2013 .

(30 publications)

J. Leconte, F. Forget, B. Charnay, R. Wordsworth, and A. Pottier. Increased insolation threshold for runaway greenhouse processes on Earth-like planets. Nature, 504:268-271, 2013. [ bib | DOI | arXiv | ADS link ]

The increase in solar luminosity over geological timescales should warm the Earth's climate, increasing water evaporation, which will in turn enhance the atmospheric greenhouse effect. Above a certain critical insolation, this destabilizing greenhouse feedback can `run away' until the oceans have completely evaporated. Through increases in stratospheric humidity, warming may also cause evaporative loss of the oceans to space before the runaway greenhouse state occurs. The critical insolation thresholds for these processes, however, remain uncertain because they have so far been evaluated using one-dimensional models that cannot account for the dynamical and cloud feedback effects that are key stabilizing features of the Earth's climate. Here we use a three-dimensional global climate model to show that the insolation threshold for the runaway greenhouse state to occur is about 375 W m-2, which is significantly higher than previously thought. Our model is specifically developed to quantify the climate response of Earth-like planets to increased insolation in hot and extremely moist atmospheres. In contrast with previous studies, we find that clouds have a destabilizing feedback effect on the long-term warming. However, subsident, unsaturated regions created by the Hadley circulation have a stabilizing effect that is strong enough to shift the runaway greenhouse limit to higher values of insolation than are inferred from one-dimensional models. Furthermore, because of wavelength-dependent radiative effects, the stratosphere remains sufficiently cold and dry to hamper the escape of atmospheric water, even at large fluxes. This has strong implications for the possibility of liquid water existing on Venus early in its history, and extends the size of the habitable zone around other stars.

C. Pilorget, C. S. Edwards, B. L. Ehlmann, F. Forget, and E. Millour. Material ejection by the cold jets and temperature evolution of the south seasonal polar cap of Mars from THEMIS/CRISM observations and implications for surface properties. Journal of Geophysical Research (Planets), 118:2520-2536, 2013. [ bib | DOI | ADS link ]

As the seasonal CO2 ice polar caps of Mars retreat during spring, dark spots appear on the ice in some specific regions. These features are thought to result from basal sublimation of the transparent CO2 ice followed by ejection of regolith-type material, which then covers the ice. We have used Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) reflectance data, Thermal Emission Imaging System (THEMIS) visible images, and THEMIS-derived temperature retrievals along with a thermal numerical model to constrain the physical and compositional characteristics of the seasonal cap for several areas exhibiting dark spots at both high spatial and temporal resolutions. Data analysis suggests an active period of material ejection (before solar longitude (Ls) 200), accumulation around the ejection points, and spreading of part of the ejected material over the whole area, followed by a period where no significant amount of material is ejected, followed by complete defrosting ( Ls 245). Dark material thickness on top of the CO2 ice is estimated to range from a few hundreds of microns to a few millimeters in the warmest spots, based on numerical modeling combined with the observed temperature evolution. The nature of the venting process and the amount of material that is moved lead to the conclusion that it could have an important impact on the surface physical properties.

F. González-Galindo, J.-Y. Chaufray, M. A. López-Valverde, G. Gilli, F. Forget, F. Leblanc, R. Modolo, S. Hess, and M. Yagi. Three-dimensional Martian ionosphere model: I. The photochemical ionosphere below 180 km. Journal of Geophysical Research (Planets), 118:2105-2123, 2013. [ bib | DOI | ADS link ]

We describe the Mars ionosphere with unprecedented detail in 3-D, as simulated by a Mars general circulation model (the Laboratoire de Météorologie Dynamique Mars GCM), and compare it with recent measurements. The model includes a number of recent extensions and improvements. Different simulations for a full Martian year have been performed. The electron density at the main ionospheric peak is shown to vary with the Sun-Mars distance and with the solar variability, both in the long-term (11 year solar cycle) and on shorter temporal scales (solar rotation). The main electronic peak is shown to be located at the same pressure level during all the Martian year. As a consequence, its altitude varies with latitude, local time, and season according to the natural expansions and fluctuations of the neutral atmosphere, in agreement with previous models. The model predicts a nighttime ionosphere due only to photochemistry. The simulated ionosphere close to the evening terminator is in agreement with observations. No effort has been made to explain the patchy ionosphere observed in the deep nightside. We have compared the modeled ionosphere with Mars Global Surveyor and Mars Advanced Radar for Subsurface and Ionosphere Sounding data. The model reproduces the solar zenith angle variability of the electron density and the altitude of the peak, although it underestimates the electron density at the main peak by about 20%. The electron density at the secondary peak is strongly underestimated by the model, probably due to a very crude representation of the X-ray solar flux. This is one of the aspects that needs a revision in future versions of the model.

E. Marcq and S. Lebonnois. Simulations of the latitudinal variability of CO-like and OCS-like passive tracers below the clouds of Venus using the Laboratoire de Météorologie Dynamique GCM. Journal of Geophysical Research (Planets), 118:1983-1990, 2013. [ bib | DOI | ADS link ]

The lower atmosphere of Venus below the clouds is a transitional region between the relatively calm lowermost scale height and the superrotating atmosphere in the cloud region and above. Any observational constraint is then welcome to help in the development of general circulation models of Venus, a difficult task considering the thickness of its atmosphere. Starting from a state-of-the-art 3-D Venus General Circulation Model (GCM), we have included passive tracers in order to investigate the latitudinal variability of two minor gaseous species, carbonyl sulfide (OCS) and carbon monoxide (CO), whose vertical profiles and mixing ratios are known to vary with latitude between 30 and 40km. The relaxation to chemical equilibrium is crudely parametrized through a vertically uniform time scale τ. A satisfactory agreement with available observations is obtained with 108CO5108 s and 107OCS108 s. These results, in addition to validating the general circulation below the clouds, are also helpful in characterizing the chemical kinetics of Venus' atmosphere. This complements the much more sophisticated chemical models which focus more on thermodynamical equilibrium.

G. Monteil, S. Houweling, A. Butz, S. Guerlet, D. Schepers, O. Hasekamp, C. Frankenberg, R. Scheepmaker, I. Aben, and T. Röckmann. Journal of Geophysical Research (Atmospheres), 118:11, 2013. [ bib | DOI | ADS link ]

Over the past decade the development of Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) retrievals has increased the interest in the use of satellite measurements for studying the global sources and sinks of methane. Meanwhile, measurements are becoming available from the more advanced Greenhouse Gases Observing Satellite (GOSAT). The aim of this study is to investigate the application of GOSAT retrievals to inverse modeling, for which we make use of the TM5-4DVAR inverse modeling framework. Inverse modeling calculations are performed using data from two different retrieval approaches: a full physics and a lightpath proxy ratio method. The performance of these inversions is analyzed in comparison with inversions using SCIAMACHY retrievals and measurements from the National Oceanic and Atmospheric Administration-Earth System Research Laboratory flask-sampling network. In addition, we compare the inversion results against independent surface, aircraft, and total-column measurements. Inversions with GOSAT data show good agreement with surface measurements, whereas for SCIAMACHY a similar performance can only be achieved after significant bias corrections. Some inconsistencies between surface and total-column methane remain in the Southern Hemisphere. However, comparisons with measurements from the Total Column Carbon Observing Network in situ Fourier transform spectrometer network indicate that those may be caused by systematic model errors rather than by shortcomings in the GOSAT retrievals. The global patterns of methane emissions derived from SCIAMACHY (with bias correction) and GOSAT retrievals are in remarkable agreement and allow an increased resolution of tropical emissions. The satellite inversions increase tropical methane emission by 30 to 60 TgCH4/yr compared to initial a priori estimates, partly counterbalanced by reductions in emissions at midlatitudes to high latitudes.

I. B. Smith, J. W. Holt, A. Spiga, A. D. Howard, and G. Parker. The spiral troughs of Mars as cyclic steps. Journal of Geophysical Research (Planets), 118:1835-1857, 2013. [ bib | DOI | ADS link ]

combine observations of stratigraphy, morphology, and atmospheric processes to relate the spiral troughs on Mars' polar layered deposits to a class of features known as cyclic steps. Cyclic steps are quasi-stable, repeating, and upstream-migrating bed forms that have been studied in terrestrial and submarine environments. The repeating pattern is bounded by hydraulic jumps, which act to stabilize the form. We use radar stratigraphy from the Shallow Radar instrument on Mars Reconnaissance Orbiter to examine trough evolution and constrain lateral transport. We examine visible images from the Thermal Emission Imaging System and observe low-altitude clouds that we interpret to be the result of katabatic jumps, i.e., the Aeolian counterpart of hydraulic jumps in open channel flow. We then devise a theoretical framework for understanding the origin of the spiral troughs that agree with 10 criteria that should be explained for any scenario to satisfactorily model the spiral troughs. Finally, we use Froude and geometrical analysis to estimate the rate of upstream migration caused by katabatic winds for the spiral troughs.

B. Charnay, F. Forget, R. Wordsworth, J. Leconte, E. Millour, F. Codron, and A. Spiga. Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3-D GCM. Journal of Geophysical Research (Atmospheres), 118:10, 2013. [ bib | DOI | arXiv | ADS link ]

Different solutions have been proposed to solve the “faint young Sun problem,” defined by the fact that the Earth was not fully frozen during the Archean despite the fainter Sun. Most previous studies were performed with simple 1-D radiative convective models and did not account well for the clouds and ice-albedo feedback or the atmospheric and oceanic transport of energy. We apply a global climate model (GCM) to test the different solutions to the faint young Sun problem. We explore the effect of greenhouse gases (CO2 and CH4), atmospheric pressure, cloud droplet size, land distribution, and Earth's rotation rate. We show that neglecting organic haze, 100 mbar of CO2 with 2 mbar of CH4 at 3.8 Ga and 10 mbar of CO2 with 2 mbar of CH4 at 2.5 Ga allow a temperate climate (mean surface temperature between 10degC and 20degC). Such amounts of greenhouse gases remain consistent with the geological data. Removing continents produces a warming lower than +4degC. The effect of rotation rate is even more limited. Larger droplets (radii of 17 μm versus 12 μm) and a doubling of the atmospheric pressure produce a similar warming of around +7degC. In our model, ice-free water belts can be maintained up to 25degN/S with less than 1 mbar of CO2 and no methane. An interesting cloud feedback appears above cold oceans, stopping the glaciation. Such a resistance against full glaciation tends to strongly mitigate the faint young Sun problem.

A. Butz, S. Guerlet, O. P. Hasekamp, A. Kuze, and H. Suto. Using ocean-glint scattered sunlight as a diagnostic tool for satellite remote sensing of greenhouse gases. Atmospheric Measurement Techniques, 6:2509-2520, 2013. [ bib | DOI | ADS link ]

Spectroscopic measurements of sunlight backscattered by the Earth's surface is a technique widely used for remote sensing of atmospheric constituent concentrations from space. Thereby, remote sensing of greenhouse gases poses particularly challenging accuracy requirements for instrumentation and retrieval algorithms which, in general, suffer from various error sources. Here, we investigate a method that helps disentangle sources of error for observations of sunlight backscattered from the glint spot on the ocean surface. The method exploits the backscattering characteristics of the ocean surface, which is bright for glint geometry but dark for off-glint angles. This property allows for identifying a set of clean scenes where light scattering due to particles in the atmosphere is negligible such that uncertain knowledge of the lightpath can be excluded as a source of error. We apply the method to more than 3 yr of ocean-glint measurements by the Thermal And Near infrared Sensor for carbon Observation (TANSO) Fourier Transform Spectrometer (FTS) onboard the Greenhouse Gases Observing Satellite (GOSAT), which aims at measuring carbon dioxide (CO2) and methane (CH4) concentrations. The proposed method is able to clearly monitor recent improvements in the instrument calibration of the oxygen (O2) A-band channel and suggests some residual uncertainty in our knowledge about the instrument. We further assess the consistency of CO2 retrievals from several absorption bands between 6400 cm-1 (1565 nm) and 4800 cm-1 (2100 nm) and find that the absorption bands commonly used for monitoring of CO2 dry air mole fractions from GOSAT allow for consistency better than 1.5 ppm. Usage of other bands reveals significant inconsistency among retrieved CO2 concentrations pointing at inconsistency of spectroscopic parameters.

S. Basu, S. Guerlet, A. Butz, S. Houweling, O. Hasekamp, I. Aben, P. Krummel, P. Steele, R. Langenfelds, M. Torn, S. Biraud, B. Stephens, A. Andrews, and D. Worthy. Global CO2 fluxes estimated from GOSAT retrievals of total column CO2. Atmospheric Chemistry & Physics, 13:8695-8717, 2013. [ bib | DOI | ADS link ]

We present one of the first estimates of the global distribution of CO2 surface fluxes using total column CO2 measurements retrieved by the SRON-KIT RemoTeC algorithm from the Greenhouse gases Observing SATellite (GOSAT). We derive optimized fluxes from June 2009 to December 2010. We estimate fluxes from surface CO2 measurements to use as baselines for comparing GOSAT data-derived fluxes. Assimilating only GOSAT data, we can reproduce the observed CO2 time series at surface and TCCON sites in the tropics and the northern extra-tropics. In contrast, in the southern extra-tropics GOSAT XCO<SUB>2</SUB> leads to enhanced seasonal cycle amplitudes compared to independent measurements, and we identify it as the result of a land-sea bias in our GOSAT XCO<SUB>2</SUB> retrievals. A bias correction in the form of a global offset between GOSAT land and sea pixels in a joint inversion of satellite and surface measurements of CO2 yields plausible global flux estimates which are more tightly constrained than in an inversion using surface CO2 data alone. We show that assimilating the bias-corrected GOSAT data on top of surface CO2 data (a) reduces the estimated global land sink of CO2, and (b) shifts the terrestrial net uptake of carbon from the tropics to the extra-tropics. It is concluded that while GOSAT total column CO2 provide useful constraints for source-sink inversions, small spatiotemporal biases - beyond what can be detected using current validation techniques - have serious consequences for optimized fluxes, even aggregated over continental scales.

S. M. Clifford, K. Yoshikawa, S. Byrne, W. Durham, D. Fisher, F. Forget, M. Hecht, P. Smith, L. Tamppari, T. Titus, and R. Zurek. Introduction to the fifth Mars Polar Science special issue: Key questions, needed observations, and recommended investigations. Icarus, 225:864-868, 2013. [ bib | DOI | ADS link ]

K. E. Scanlon, J. W. Head, J.-B. Madeleine, R. D. Wordsworth, and F. Forget. Orographic precipitation in valley network headwaters: Constraints on the ancient Martian atmosphere. Geophysical Research Letters, 40:4182-4187, 2013. [ bib | DOI | ADS link ]

We examine the Martian valley networks in the framework of topographic influences on precipitation. We use an analytical model and the Laboratoire de Météorologie Dynamique (LMD) early Mars global circulation model (GCM) to explore the local-scale distribution of orographically forced precipitation as a function of atmospheric pressure. In simulations with 500 mbar and 1 bar CO2 atmospheres, orographic lifting results in enhanced snowfall upslope of the observed valley network tributaries. Our framework also suggests that a 2 bar atmosphere cannot create the observed valley pattern at the highest-relief valley network, Warrego Valles. As in previous work, the GCM does not generate temperatures warm enough for rain or significant snowmelt in the highlands with CO2 greenhouse warming alone. Thus while transient periods of unusual warming are still required to melt the deposits and carve the valleys, our model predicts snow deposition in the correct locations.

A. Colaïtis, A. Spiga, F. Hourdin, C. Rio, F. Forget, and E. Millour. A thermal plume model for the Martian convective boundary layer. Journal of Geophysical Research (Planets), 118:1468-1487, 2013. [ bib | DOI | arXiv | ADS link ]

The Martian planetary boundary layer (PBL) is a crucial component of the Martian climate system. Global climate models (GCMs) and mesoscale models (MMs) lack the resolution to predict PBL mixing which is therefore parameterized. Here we propose to adapt the “thermal plume” model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single-column models. The aim of this physically based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in large-eddy simulations (LESs). We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near-surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking into account stability and turbulent gustiness to calculate surface-atmosphere fluxes. Those new parameterizations for the surface and mixed layers are validated against near-surface lander measurements. Using a thermal plume model moreover enables a first-order estimation of key turbulent quantities (e.g., PBL height and convective plume velocity) in Martian GCMs and MMs without having to run costly LESs.

F. Forget. On the probability of habitable planets. International Journal of Astrobiology, 12:177-185, 2013. [ bib | DOI | arXiv | ADS link ]

In the past 15 years, astronomers have revealed that a significant fraction of the stars should harbour planets and that it is likely that terrestrial planets are abundant in our galaxy. Among these planets, how many are habitable, i.e. suitable for life and its evolution? These questions have been discussed for years and we are slowly making progress. Liquid water remains the key criterion for habitability. It can exist in the interior of a variety of planetary bodies, but it is usually assumed that liquid water at the surface interacting with rocks and light is necessary for emergence of a life able to modify its environment and evolve. The first key issue is thus to understand the climatic conditions allowing surface liquid water assuming a suitable atmosphere. These have been studied with global mean one-dimensional (1D) models which have defined the `classical habitable zone', the range of orbital distances within which worlds can maintain liquid water on their surfaces (Kasting et al. 1993). A new generation of 3D climate models based on universal equations and tested on bodies in the solar system are now available to explore with accuracy climate regimes that could locally allow liquid water. The second key issue is now to better understand the processes which control the composition and the evolution of the atmospheres of exoplanets, and in particular the geophysical feedbacks that seem to be necessary to maintain a continuously habitable climate. From that point of view, it is not impossible that the Earth's case may be special and uncommon.

N. C. Parazoo, K. Bowman, C. Frankenberg, J.-E. Lee, J. B. Fisher, J. Worden, D. B. A. Jones, J. Berry, G. J. Collatz, I. T. Baker, M. Jung, J. Liu, G. Osterman, C. O'Dell, A. Sparks, A. Butz, S. Guerlet, Y. Yoshida, H. Chen, and C. Gerbig. Interpreting seasonal changes in the carbon balance of southern Amazonia using measurements of XCO2 and chlorophyll fluorescence from GOSAT. Geophysical Research Letters, 40:2829-2833, 2013. [ bib | DOI | ADS link ]

Amazon forests exert a major influence on the global carbon cycle, but quantifying the impact is complicated by diverse landscapes and sparse data. Here we examine seasonal carbon balance in southern Amazonia using new measurements of column-averaged dry air mole fraction of CO2 (XCO2) and solar induced chlorophyll fluorescence (SIF) from the Greenhouse Gases Observing Satellite (GOSAT) from July 2009 to December 2010. SIF, which reflects gross primary production (GPP), is used to disentangle the photosynthetic component of land-atmosphere carbon exchange. We find that tropical transitional forests in southern Amazonia exhibit a pattern of low XCO2 during the wet season and high XCO2 in the dry season that is robust to retrieval methodology and with seasonal amplitude double that of cerrado ecosystems to the east (4 ppm versus 2 ppm), including enhanced dilution of 2.5 ppm in the wet season. Concomitant measurements of SIF, which are inversely correlated with XCO2 in southern Amazonia (r = -0.53, p 0.001), indicate that the enhanced variability is driven by seasonal changes in GPP due to coupling of strong vertical mixing with seasonal changes in underlying carbon exchange. This finding is supported by forward simulations of the Goddard Chemistry Transport Model (GEOS-Chem) which show that local carbon uptake in the wet season and loss in the dry season due to emissions by ecosystem respiration and biomass burning produces best agreement with observed XCO2. We conclude that GOSAT provides critical measurements of carbon exchange in southern Amazonia, but more samples are needed to examine moist Amazon forests farther north.

J. Leconte, F. Forget, B. Charnay, R. Wordsworth, F. Selsis, E. Millour, and A. Spiga. 3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability, and habitability. Astronomy Astrophysics, 554:A69, 2013. [ bib | DOI | arXiv | ADS link ]

The inner edge of the classical habitable zone is often defined by the critical flux needed to trigger the runaway greenhouse instability. This 1D notion of a critical flux, however, may not be all that relevant for inhomogeneously irradiated planets, or when the water content is limited (land planets). Based on results from our 3D global climate model, we present general features of the climate and large-scale circulation on close-in terrestrial planets. We find that the circulation pattern can shift from super-rotation to stellar/anti stellar circulation when the equatorial Rossby deformation radius significantly exceeds the planetary radius, changing the redistribution properties of the atmosphere. Using analytical and numerical arguments, we also demonstrate the presence of systematic biases among mean surface temperatures and among temperature profiles predicted from either 1D or 3D simulations. After including a complete modeling of the water cycle, we further demonstrate that two stable climate regimes can exist for land planets closer than the inner edge of the classical habitable zone. One is the classical runaway state where all the water is vaporized, and the other is a collapsed state where water is captured in permanent cold traps. We identify this “moist” bistability as the result of a competition between the greenhouse effect of water vapor and its condensation on the night side or near the poles, highlighting the dynamical nature of the runaway greenhouse effect. We also present synthetic spectra showing the observable signature of these two states. Taking the example of two prototype planets in this regime, namely Gl 581 c and HD 85512 b, we argue that depending on the rate of water delivery and atmospheric escape during the life of these planets, they could accumulate a significant amount of water ice at their surface. If such a thick ice cap is present, various physical mechanisms observed on Earth (e.g., gravity driven ice flows, geothermal flux) should come into play to produce long-lived liquid water at the edge and/or bottom of the ice cap. Consequently, the habitability of planets at smaller orbital distance than the inner edge of the classical habitable zone cannot be ruled out. Transiting planets in this regime represent promising targets for upcoming exoplanet characterization observatories, such as EChO and JWST.

R. T. Clancy, B. J. Sandor, M. J. Wolff, M. D. Smith, F. LefèVre, J.-B. Madeleine, F. Forget, S. L. Murchie, F. P. Seelos, K. D. Seelos, H. Nair, A. D. Toigo, D. Humm, D. M. Kass, A. KleinböHl, and N. Heavens. Correction to “Extensive MRO CRISM observations of 1.27 m O2 airglow in Mars polar night and their comparison to MRO MCS temperature profiles and LMD GCM simulations”. Journal of Geophysical Research (Planets), 118:1148-1154, 2013. [ bib | DOI | ADS link ]

S. Guerlet, A. Butz, D. Schepers, S. Basu, O. P. Hasekamp, A. Kuze, T. Yokota, J.-F. Blavier, N. M. Deutscher, D. W. T. Griffith, F. Hase, E. Kyro, I. Morino, V. Sherlock, R. Sussmann, A. Galli, and I. Aben. Impact of aerosol and thin cirrus on retrieving and validating XCO2 from GOSAT shortwave infrared measurements. Journal of Geophysical Research (Atmospheres), 118:4887-4905, 2013. [ bib | DOI | ADS link ]

Inadequate treatment of aerosol scattering can be a significant source of error when retrieving column-averaged dry-air mole fractions of CO2 (XCO2) from space-based measurements of backscattered solar shortwave radiation. We have developed a retrieval algorithm, RemoTeC, that retrieves three aerosol parameters (amount, size, and height) simultaneously with XCO2. Here we evaluate the ability of RemoTeC to account for light path modifications by clouds, subvisual cirrus, and aerosols when retrieving XCO2 from Greenhouse Gases Observing Satellite (GOSAT) Thermal and Near-infrared Sensor for carbon Observation (TANSO)-Fourier Transform Spectrometer (FTS) measurements. We first evaluate a cloud filter based on measurements from the Cloud and Aerosol Imager and a cirrus filter that uses radiances measured by TANSO-FTS in the 2 micron spectral region, with strong water absorption. For the cloud-screened scenes, we then evaluate errors due to aerosols. We find that RemoTeC is well capable of accounting for scattering by aerosols for values of aerosol optical thickness at 750 nm up to 0.25. While no significant correlation of errors is found with albedo, correlations are found with retrieved aerosol parameters. To further improve the XCO2 accuracy, we propose and evaluate a bias correction scheme. Measurements from 12 ground-based stations of the Total Carbon Column Observing Network (TCCON) are used as a reference in this study. We show that spatial colocation criteria may be relaxed using additional constraints based on modeled XCO2 gradients, to increase the size and diversity of validation data and provide a more robust evaluation of GOSAT retrievals. Global-scale validation of satellite data remains challenging and would be improved by increasing TCCON coverage.

S. Guerlet, S. Basu, A. Butz, M. Krol, P. Hahne, S. Houweling, O. P. Hasekamp, and I. Aben. Reduced carbon uptake during the 2010 Northern Hemisphere summer from GOSAT. Geophysical Research Letters, 40:2378-2383, 2013. [ bib | DOI | ADS link ]

Column-averaged dry air mole fractions of carbon dioxide (XCO2) measured by the Greenhouse Gases Observing Satellite (GOSAT) reveal significant interannual variation (IAV) of CO2uptake during the Northern Hemisphere summer between 2009 and 2010. The XCO2drawdown in 2010 is shallower than in 2009 by 2.4 ppm and 3.0 ppm over North America and Eurasia, respectively. Reduced carbon uptake in the summer of 2010 is most likely due to the heat wave in Eurasia driving biospheric fluxes and fire emissions. A joint inversion of GOSAT and surface data estimates an integrated biospheric and fire emission anomaly in April-September of 0.89 0.20 PgC over Eurasia. In contrast, inversions of surface measurements alone fail to replicate the observed XCO2IAV and underestimate emission IAV over Eurasia. This shows the value of GOSAT XCO2in constraining the response of land-atmosphere exchange of CO2 to climate events.

A. Spiga, J. Faure, J.-B. Madeleine, A. Määttänen, and F. Forget. Rocket dust storms and detached dust layers in the Martian atmosphere. Journal of Geophysical Research (Planets), 118:746-767, 2013. [ bib | DOI | arXiv | ADS link ]

Airborne dust is the main climatic agent in the Martian environment. Local dust storms play a key role in the dust cycle; yet their life cycle is poorly known. Here we use mesoscale modeling that includes the transport of radiatively active dust to predict the evolution of a local dust storm monitored by OMEGA on board Mars Express. We show that the evolution of this dust storm is governed by deep convective motions. The supply of convective energy is provided by the absorption of incoming sunlight by dust particles, rather than by latent heating as in moist convection on Earth. We propose to use the terminology “rocket dust storm,” or conio-cumulonimbus, to describe those storms in which rapid and efficient vertical transport takes place, injecting dust particles at high altitudes in the Martian troposphere (30-50 km). Combined to horizontal transport by large-scale winds, rocket dust storms produce detached layers of dust reminiscent of those observed with Mars Global Surveyor and Mars Reconnaissance Orbiter. Since nighttime sedimentation is less efficient than daytime convective transport, and the detached dust layers can convect during the daytime, these layers can be stable for several days. The peak activity of rocket dust storms is expected in low-latitude regions at clear seasons (late northern winter to late northern summer), which accounts for the high-altitude tropical dust maxima unveiled by Mars Climate Sounder. Dust-driven deep convection has strong implications for the Martian dust cycle, thermal structure, atmospheric dynamics, cloud microphysics, chemistry, and robotic and human exploration.

L. Maltagliati, F. Montmessin, O. Korablev, A. Fedorova, F. Forget, A. Määttänen, F. Lefèvre, and J.-L. Bertaux. Annual survey of water vapor vertical distribution and water-aerosol coupling in the martian atmosphere observed by SPICAM/MEx solar occultations. Icarus, 223:942-962, 2013. [ bib | DOI | ADS link ]

The vertical distribution of water vapor is a very important diagnostic to determine the physical and chemical processes that drive the martian water cycle. Yet, very few direct measurements have been performed so far, and our knowledge of the H2O vertical distribution on Mars relies on General Circulation Models (GCMs). The study presented here follows for the first time the evolution of water vapor profile during a martian year. 120 profiles, obtained by the SPICAM spectrometer onboard Mars Express with the solar occultations technique, are retrieved. They cover the northern spring-summer season and the southern spring of Mars Year (MY) 29. The seasonal evolution of H2O mixing ratio vertical distribution reveals its strong dynamism, especially during southern spring. There are significant discrepancies with the predictions of the General Circulation Model developed at the Laboratoire de Météorologie Dynamique (LMD-GCM). The LMD-GCM underestimates the water vapor content in the middle atmosphere. The measured profiles also exhibit often abrupt temporal variations and a greater variety of shapes, with the frequent presence of detached layers. We believe that the model underestimates the strength of the coupling between water vapor and aerosols, whose slant optical depth profile is obtained by SPICAM simultaneously with H2O. The SPICAM measurements can be grouped according to the mutual behavior of the two profiles. Individual features are often related too. The presence of water supersaturation and of correlated aerosol-water detached layers highlights the role of water ice clouds as a favorable location for the dust-water coupling. The water vapor vertical distribution is more reactive than expected to regional perturbations, which can propagate rapidly through the atmosphere, create abrupt water vapor and aerosol upsurges and influence the large-scale vertical evolution of these two constituents. This phenomenon has been observed thrice during MY29. The martian annual water cycle revealed by the SPICAM profiles exhibits a different behavior with respect to nadir observations. This result suggests a generally weak connection between the upper atmosphere and the lower atmospheric layers, to whom the nadir measurements are most sensitive and that are not resolved by SPICAM occultations, and hints at a significant influence of surface-atmosphere interactions on the water cycle.

T. C. Brothers, J. W. Holt, and A. Spiga. Orbital radar, imagery, and atmospheric modeling reveal an aeolian origin for Abalos Mensa, Mars. Geophysical Research Letters, 40:1334-1339, 2013. [ bib | DOI | ADS link ]

Icy deposits surrounding Planum Boreum, Mars, contain crucial information for deciphering paleoclimate and past geologic processes at the martian north pole. One such deposit, Abalos Mensa, is an enigmatic wedge of material located near the ˜ 1 km high Rupes Tenuis. Its unique location and lobate morphology have fostered formation hypotheses that assume either fluvial or aeolian erosion of a once-larger ice deposit. The aeolian scenario posed previously requires impact shielding of ancient basal unit material to provide an erosional remnant which seeds later deposition, while the fluvial hypotheses invoke cryovolcanism beneath the younger north polar layered deposits (NPLD) and associated outflow to erode the adjacent chasmata. Here we combine newly available radar sounding data, high-resolution imagery, digital elevation models, and atmospheric modeling to examine internal structure and infer both the mechanisms for, and timing of, Abalos Mensa formation. From this integrative approach, we conclude that Abalos Mensa formed as a distinct feature via atmospheric deposition following erosion of Rupes Tenuis and grew concurrently with the rest of Planum Boreum as the NPLD accumulated. The required processes are consistent with those observed today: no exotic phenomena (cryovolcanism, fluvial activity, or impact shielding) appear necessary to explain the formation of Abalos Mensa.

B. Gans, Z. Peng, N. Carrasco, D. Gauyacq, S. Lebonnois, and P. Pernot. Impact of a new wavelength-dependent representation of methane photolysis branching ratios on the modeling of Titans atmospheric photochemistry. Icarus, 223:330-343, 2013. [ bib | DOI | ADS link ]

A new wavelength-dependent model for CH4 photolysis branching ratios is proposed, based on the values measured recently by Gans et al. (Gans, B. et al. [2011]. Phys. Chem. Chem. Phys. 13, 8140-8152). We quantify the impact of this representation on the predictions of a photochemical model of Titans atmosphere, on their precision, and compare to earlier representations. Although the observed effects on the mole fraction of the species are small (never larger than 50%), it is possible to draw some recommendations for further studies: (i) the Ly-α branching ratios of Wang et al. (Wang, J.H. et al. [2000]. J. Chem. Phys. 113, 4146-4152) used in recent models overestimate the CH2:CH3 ratio, a factor to which a lot of species are sensitive; (ii) the description of out-of-Ly-α branching ratios by the 100% CH3 scenario has to be avoided, as it can bias significantly the mole fractions of some important species (C3H8); and (iii) complementary experimental data in the 130-140 nm range would be useful to constrain the models in the Ly-α deprived 500-700 km altitude range.

L. Kerber, F. Forget, J.-B. Madeleine, R. Wordsworth, J. W. Head, and L. Wilson. The effect of atmospheric pressure on the dispersal of pyroclasts from martian volcanoes. Icarus, 223:149-156, 2013. [ bib | DOI | ADS link ]

A planetary global circulation model developed by the Laboratoire de Météorologie Dynamique (LMD) was used to simulate explosive eruptions of ancient martian volcanoes into paleo-atmospheres with higher atmospheric pressures than that of present-day Mars. Atmospheric pressures in the model were varied between 50 mbar and 2 bars. In this way it was possible to investigate the sensitivity of the volcanic plume dispersal model to atmospheric pressure. It was determined that the model has a sensitivity to pressure that is similar to its sensitivity to other atmospheric parameters such as planetary obliquity and season of eruption. Higher pressure atmospheres allow volcanic plumes to convect to higher levels, meaning that volcanic pyroclasts have further to fall through the atmosphere. Changes in atmospheric circulation due to pressure cause pyroclasts to be dispersed in narrower latitudinal bands compared with pyroclasts in a modern atmosphere. Atmospheric winds are generally slower under higher pressure regimes; however, the final distance traveled by the pyroclasts depends greatly on the location of the volcano and can either increase or decrease with pressure. The directionality of the pyroclast transport, however, remains dominantly east or west along lines of latitude. Augmentation of the atmospheric pressure improves the fit between possible ash sources Arsia and Pavonis Mons and the Medusae Fossae Formation, a hypothesized ash deposit.

T. McDunn, S. Bougher, J. Murphy, A. KleinböHl, F. Forget, and M. Smith. Characterization of middle-atmosphere polar warming at Mars. Journal of Geophysical Research (Planets), 118:161-178, 2013. [ bib | DOI | ADS link ]

We characterize middle-atmosphere polar warming (PW) using nearly three Martian years of temperature observations by the Mars Climate Sounder. We report the observed structure of PW and share hypotheses as to possible explanations, which have yet to be tested with global dynamical models. In the data, PW manifested between p = 15 Pa and p = 4.8×10-3 Pa. The latitude where PW maximized shifted poleward with decreasing pressure. The nightside magnitude was larger than the dayside magnitude. The maximum nightside magnitudes ranged from 22 to 67 K. As expected, the annual maximum magnitude in the north occurred during late-local fall to middle-local winter. In the south it occurred during late-local winter. Also as expected, the maximum magnitude near MY 28's southern winter solstice was smaller than that at that same year's northern winter solstice, when a global dust storm was occurring. Unexpectedly, the maximum magnitude at southern winter solstice was comparable to that at northern winter solstice for both MY 29 and MY 30, years that did not experience global dust storms but certainly experienced greater dust loading during Ls = 270deg than Ls = 90deg. Another unexpected result was a hemispheric asymmetry in PW magnitude during most of the observed equinoxes. This paper also provides tables of (1) averaged temperatures as a function of latitude, pressure, and season, and (2) the maximum polar warming features as a function of pressure and season. These tables can be used to validate GCM calculations of middle-atmosphere temperatures and constrain calculations of unobserved winds.

S. Oshchepkov, A. Bril, T. Yokota, P. O. Wennberg, N. M. Deutscher, D. Wunch, G. C. Toon, Y. Yoshida, C. W. O'Dell, D. Crisp, C. E. Miller, C. Frankenberg, A. Butz, I. Aben, S. Guerlet, O. Hasekamp, H. Boesch, A. Cogan, R. Parker, D. Griffith, R. Macatangay, J. Notholt, R. Sussmann, M. Rettinger, V. Sherlock, J. Robinson, E. Kyrö, P. Heikkinen, D. G. Feist, I. Morino, N. Kadygrov, D. Belikov, S. Maksyutov, T. Matsunaga, O. Uchino, and H. Watanabe. Effects of atmospheric light scattering on spectroscopic observations of greenhouse gases from space. Part 2: Algorithm intercomparison in the GOSAT data processing for CO2 retrievals over TCCON sites. Journal of Geophysical Research (Atmospheres), 118:1493-1512, 2013. [ bib | DOI | ADS link ]

This report is the second in a series of companion papers describing the effects of atmospheric light scattering in observations of atmospheric carbon dioxide (CO2) by the Greenhouse gases Observing SATellite (GOSAT), in orbit since 23 January 2009. Here we summarize the retrievals from six previously published algorithms; retrieving column-averaged dry air mole fractions of CO2 First, we compare data products from each algorithm with ground-based remote sensing observations by Total Carbon Column Observing Network (TCCON). Our GOSAT-TCCON coincidence criteria select satellite observations within a 5deg radius of 11 TCCON sites. We have compared the GOSAT-TCCON XCO2 regression slope, standard deviation, correlation and determination coefficients, and global and station-to-station biases. The best agreements with TCCON measurements were detected for NIES 02.xx and RemoTeC. Next, the impact of atmospheric light scattering on XCO2 retrievals was estimated for each data product using scan by scan retrievals of light path modification with the photon path length probability density function (PPDF) method. After a cloud pre-filtering test, approximately 25% of GOSAT soundings processed by NIES 02.xx, ACOS B2.9, and UoL-FP: 3G and 35% processed by RemoTeC were found to be contaminated by atmospheric light scattering. This study suggests that NIES 02.xx and ACOS B2.9 algorithms tend to overestimate aerosol amounts over bright surfaces, resulting in an underestimation of XCO2 for GOSAT observations. Cross-comparison between algorithms shows that ACOS B2.9 agrees best with NIES 02.xx and UoL-FP: 3G while RemoTeC XCO2 retrievals are in a best agreement with NIES PPDF-D.

M. Reuter, H. Bösch, H. Bovensmann, A. Bril, M. Buchwitz, A. Butz, J. P. Burrows, C. W. O'Dell, S. Guerlet, O. Hasekamp, J. Heymann, N. Kikuchi, S. Oshchepkov, R. Parker, S. Pfeifer, O. Schneising, T. Yokota, and Y. Yoshida. A joint effort to deliver satellite retrieved atmospheric CO2 concentrations for surface flux inversions: the ensemble median algorithm EMMA. Atmospheric Chemistry & Physics, 13:1771-1780, 2013. [ bib | DOI | ADS link ]

We analyze an ensemble of seven XCO2 retrieval algorithms for SCIAMACHY (scanning imaging absorption spectrometer of atmospheric chartography) and GOSAT (greenhouse gases observing satellite). The ensemble spread can be interpreted as regional uncertainty and can help to identify locations for new TCCON (total carbon column observing network) validation sites. Additionally, we introduce the ensemble median algorithm EMMA combining individual soundings of the seven algorithms into one new data set. The ensemble takes advantage of the algorithms' independent developments. We find ensemble spreads being often 1 ppm but rising up to 2 ppm especially in the tropics and all individual algorithms with TCCON and CarbonTracker model results (potential outliers, north/south gradient, seasonal (peak-to-peak) amplitude, standard deviation of the difference). Our findings show that EMMA is a promising candidate for inverse modeling studies. Compared to CarbonTracker, the satellite retrievals find consistently larger north/south gradients (by 0.3-0.9 ppm) and seasonal amplitudes (by 1.5-2.0 ppm).

R. M. Haberle, F. Forget, J. Head, M. A. Kahre, M. Kreslavsky, and S. J. Owen. Summary of the Mars recent climate change workshop NASA/Ames Research Center, May 15-17, 2012. Icarus, 222:415-418, 2013. [ bib | DOI | ADS link ]

This note summarizes the results from the Mars recent climate change workshop at NASA/Ames Research Center, May 15-17, 2012.

F. Forget, R. Wordsworth, E. Millour, J.-B. Madeleine, L. Kerber, J. Leconte, E. Marcq, and R. M. Haberle. 3D modelling of the early martian climate under a denser CO2 atmosphere: Temperatures and CO2 ice clouds. Icarus, 222:81-99, 2013. [ bib | DOI | arXiv | ADS link ]

On the basis of geological evidence, it is often stated that the early martian climate was warm enough for liquid water to flow on the surface thanks to the greenhouse effect of a thick atmosphere. We present 3D global climate simulations of the early martian climate performed assuming a faint young Sun and a CO2 atmosphere with surface pressure between 0.1 and 7 bars. The model includes a detailed radiative transfer model using revised CO2 gas collision induced absorption properties, and a parameterisation of the CO2 ice cloud microphysical and radiative properties. A wide range of possible climates is explored using various values of obliquities, orbital parameters, cloud microphysic parameters, atmospheric dust loading, and surface properties. Unlike on present day Mars, for pressures higher than a fraction of a bar, surface temperatures vary with altitude because of the adiabatic cooling and warming of the atmosphere when it moves vertically. In most simulations, CO2 ice clouds cover a major part of the planet. Previous studies had suggested that they could have warmed the planet thanks to their scattering greenhouse effect. However, even assuming parameters that maximize this effect, it does not exceed +15 K. Combined with the revised CO2 spectroscopy and the impact of surface CO2 ice on the planetary albedo, we find that a CO2 atmosphere could not have raised the annual mean temperature above 0 degC anywhere on the planet. The collapse of the atmosphere into permanent CO2 ice caps is predicted for pressures higher than 3 bar, or conversely at pressure lower than 1 bar if the obliquity is low enough. Summertime diurnal mean surface temperatures above 0 degC (a condition which could have allowed rivers and lakes to form) are predicted for obliquity larger than 40deg at high latitudes but not in locations where most valley networks or layered sedimentary units are observed. In the absence of other warming mechanisms, our climate model results are thus consistent with a cold early Mars scenario in which nonclimatic mechanisms must occur to explain the evidence for liquid water. In a companion paper by Wordsworth et al. we simulate the hydrological cycle on such a planet and discuss how this could have happened in more detail.

R. Wordsworth, F. Forget, E. Millour, J. W. Head, J.-B. Madeleine, and B. Charnay. Global modelling of the early martian climate under a denser CO2 atmosphere: Water cycle and ice evolution. Icarus, 222:1-19, 2013. [ bib | DOI | arXiv | ADS link ]

We discuss 3D global simulations of the early martian climate that we have performed assuming a faint young Sun and denser CO2 atmosphere. We include a self-consistent representation of the water cycle, with atmosphere-surface interactions, atmospheric transport, and the radiative effects of CO2 and H2O gas and clouds taken into account. We find that for atmospheric pressures greater than a fraction of a bar, the adiabatic cooling effect causes temperatures in the southern highland valley network regions to fall significantly below the global average. Long-term climate evolution simulations indicate that in these circumstances, water ice is transported to the highlands from low-lying regions for a wide range of orbital obliquities, regardless of the extent of the Tharsis bulge. In addition, an extended water ice cap forms on the southern pole, approximately corresponding to the location of the Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2 atmosphere, conditions are too cold to allow long-term surface liquid water. Limited melting occurs on warm summer days in some locations, but only for surface albedo and thermal inertia conditions that may be unrealistic for water ice. Nonetheless, meteorite impacts and volcanism could potentially cause intense episodic melting under such conditions. Because ice migration to higher altitudes is a robust mechanism for recharging highland water sources after such events, we suggest that this globally sub-zero, 'icy highlands' scenario for the late Noachian climate may be sufficient to explain most of the fluvial geology without the need to invoke additional long-term warming mechanisms or an early warm, wet Mars.

F. Forget and S. Lebonnois. Global Climate Models of the Terrestrial Planets. Comparative Climatology of Terrestrial Planets, pages 213-229, 2013. [ bib | DOI | ADS link ]

On the basis of the global climate models (GCMs) originally developed for Earth, several teams around the world have been able to develop GCMs for the atmospheres of the other terrestrial bodies in our solar system: Venus, Mars, Titan, Triton, and Pluto. In spite of the apparent complexity of climate systems and meteorology, GCMs are based on a limited number of equations. In practice, relatively complete climate simulators can be developed by combining a few components such as a dynamical core, a radiative transfer solver, a parameterization of turbulence and convection, a thermal ground model, and a volatile phase change code, possibly completed by a few specific schemes. It can be shown that many of these GCM components are “universal” so that we can envisage building realistic climate models for any kind of terrestrial planets and atmospheres that we can imagine. Such a tool is useful for conducting scientific investigations on the possible climates of terrestrial extrasolar planets, or to study past environments in the solar system. The ambition behind the development of GCMs is high: The ultimate goal is to build numerical simulators based only on universal physical or chemical equations, yet able to reproduce or predict all the available observations on a given planet, without any ad hoc forcing. In other words, we aim to virtually create in our computers planets that “behave” exactly like the actual planets themselves. In reality, of course, nature is always more complex than expected, but we learn a lot in the process. In this chapter we detail some lessons learned in the solar system: In many cases, GCMs work. They have been able to simulate many aspects of planetary climates without difficulty. In some cases, however, problems have been encountered, sometimes simply because a key process has been forgotten in the model or is not yet correctly parameterized, but also because sometimes the climate regime seems to be result of a subtle balance between processes that remain highly model sensitive, or are the subject of positive feedback and unstability. In any case, building virtual planets with GCMs, in light of the observations obtained by spacecraft or from Earth, is a true scientific endeavor that can teach us a lot about the complex nature of climate systems.