Three years of greenhouse gas column-averaged dry air mole fractions retrieved from satellite – Part 2: Methane

Carbon dioxide (CO2) and methane (CH4) are the two most important anthropogenic greenhouse gases. SCIAMACHY on ENVISAT is the first satellite instrument whose measurements are sensitive to concentration changes of the two gases at all altitude levels down to the Earth's surface where the source/sink signals are largest. We have processed three years (2003–2005) of SCIAMACHY near-infrared nadir measurements to simultaneously retrieve vertical columns of CO2 (from the 1.58 µm absorption band), CH4 (1.66 µm) and oxygen (O2 A-band at 0.76 µm) using the scientific retrieval algorithm WFM-DOAS. We show that the latest version of WFM-DOAS, version 1.0, which is used for this study, has been significantly improved with respect to its accuracy compared to the previous versions while essentially maintaining its high processing speed (~1 min per orbit, corresponding to ~6000 single measurements, and per gas on a standard PC). The greenhouse gas columns are converted to dry air column-averaged mole fractions, denoted XCO2 (in ppm) and XCH4 (in ppb), by dividing the greenhouse gas columns by simultaneously retrieved dry air columns. For XCO2 dry air columns are obtained from the retrieved O2 columns. For XCH4 dry air columns are obtained from the retrieved CO2 columns because of better cancellation of light path related errors compared to using O2 columns retrieved from the spectrally distant O2 A-band. Here we focus on a discussion of the XCH4 data set. The XCO2 data set is discussed in a separate paper (Part 1). For 2003 we present detailed comparisons with the TM5 model which has been optimally matched to highly accurate but sparse methane surface observations. After accounting for a systematic low bias of ~2% agreement with TM5 is typically within 1–2%. We investigated to what extent the SCIAMACHY XCH4 is influenced by the variability of atmospheric CO2 using global CO2 fields from NOAA's CO2 assimilation system CarbonTracker. We show that the CO2 corrected and uncorrected XCH4 spatio-temporal pattern are very similar but that agreement with TM5 is better for the CarbonTracker CO2 corrected XCH4. In line with previous studies (e.g., Frankenberg et al., 2005b) we find higher methane over the tropics compared to the model. We show that tropical methane is also higher when normalizing the CH4 columns with retrieved O2 columns instead of CO2. In consistency with recent results of Frankenberg et al. (2008b) it is shown that the magnitude of the retrieved tropical methane is sensitive to the choice of the spectroscopic line parameters of water vapour. Concerning inter-annual variability we find similar methane spatio-temporal pattern for 2003 and 2004. For 2005 the retrieved methane shows significantly higher variability compared to the two previous years, most likely due to somewhat larger noise of the spectral measurement

Retrieval of nitric oxide in the mesosphere and lower thermosphere from SCIAMACHY limb spectra

We use the ultra-violet (UV) spectra in the range 230-300 nm from the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) to retrieve the nitric oxide (NO) number densities from atmospheric emissions in the gamma-bands in the mesosphere and lower thermosphere. Using 3-D ray tracing, a 2-D retrieval grid, and regularisation with respect to altitude and latitude, we retrieve a whole semi-orbit simultaneously for the altitude range from 60 to 160 km. We present details of the retrieval algorithm, first results, and initial comparisons to data from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). Our results agree on average well with MIPAS data and are in line with previously published measurements from other instruments. For the time of available measurements in 2008-2011, we achieve a vertical resolution of 5-10 km in the altitude range 70-140 km and a horizontal resolution of about 9 from 60 S-60 N. With this we have independent measurements of the NO densities in the mesosphere and lower thermosphere with approximately global coverage. This data can be further used to validate climate models or as input for them. © 2013 Author(s).S. Bender and M. Sinnhuber thank the Helmholtz-society for funding this project under the grant number VH-NG-624. The IAA team (M. Lopez-Puertas and B. Funke) was supported by the Spanish MINECO under grant AYA2011-23552 and EC FEDER funds. The SCIAMACHY project was funded by German Aerospace (DLR), the Dutch Space Agency, SNO, and the Belgium ministry. ESA funded the Envisat project. The University of Bremen as Principal Investigator has led the scientific support and development of SCIAMACHY and the scientific exploitation of its data products. We acknowledge support by Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of Karlsruhe Institute of Technology.Peer Reviewe

Satellite remote sensing of regional and seasonal Arctic cooling showing a multi-decadal trend towards brighter and more liquid clouds

Two decades of measurements of spectral reflectance of solar radiation at the top of the atmosphere and a complementary record of cloud properties from satellite passive remote sensing have been analyzed for their pan-Arctic, regional, and seasonal changes. The pan-Arctic loss of brightness, which is explained by the retreat of sea ice during the current warming period, is not compensated by a corresponding increase in cloud cover. A systematic change in the thermodynamic phase of clouds has taken place, shifting towards the liquid phase at the expense of the ice phase. Without significantly changing the total cloud optical thickness or the mass of condensed water in the atmosphere, liquid water content has increased, resulting in positive trends in liquid cloud optical thickness and albedo. This leads to a cooling trend by clouds being superimposed on top of the pan-Arctic amplified warming, induced by the anthropogenic release of greenhouse gases, the ice–albedo feedback, and related effects. Except over the permanent and parts of the marginal sea ice zone around the Arctic Circle, the rate of surface cooling by clouds has increased, both in spring (−32 % in total radiative forcing for the whole Arctic) and in summer (−14 %). The magnitude of this effect depends on both the underlying surface type and changes in the regional Arctic climate

Simulating tropospheric BrO in the Arctic using an artificial neural network

An intriguing natural phenomenon occurs every polar spring, namely the bromine explosion, in which plumes of tropospheric bromine monoxide (BrO) are formed. These plumes are observed in the BrO vertical column densities (VCDs), retrieved from satellite sensors. Tropospheric BrO depletes tropospheric ozone and facilitates the deposition of mercury. Bromine molecules are mainly released from young sea ice, and meteorological parameters determine the formation and evolution of enhanced BrO VCD plumes. Due to the complexity of the physicochemical processes involved in the bromine explosion, the modeling of tropospheric BrO VCDs in chemical transport models is challenging and not yet adequate. The first of its type, this study demonstrates the potential of using an artificial neural network (ANN), which uses meteorological parameters and sea ice age as inputs to simulate and predict tropospheric BrO VCDs in the Arctic. The ANN is trained and validated using a 22-year satellite remote sensing dataset of Arctic tropospheric BrO VCDs. A generally satisfactory spatial agreement between observed and simulated tropospheric BrO VCDs is observed. However, the magnitude of the observed BrO VCD plumes is underestimated. Air temperature and mean sea level pressure are the most important parameters influencing the magnitude of tropospheric BrO VCD simulations. Although the changing spatial distribution of tropospheric BrO VCDs over time is well captured, the trend reported in the observations of tropospheric BrO VCDs is not reproduced by the ANN, suggesting that additional parameters not included in the ANN also influence the formation of tropospheric BrO VCD plumes

Retrieval algorithm for densities of mesospheric and lower thermospheric metal atom and ion species from satellite-borne limb emission signals

Meteoroids bombard Earth's atmosphere during its orbit around the Sun, depositing a highly varying and significant amount of matter into the thermosphere and mesosphere. The strength of the material source needs to be characterized and its impact on atmospheric chemistry assessed. In this study an algorithm for the retrieval of metal atom and ion number densities for a two-dimensional (latitude, altitude) grid is described and explained. Dayglow emission spectra of the mesosphere and lower thermosphere are used, which are obtained by passive satellite remote sensing with the SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) instrument on board Envisat. The limb scans cover the tangent altitude range from 50 to 150 km. Metal atoms and ions are strong emitters in this region and form sharply peaked layers with a FWHM (full width at half maximum) of several 10 km in the mesosphere and lower thermosphere measuring peak altitudes between 90 to 110 km. The emission signal is first separated from the background signal, arising from Rayleigh and Raman scattering of solar radiation by air molecules. A forward radiative transfer model calculating the slant column density (SCD) from a given vertical distribution was developed. This nonlinear model is inverted in an iterative procedure to yield the vertical profiles for the emitting species. Several constraints are applied to the solution for numerical stability reasons and to get physically reasonable solutions. The algorithm is applied to SCIAMACHY limb-emission observations for the retrieval of Mg and Mg+ using emission signatures at 285.2 and 279.6/280.4 nm, respectively. Results are presented for these three lines as well as error estimations and sensitivity tests on different constraint strength and different separation approaches for the background signal

Earthshine observation of vegetation and implication for life detection on other planets - A review of 2001 - 2006 works

The detection of exolife is one of the goals of very ambitious future space missions that aim to take direct images of Earth-like planets. While associations of simple molecules present in the planet's atmosphere ($O_2$, $O_3$, $CO_2$ etc.) have been identified as possible global biomarkers, we review here the detectability of a signature of life from the planet's surface, i.e. the green vegetation. The vegetation reflectance has indeed a specific spectrum, with a sharp edge around 700 nm, known as the "Vegetation Red Edge" (VRE). Moreover vegetation covers a large surface of emerged lands, from tropical evergreen forest to shrub tundra. Thus considering it as a potential global biomarker is relevant. Earthshine allows to observe the Earth as a distant planet, i.e. without spatial resolution. Since 2001, Earthshine observations have been used by several authors to test and quantify the detectability of the VRE in the Earth spectrum. The egetation spectral signature is detected as a small 'positive shift' of a few percents above the continuum, starting at 700 nm. This signature appears in most spectra, and its strength is correlated with the Earth's phase (visible land versus visible ocean). The observations show that detecting the VRE on Earth requires a photometric relative accuracy of 1% or better. Detecting something equivalent on an Earth-like planet will therefore remain challenging, moreover considering the possibility of mineral artifacts and the question of 'red edge' universality in the Universe.Comment: Invited talk in "Strategies for Life Detection" (ISSI Bern, 24-28 April 2006) to appear in a hardcopy volume of the ISSI Space Science Series, Eds, J. Bada et al., and also in an issue of Space Science Reviews. 13 pages, 8 figures, 1 tabl

Comparison of the HadGEM2 climate-chemistry model against in situ and SCIAMACHY atmospheric methane data

Wetlands are a major emission source of methane (CH4) globally. In this study, we evaluate wetland emission estimates derived using the UK community land surface model (JULES, the Joint UK Land Earth Simulator) against atmospheric observations of methane, including, for the first time, total methane columns derived from the SCIAMACHY instrument on board the ENVISAT satellite. Two JULES wetland emission estimates are investigated: (a) from an offline run driven with Climatic Research Unit–National Centers for Environmental Prediction (CRU-NCEP) meteorological data and (b) from the same offline run in which the modelled wetland fractions are replaced with those derived from the Global Inundation Extent from Multi-Satellites (GIEMS) remote sensing product. The mean annual emission assumed for each inventory (181 Tg CH4 per annum over the period 1999–2007) is in line with other recently published estimates. There are regional differences as the unconstrained JULES inventory gives significantly higher emissions in the Amazon (by ~36 Tg CH4 yr−1) and lower emissions in other regions (by up to 10 Tg CH4 yr−1) compared to the JULES estimates constrained with the GIEMS product. Using the UK Hadley Centre's Earth System model with atmospheric chemistry (HadGEM2), we evaluate these JULES wetland emissions against atmospheric observations of methane. We obtain improved agreement with the surface concentration measurements, especially at high northern latitudes, compared to previous HadGEM2 runs using the wetland emission data set of Fung et al. (1991). Although the modelled monthly atmospheric methane columns reproduce the large-scale patterns in the SCIAMACHY observations, they are biased low by 50 part per billion by volume (ppb). Replacing the HadGEM2 modelled concentrations above 300 hPa with HALOE–ACE assimilated TOMCAT output results in a significantly better agreement with the SCIAMACHY observations. The use of the GIEMS product to constrain the JULES-derived wetland fraction improves the representation of the wetland emissions in JULES and gives a good description of the seasonality observed at surface sites influenced by wetlands, especially at high latitudes. We find that the annual cycles observed in the SCIAMACHY measurements and at many of the surface sites influenced by non-wetland sources cannot be reproduced in these HadGEM2 runs. This suggests that the emissions over certain regions (e.g. India and China) are possibly too high and/or the monthly emission patterns for specific sectors are incorrect. The comparisons presented in this paper show that the performance of the JULES wetland scheme is comparable to that of other process-based land surface models. We identify areas for improvement in this and the atmospheric chemistry components of the HadGEM Earth System model. The Earth Observation data sets used here will be of continued value in future evaluations of JULES and the HadGEM family of models

Impact of forest fires, biogenic emissions and high temperatures on the elevated Eastern Mediterranean ozone levels during the hot summer of 2007

International audienceThe hot summer of 2007 in southeast Europe has been studied using two regional atmospheric chemistry models; WRF-Chem and EMEP MSC-W. The region was struck by three heat waves and a number of forest fire episodes, greatly affecting air pollution levels. We have focused on ozone and its precursors using state-of-the-art inventories for anthropogenic, biogenic and forest fire emissions. The models have been evaluated against measurement data, and processes leading to ozone formation have been quantified. Heat wave episodes are projected to occur more frequently in a future climate, and therefore this study also makes a contribution to climate change impact research. The plume from the Greek forest fires in August 2007 is clearly seen in satellite observations of CO and NO2 columns, showing extreme levels of CO in and downwindof the fires. Model simulations reflect the location and influence of the fires relatively well, but the modelled magnitude of CO in the plume core is too low. Most likely, this is caused by underestimation of CO in the emission inventories, suggesting that the CO/NOx ratios of fire emissions should be re-assessed. Moreover, higher maximum values are seen in WRF-Chem than in EMEP MSC-W, presumably due to differences in plume rise altitudes as the first model emits a larger fraction of the fire emissions in the lowermost model layer. The model results are also in fairly good agreement with surface ozone measurements. Biogenic VOC emissions reacting with anthropogenic NOx emissions are calculated to contribute significantly to the levels of ozone in the region, but the magnitude and geographical distribution depend strongly on the model and biogenic emission module used. During the July and August heat waves, ozone levels increased substantially due to a combination of forest fire emissions and the effect of high temperatures. We found that the largest temperature impact on ozone was through the temperature dependence of the biogenic emissions, closely followed by the effect of reduced dry deposiion caused by closing of the plants' stomata at very high temperatures. The impact of high temperatures on the ozone chemistry was much lower. The results suggest that forest fire emissions, and the temperature effect on biogenic emissions and dry deposition, will potentially lead to substantial ozone increases in a warmer climate

Transiting Exoplanets with JWST

The era of exoplanet characterization is upon us. For a subset of exoplanets -- the transiting planets -- physical properties can be measured, including mass, radius, and atmosphere characteristics. Indeed, measuring the atmospheres of a further subset of transiting planets, the hot Jupiters, is now routine with the Spitzer Space Telescope. The James Webb Space Telescope (JWST) will continue Spitzer's legacy with its large mirror size and precise thermal stability. JWST is poised for the significant achievement of identifying habitable planets around bright M through G stars--rocky planets lacking extensive gas envelopes, with water vapor and signs of chemical disequilibrium in their atmospheres. Favorable transiting planet systems, are, however, anticipated to be rare and their atmosphere observations will require tens to hundreds of hours of JWST time per planet. We review what is known about the physical characteristics of transiting planets, summarize lessons learned from Spitzer high-contrast exoplanet measurements, and give several examples of potential JWST observations.Comment: 22 pages, 11 figures. In press in "Astrophysics in the Next Decade: JWST and Concurrent Facilities, Astrophysics & Space Science Library, Thronson, H. A., Tielens, A., Stiavelli, M., eds., Springer: Dordrecht (2008)." The original publication will be available at http://www.springerlink.co