204 research outputs found
A linear method for the retrieval of sun-induced chlorophyll fluorescence from GOME-2 and SCIAMACHY data
Global retrievals of near-infrared sun-induced chlorophyll fluorescence (SIF)
have been achieved in the last few years by means of a number of space-borne
atmospheric spectrometers. Here, we present a new retrieval method for medium
spectral resolution instruments such as the Global Ozone Monitoring
Experiment-2 (GOME-2) and the SCanning Imaging Absorption SpectroMeter for
Atmospheric CHartographY (SCIAMACHY). Building upon the previous work by
Guanter et al. (2013) and Joiner et al. (2013), our approach provides a
solution for the selection of the number of free parameters. In particular, a
backward elimination algorithm is applied to optimize the number of
coefficients to fit, which reduces also the retrieval noise and selects the
number of state vector elements automatically. A sensitivity analysis with
simulated spectra has been utilized to evaluate the performance of our
retrieval approach. The method has also been applied to estimate SIF at 740 nm
from real spectra from GOME-2 and for the first time, from SCIAMACHY. We find
a good correspondence of the absolute SIF values and the spatial patterns from
the two sensors, which suggests the robustness of the proposed retrieval
method. In addition, we compare our results to existing SIF data sets, examine
uncertainties and use our GOME-2 retrievals to show empirically the relatively
low sensitivity of the SIF retrieval to cloud contamination
Remote sensing of near-infrared chlorophyll fluorescence from space in scattering atmospheres: implications for its retrieval and interferences with atmospheric CO_2 retrievals
With the advent of dedicated greenhouse gas space-borne spectrometers sporting high resolution spectra in the O_2 A-band spectral region (755–774 nm), the retrieval of chlorophyll fluorescence has become feasible on a global scale. If unaccounted for, however, fluorescence can indirectly perturb the greenhouse gas retrievals as it perturbs the oxygen absorption features. As atmospheric CO_2 measurements are used to invert net fluxes at the land–atmosphere interface, a bias caused by fluorescence can be crucial as it will spatially correlate with the fluxes to be inverted. Avoiding a bias and retrieving fluorescence accurately will provide additional constraints on both the net and gross fluxes in the global carbon cycle. We show that chlorophyll fluorescence, if neglected, systematically interferes with full-physics multi-band X_(CO_2) retrievals using the O_2 A-band. Systematic biases in X_(CO_2) can amount to +1 ppm if fluorescence constitutes 1% to the continuum level radiance. We show that this bias can be largely eliminated by simultaneously fitting fluorescence in a full-physics based retrieval.
If fluorescence is the primary target, a dedicated but very simple retrieval based purely on Fraunhofer lines is shown to be more accurate and very robust even in the presence of large scattering optical depths. We find that about 80% of the surface fluorescence is retained at the top-of-atmosphere, even for cloud optical thicknesses around 2–5. We further show that small instrument modifications to future O_2 A-band spectrometer spectral ranges can result in largely reduced random errors in chlorophyll fluorescence, paving the way towards a more dedicated instrument exploiting solar absorption features only
Reviews and syntheses: Systematic Earth observations for use in terrestrial carbon cycle data assimilation systems
The global carbon cycle is an important component of the Earth system and it interacts with the hydrology, energy and nutrient cycles as well as ecosystem dynamics. A better understanding of the global carbon cycle is required for improved projections of climate change including corresponding changes in water and food resources and for the verification of measures to reduce anthropogenic greenhouse gas emissions. An improved understanding of the carbon cycle can be achieved by data assimilation systems, which integrate observations relevant to the carbon cycle into coupled carbon, water, energy and nutrient models. Hence, the ingredients for such systems are a carbon cycle model, an algorithm for the assimilation and systematic and well error-characterised observations relevant to the carbon cycle. Relevant observations for assimilation include various in situ measurements in the atmosphere (e.g. concentrations of CO2 and other gases) and on land (e.g. fluxes of carbon water and energy, carbon stocks) as well as remote sensing observations (e.g. atmospheric composition, vegetation and surface properties). We briefly review the different existing data assimilation techniques and contrast them to model benchmarking and evaluation efforts (which also rely on observations). A common requirement for all assimilation techniques is a full description of the observational data properties. Uncertainty estimates of the observations are as important as the observations themselves because they similarly determine the outcome of such assimilation systems. Hence, this article reviews the requirements of data assimilation systems on observations and provides a non-exhaustive overview of current observations and their uncertainties for use in terrestrial carbon cycle data assimilation. We report on progress since the review of model-data synthesis in terrestrial carbon observations by Raupach et al.(2005), emphasising the rapid advance in relevant space-based observations
Potential of the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor for the monitoring of terrestrial chlorophyll fluorescence
Global monitoring of sun-induced chlorophyll fluorescence (SIF) is improving our knowledge about the photosynthetic functioning of terrestrial ecosystems. The feasibility of SIF retrievals from spaceborne atmospheric spectrometers has been demonstrated by a number of studies in the last years. In this work, we investigate the potential of the upcoming TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite mission for SIF retrieval. TROPOMI will sample the 675–775 nm spectral window with a spectral resolution of 0.5 nm and a pixel size of 7 km × 7 km. We use an extensive set of simulated TROPOMI data in order to assess the uncertainty of single SIF retrievals and subsequent spatio-temporal composites. Our results illustrate the enormous improvement in SIF monitoring achievable with TROPOMI with respect to comparable spectrometers currently in-flight, such as the Global Ozone Monitoring Experiment-2 (GOME-2) instrument. We find that TROPOMI can reduce global uncertainties in SIF mapping by more than a factor of 2 with respect to GOME-2, which comes together with an approximately 5-fold improvement in spatial sampling. Finally, we discuss the potential of TROPOMI to map other important vegetation parameters at a global scale with moderate spatial resolution and short revisit time. Those include leaf photosynthetic pigments and proxies for canopy structure, which will complement SIF retrievals for a self-contained description of vegetation condition and functioning
Exploiting the entire near-infrared spectral range to improve the detection of methane plumes with high-resolution imaging spectrometers
Remote sensing emerges as an important tool for the detection of methane plumes emitted by so-called point sources, which are common in the energy sector (e.g., oil and gas extraction and coal mining activities). In particular, satellite imaging spectroscopy missions covering the shortwave infrared part of the solar spectrum are very effective for this application. These instruments sample the methane absorption features at the spectral regions around 1700 and 2300 nm, which enables the retrieval of methane concentration enhancements per pixel. Data-driven retrieval methods, in particular those based on the matched filter concept, are widely used to produce maps of methane concentration enhancements from imaging spectroscopy data. Using these maps enables the detection of plumes and the subsequent identification of active sources. However, retrieval artifacts caused by particular surface components may sometimes appear as false plumes or disturbing elements in the methane maps, which complicates the identification of real plumes. In this work, we use a matched filter that exploits a wide spectral window (1000–2500 nm) instead of the usual 2100–2450 nm window with the aim of reducing the occurrence of retrieval artifacts and background noise. This enables a greater ability to discriminate between surface elements and methane. The improvement in plume detection is evaluated through an analysis derived from both simulated data and real data from areas including active point sources, such as the oil and gas (O&G) industry from San Joaquin Valley (US) and the coal mines from the Shanxi region (China). We use datasets from the Precursore IperSpettrale della Missione Applicativa (PRISMA) and the Environmental Mapping and Analysis Program (EnMAP) satellite imaging spectrometer missions and from the Airborne Visible/Infrared Imaging Spectrometer – Next Generation (AVIRIS-NG) instrument. We find that the interference with atmospheric carbon dioxide and water vapor is generally almost negligible, while co-emission or overlapping of these trace gases with methane plumes leads to a reduction in the retrieved concentration values. Attenuation will also occur in the case of methane emissions situated above surface structures that are associated with retrieval artifacts. The results show that the new approach is an optimal trade-off between the reduction in background noise and retrieval artifacts. This is illustrated by a comprehensive analysis in a PRISMA dataset with 15 identified plumes, where the output mask from an automatic detection algorithm shows an important reduction in the number of clusters not related to CH4 emissions.</p
Assessing the dynamics of vegetation productivity in circumpolar regions with different satellite indicators of greenness and photosynthesis
High-latitude treeless ecosystems represent spatially highly heterogeneous
landscapes with small net carbon fluxes and a short growing season. Reliable
observations and process understanding are critical for projections of the
carbon balance of the climate-sensitive tundra. Space-borne remote sensing is the
only tool to obtain spatially continuous and temporally resolved information
on vegetation greenness and activity in remote circumpolar areas. However,
confounding effects from persistent clouds, low sun elevation angles,
numerous lakes, widespread surface inundation, and the sparseness of the
vegetation render it highly challenging. Here, we conduct an extensive
analysis of the timing of peak vegetation productivity as shown by satellite
observations of complementary indicators of plant greenness and
photosynthesis. We choose to focus on productivity during the peak of the
growing season, as it importantly affects the total annual carbon uptake. The
suite of indicators are as follows: (1) MODIS-based vegetation indices (VIs) as proxies
for the fraction of incident photosynthetically active radiation (PAR) that is absorbed (fPAR), (2) VIs combined with estimates
of PAR as a proxy of the total absorbed radiation (APAR), (3) sun-induced chlorophyll fluorescence (SIF) serving as a proxy
for photosynthesis, (4) vegetation optical depth (VOD), indicative of total
water content and (5) empirically upscaled modelled gross primary
productivity (GPP). Averaged over the pan-Arctic we find a clear order of the
annual peak as APAR ≦ GPP < SIF < VIs∕VOD. SIF as an indicator of
photosynthesis is maximised around the time of highest annual temperatures.
The modelled GPP peaks at a similar time to APAR. The time lag of the annual peak
between APAR and instantaneous SIF fluxes indicates that the SIF data do
contain information on light-use efficiency of tundra vegetation, but further
detailed studies are necessary to verify this. Delayed peak greenness
compared to peak photosynthesis is consistently found across years and land-cover classes. A particularly late peak of the normalised difference
vegetation index (NDVI) in regions with very small seasonality in greenness
and a high amount of lakes probably originates from artefacts. Given the very
short growing season in circumpolar areas, the average time difference in
maximum annual photosynthetic activity and greenness or growth of 3 to 25 days
(depending on the data sets chosen) is important and needs to be considered
when using satellite observations as drivers in vegetation models.</p
CH4Net: a deep learning model for monitoring methane super-emitters with Sentinel-2 imagery
We present a deep learning model, CH4Net, for automated monitoring of methane super-emitters from Sentinel-2 data. When trained on images of 23 methane super-emitter locations from 2017–2020 and evaluated on images from 2021, this model detects 84 % of methane plumes compared with 24 % of plumes for a state-of-the-art baseline while maintaining a similar false positive rate. We present an in-depth analysis of CH4Net over the complete dataset and at each individual super-emitter site. In addition to the CH4Net model, we compile and make open source a hand-annotated training dataset consisting of 925 methane plume masks as a machine learning baseline to drive further research in this field.</p
Duality and hidden equilibrium in transport models
A large family of diffusive models of transport that have been considered in the past years admit a transformation into the same model in contact with an equilibrium bath. This mapping holds at the full dynamical level, and is independent of dimension or topology. It provides a good opportunity to discuss questions of time reversal in out of equilibrium contexts. In particular, thanks to the mapping one may define the free energy in the non-equilibrium states very naturally as the (usual) free energy of the mapped system
Space-based remote imaging spectroscopy of the Aliso Canyon CH_4 superemitter
The Aliso Canyon gas storage facility near Porter Ranch, California, produced a large accidental CH_4 release from October 2015 to February 2016. The Hyperion imaging spectrometer on board the EO-1 satellite successfully detected this event, achieving the first orbital attribution of CH_4 to a single anthropogenic superemitter. Hyperion measured shortwave infrared signatures of CH_4 near 2.3 μm at 0.01 μm spectral resolution and 30 m spatial resolution. It detected the plume on three overpasses, mapping its magnitude and morphology. These orbital observations were consistent with measurements by airborne instruments. We evaluate Hyperion instrument performance, draw implications for future orbital instruments, and extrapolate the potential for a global survey of CH_4 superemitters
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