosphere of Mars (between 60 and 130 km) obtained by the Mars Express ultraviolet spectrometer Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM). Six hundred sixteen profiles were retrieved using stellar occultations technique at various latitude and longitude. The atmospheric densities exhibit large seasonal fluctuations due to variations in the dust content of the lower atmosphere which controls the temperature and, thus, the atmospheric scale height, below 50 km. In particular, the year observed by SPICAM was affected by an unexpected dust loading around Ls = 130° which induced a sudden increase of density above 60 km. The diurnal cycle could not be analyzed in detail because most data were obtained at nighttime, except for a few occultations observed around noon during northern winter. There, the averaged midday profile is found to slightly differ from the corresponding midnight profile, with the observed differences being consistent with propagating thermal tides and variations in local solar heating. About 6% of the observed mesopause temperatures exhibits temperature below the CO 2 frost point, especially during northern summer in the tropics. Comparison with atmospheric general circulation model predictions shows that the existing models overestimate the temperature around the mesopause (above 80 to 100 km) by up to 30 K, probably because of an underestimation of the atomic oxygen concentration which controls the CO 2 infrared cooling.

Polar warming in the Mars thermosphere: Seasonal variations owing to changing insolation and dust distributions

Received 13 July 2005; revised 16 December 2005; accepted 27 December 2005; published 27 January 2006. [1] Warming of the martian lower thermosphere (100–130 km) at north polar latitudes near the perihelion/winter solstice (Ls = 270) was recently observed. No analogous warming has been observed within the south polar thermosphere during its aphelion/winter season (Ls 90). Detailed global model simulations are required to investigate the physical processes driving these seasonal variations. New simulations are conducted for conditions approximating the atmosphere during these Mars Global Surveyor (MGS) and Odyssey (ODY) aerobraking periods. Strong northern winter polar warming features are calculated near 120 km, yielding nightside mean temperatures 10–15 K warmer than observed ODY values. No southern winter polar warming trend is simulated; however, nightside mean temperatures are 20– 30 K warmer than observed by MGS. The stronger interhemispheric circulation during northern winter is clearly driven by stronger insolation and dust heating near perihelion, resulting in subsidence and warmer temperatures in the northern polar night. Citation: Bougher, S. W., J. M. Bell, J. R. Murphy, M. A. Lopez-Valverde, and P. G. Withers (2006), Polar warming in the Mars thermosphere: Seasonal variations owing to changing insolation and dust distributions, Geophys. Res. Lett., 33, L02203, doi:10.1029/2005GL024059.

South-polar features on Venus similar to those near the north pole

Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright ‘dipole’ feature surrounded by a cold ‘collar’ at its north pole. The polar dipole is a ‘double-eye’ feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus’ south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express

The upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space. Molecular emissions from the upper atmosphere (90–120 km altitude) of Venus can be used to investigate the energetics and to trace the circulation of this hitherto little-studied region. Previous spacecraft and ground-based observations of infrared emission from CO2, O2 and NO have established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus. These data, however, have left unresolved the precise altitude of the emission owing to a lack of data and of an adequate observing geometry. Here we report measurements of day-side CO2 non-local thermodynamic equilibrium emission at 4.3 µm, extending from 90 to 120 km altitude, and of night-side O2 emission extending from 95 to 100 km. The CO2 emission peak occurs at ∼115 km and varies with solar zenith angle over a range of ∼10 km. This confirms previous modelling, and permits the beginning of a systematic study of the variability of the emission. The O2 peak emission happens at 96 km ± 1 km, which is consistent with three-body recombination of oxygen atoms transported from the day side by a global thermospheric sub-solar to anti-solar circulation, as previously predicted.

Non-local thermodynamic equilibrium in general circulation models of the Martian atmosphere 1. Effects of the local thermodynamic equilibrium approximation on thermal cooling and solar heating

Calculations of CO2 thermal cooling and near-IR solar heating rates under non-local thermodynamic equilibrium (non-LTE) situations have been performed to understand and evaluate the effects of non-LTE on the energy balance of the upper atmosphere of Mars. We find that the 15-μm cooling rates can be in error if LTE is assumed above 80 km. In general, the correct non-LTE values are significantly smaller than the LTE values above about 85 km, but the magnitude and sign of the error depend on the temperature structure and the top altitude of the model and, to a lesser extent, on the collisions with atomic oxygen. A detailed analysis of the relevance of the upper boundary layer and a suggested buffer region are presented for both LTE and non-LTE. Based on general considerations of the thermal profile in the mesosphere and lower thermosphere, recommendations for general circulation models (GCM) are presented as a first guide for minimizing the LTE cooling rates inaccuracies. The error of assuming LTE on the CO2 near-IR solar heating rates is found to be about 20% at 85 km and increases strongly above this altitude. The dependences of this LTE-non-LTE difference on rate coefficients, thermal structure, surface pressure, and solar zenith angle (SZA) are studied. In contrast to the large effect of the SZA on the solar heating rate, we find it is not important for the LTE-non-LTE relative difference, which permits a simple tabulation of the non-LTE effect as a function of pressure only. A table of LTE correction factors for the solar heating rate is included for its potential use as a fast yet accurate operative scheme within GCMs.

Non-local thermodynamic equilibrium studies of the 15-μm bands of CO2 for atmospheric remote sensing

A new line-by-line non-local thermodynamic equilibrium (non-LTE) radiance model based on the GENLN2 radiative transfer code is presented. This is capable of high resolution spectral radiance calculations for the upper atmosphere. We describe the non-LTE model implementation and discuss the molecular state vibrational temperature input requirements for studies of the 15-μm ν2 bands of CO2. Monochromatic and band-integrated radiance calculations have been performed for atmospheric limb view tangent heights between 50 and 120 km for non-LTE nighttime and daytime conditions. Two model atmospheres are considered, the U.S. 1976 Standard and a subarctic summer, which show, respectively, mean and large radiance differences from a reference LTE radiance calculation. Non-LTE radiance considerations are shown to be important for the 15-μm CO2 bands for tangent heights greater than 70 km, the magnitude of the divergence from LTE values and diurnal variation being dependent on the band and kinetic temperature profile. We show that the use of the Voigt line shape, rather than the Doppler, is important for tangent heights below 85 km, and that the inclusion of the effect of overlapping lines is a consideration for tangent heights below 75 km. We present calculations of synthetic radiance spectra showing the non-LTE effect for two CO2 temperature sounding channels of instruments aboard the Upper Atmosphere Research Satellite as a demonstration of the model capability.

Local thermodynamic equilibrium of carbon dioxide in the upper atmosphere

The rotational and vibrational temperatures of the v2 and 2v2 modes of carbon dioxide in the upper atmosphere have been derived from transmission spectra measured by the ATMOS instrument on Spacelab 3 over the height range 60–110 km. The rotational and vibrational temperature profiles are found to be nearly identical, which leads to the conclusion that the CO2 (v2) level is in local thermodynamic equilibrium (LTE) within the experimental errors up to 95 km, and very nearly so to at least 110 km. The CO2 (2v2) level is found to be close to LTE up to at least the upper height limit of its retrieval, i.e., 93 km. The interpretation of those measurements using a non-LTE model [Lopez-Puertas et al., 1986] supports a fast rate for the deactivation of CO2(v2) by atomic oxygen, similar to that derived by Sharma and Wintersteiner [1990]. This provides independent confirmation, from a qualitatively different type of measurement, of the conclusion of those authors that the radiative cooling of the upper atmospheres of the Earth is around an order of magnitude greater than our 1986 model predicted. It also has implications for thermospheric cooling on the other terrestrial planets, as well as expanding the possibilities for future remote sensing of the temperature of the upper mesosphere and lower thermosphere, since the upper limit which can be sounded is higher than previously believed.

Remote sensing of the middle atmosphere with MIPAS

On 1 March 2002 the Envisat research satellite has been launched successfully into its sun-synchronous orbit. One of its instruments for atmospheric composition sounding is the Michelson Interferometer for Passive Atmospheric Sounding, a limb-scanning mid-infrared Fourier transform spectrometer. Different scientific objectives of data users require different approaches to data analysis, which are discussed. A strategy on how to validate the involved algorithms and relevant strategies is presented.

Titan’s 5-μm window: observations with the Very Large Telescope

Abstract We report on mid-resolution ( R ∼2000) spectroscopic observations of Titan, acquired in November 2000 with the Very Large Telescope and covering the range 4.75–5.07 μm. These observations provide a detailed characterization of the CO (1–0) vibrational band, clearly separating for the first time individual CO lines (P10 to P19 lines of 13 CO). They indicate that the CO/N 2 mixing ratio in Titan’s troposphere is 32±10 ppm. Comparison with photochemical models indicates that CO is not in a steady state in Titan’s atmosphere. The observations confirm that Titan’s 5-μm continuum geometric albedo is ∼0.06, and further indicates a ∼20% albedo decrease over 4.98–5.07 μm. Nonzero flux is detected at the 0.01 geometric albedo level in the saturated core of the 12 CO (1–0) band, at 4.75–4.85 μm, providing evidence for backscattering on the stratospheric haze. Finally, emission lines are detected at 4.75–4.835 μm, coinciding in position with lines from the CO(1–0) and/or CO(2–1) bands. Matching them by thermal emission would require Titan’s stratosphere to be much warmer (by ∼ 25 K at 0.1 mbar) than indicated by the methane 7.7-μm emission and the Voyager radio-occultation. We show instead that a nonthermal mechanism, namely solar-excited fluorescence, is a more plausible source for these emissions. Improved observations and laboratory measurements on the vibrational–translational relaxation of CO are needed for further interpretation of these emissions in terms of a CO stratospheric mixing ratio.