ALBERT

Description: We present the result of the third Marine Ice Sheet Model Intercomparison Project, MISMIP+. MISMIP+ is intended to be a benchmark for ice-flow models which include fast sliding marine ice streams and floating ice shelves and in particular a treatment of viscous stress that is sufficient to model buttressing, where upstream ice flow is restrained by a downstream ice shelf. A set of idealized experiments first tests that models are able to maintain a steady state with the grounding line located on a retrograde slope due to buttressing and then explore scenarios where a reduction in that buttressing causes ice stream acceleration, thinning, and grounding line retreat. The majority of participating models passed the first test and then produced similar responses to the loss of buttressing. We find that the most important distinction between models in this particular type of simulation is in the treatment of sliding at the bed, with other distinctions – notably the difference between the simpler and more complete treatments of englacial stress but also the differences between numerical methods – taking a secondary role.

Description: Carbon monoxide (CO) is an important atmospheric constituent affecting air quality, and methane (CH4) is the second most important greenhouse gas contributing to human-induced climate change. Detailed and continuous observations of these gases are necessary to better assess their impact on climate and atmospheric pollution. While surface and airborne measurements are able to accurately determine atmospheric abundances on local scales, global coverage can only be achieved using satellite instruments. The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite, which was successfully launched in October 2017, is a spaceborne nadir-viewing imaging spectrometer measuring solar radiation reflected by the Earth in a push-broom configuration. It has a wide swath on the terrestrial surface and covers wavelength bands between the ultraviolet (UV) and the shortwave infrared (SWIR), combining a high spatial resolution with daily global coverage. These characteristics enable the determination of both gases with an unprecedented level of detail on a global scale, introducing new areas of application. Abundances of the atmospheric column-averaged dry air mole fractions XCO and XCH4 are simultaneously retrieved from TROPOMI's radiance measurements in the 2.3 µm spectral range of the SWIR part of the solar spectrum using the scientific retrieval algorithm Weighting Function Modified Differential Optical Absorption Spectroscopy (WFM-DOAS). This algorithm is intended to be used with the operational algorithms for mutual verification and to provide new geophysical insights. We introduce the algorithm in detail, including expected error characteristics based on synthetic data, a machine-learning-based quality filter, and a shallow learning calibration procedure applied in the post-processing of the XCH4 data. The quality of the results based on real TROPOMI data is assessed by validation with ground-based Fourier transform spectrometer (FTS) measurements providing realistic error estimates of the satellite data: the XCO data set is characterised by a random error of 5.1 ppb (5.8 %) and a systematic error of 1.9 ppb (2.1 %); the XCH4 data set exhibits a random error of 14.0 ppb (0.8 %) and a systematic error of 4.3 ppb (0.2 %). The natural XCO and XCH4 variations are well-captured by the satellite retrievals, which is demonstrated by a high correlation with the validation data (R=0.97 for XCO and R=0.91 for XCH4 based on daily averages). We also present selected results from the mission start until the end of 2018, including a first comparison to the operational products and examples of the detection of emission sources in a single satellite overpass, such as CO emissions from the steel industry and CH4 emissions from the energy sector, which potentially allows for the advance of emission monitoring and air quality assessments to an entirely new level.

Description: NASA's Orbiting Carbon Observatory-2 (OCO-2) has been measuring carbon dioxide column-averaged dry-air mole fraction, XCO2, in the Earth's atmosphere for over 2 years. In this paper, we describe the comparisons between the first major release of the OCO-2 retrieval algorithm (B7r) and XCO2 from OCO-2's primary ground-based validation network: the Total Carbon Column Observing Network (TCCON). The OCO-2 XCO2 retrievals, after filtering and bias correction, agree well when aggregated around and coincident with TCCON data in nadir, glint, and target observation modes, with absolute median differences less than 0.4 ppm and RMS differences less than 1.5 ppm. After bias correction, residual biases remain. These biases appear to depend on latitude, surface properties, and scattering by aerosols. It is thus crucial to continue measurement comparisons with TCCON to monitor and evaluate the OCO-2 XCO2 data quality throughout its mission.

Description: Nitrous oxide (N2O) is an important greenhouse gas and it can also generate nitric oxide, which depletes ozone in the stratosphere. It is a common target species of ground-based Fourier transform infrared (FTIR) near-infrared (TCCON) and mid-infrared (NDACC) measurements. Both TCCON and NDACC networks provide a long-term global distribution of atmospheric N2O mole fraction. In this study, the dry-air column-averaged mole fractions of N2O (XN2O) from the TCCON and NDACC measurements are compared against each other at seven sites around the world (Ny-Ålesund, Sodankylä, Bremen, Izaña, Réunion, Wollongong, Lauder) in the time period of 2007–2017. The mean differences in XN2O between TCCON and NDACC (NDACC–TCCON) at these sites are between −3.32 and 1.37 ppb (−1.1 %–0.5 %) with standard deviations between 1.69 and 5.01 ppb (0.5 %–1.6 %), which are within the uncertainties of the two datasets. The NDACC N2O retrieval has good sensitivity throughout the troposphere and stratosphere, while the TCCON retrieval underestimates a deviation from the a priori in the troposphere and overestimates it in the stratosphere. As a result, the TCCON XN2O measurement is strongly affected by its a priori profile. Trends and seasonal cycles of XN2O are derived from the TCCON and NDACC measurements and the nearby surface flask sample measurements and compared with the results from GEOS-Chem model a priori and a posteriori simulations. The trends and seasonal cycles from FTIR measurement at Ny-Ålesund and Sodankylä are strongly affected by the polar winter and the polar vortex. The a posteriori N2O fluxes in the model are optimized based on surface N2O measurements with a 4D-Var inversion method. The XN2O trends from the GEOS-Chem a posteriori simulation (0.97±0.02 (1σ) ppb yr−1) are close to those from the NDACC (0.93±0.04 ppb yr−1) and the surface flask sample measurements (0.93±0.02 ppb yr−1). The XN2O trend from the TCCON measurements is slightly lower (0.81±0.04 ppb yr−1) due to the underestimation of the trend in TCCON a priori simulation. The XN2O trends from the GEOS-Chem a priori simulation are about 1.25 ppb yr−1, and our study confirms that the N2O fluxes from the a priori inventories are overestimated. The seasonal cycles of XN2O from the FTIR measurements and the model simulations are close to each other in the Northern Hemisphere with a maximum in August–October and a minimum in February–April. However, in the Southern Hemisphere, the modeled XN2O values show a minimum in February–April while the FTIR XN2O retrievals show different patterns. By comparing the partial column-averaged N2O from the model and NDACC for three vertical ranges (surface–8, 8–17, 17–50 km), we find that the discrepancy in the XN2O seasonal cycle between the model simulations and the FTIR measurements in the Southern Hemisphere is mainly due to their stratospheric differences.

Description: On 13 October 2017, the European Space Agency (ESA) successfully launched the Sentinel-5 Precursor satellite with the Tropospheric Monitoring Instrument (TROPOMI) as its single payload. TROPOMI is the first of ESA's atmospheric composition Sentinel missions, which will provide complete long-term records of atmospheric trace gases for the coming 30 years as a contribution to the European Union's Earth Observing program Copernicus. One of TROPOMI's primary products is atmospheric carbon monoxide (CO). It is observed with daily global coverage and a high spatial resolution of 7×7 km2. The moderate atmospheric resistance time and the low background concentration leads to localized pollution hotspots of CO and allows the tracking of the atmospheric transport of pollution on regional to global scales. In this contribution, we demonstrate the groundbreaking performance of the TROPOMI CO product, sensing CO enhancements above cities and industrial areas and tracking, with daily coverage, the atmospheric transport of pollution from biomass burning regions. The CO data product is validated with two months of Fourier-transform spectroscopy (FTS) measurements at nine ground-based stations operated by the Total Carbon Column Observing Network (TCCON). We found a good agreement between both datasets with a mean bias of 6 ppb (average of individual station biases) for both clear-sky and cloudy TROPOMI CO retrievals. Together with the corresponding standard deviation of the individual station biases of 3.8 ppb for clear-sky and 4.0 ppb for cloudy sky, it indicates that the CO data product is already well within the mission requirement.

Description: In this paper we describe the retrievals of atmospheric trace gases from near infrared, high resolution solar absorption spectroscopy measurements at the Lauder atmospheric research station in New Zealand and submitted to the Total Carbon Column Observing Network (TCCON) archive. The Lauder site (45.034°S, 169.68°E, 370 masl) is located within a sparsely populated region of the South Island of New Zealand, and is sheltered from the prevailing wind direction by the Southern Alps, which gives the site a high number of clear-sky days and an airmass that is largely unmodified by regional anthropogenic sources. The Lauder TCCON archive consists of data from two instruments; a Bruker IFS 120HR from June 2004 to February 2010 and a Bruker IFS 125HR from February 2010 to present. The bias between the two instrument is assessed to be 0.068% for CO2. Since measurements using the IFS 125HR began, the standard deviation about the hourly mean has been better than 0.1% for 96.81% of CO2 column retrievals. The retrievals have been calibrated against in situ airborne measurements to correct for biases and provide traceability to the World Meteorological Organisation (WMO) scales with an accuracy of 0.1% for CO2. The Lauder TCCON time series of retrieved dry-air mole fractions of CO2, CH4, N2O, HF, H2O, HDO and CO are available from the TCCON data archive. The DOIs are: doi:10.14291/tccon.ggg2014.lauder01.R0/1149293 for the IFS 120HR data doi:10.14291/tccon.ggg2014.lauder02.R0/1149298 for the IFS 125HR data.

Description: In this paper we describe the retrievals of atmospheric trace gases from near-infrared, high-resolution solar absorption spectroscopy measurements at the Lauder atmospheric research station in New Zealand and submitted to the Total Carbon Column Observing Network (TCCON) archive. The Lauder site (45.034° S, 169.68° E, 370 m a.s.l.) is located within a sparsely populated region of the South Island of New Zealand and is sheltered from the prevailing wind direction by the Southern Alps, which gives the site a high number of clear-sky days and an air mass that is largely unmodified by regional anthropogenic sources. The Lauder TCCON archive consists of data from two instruments: a Bruker IFS 120HR from June 2004 to February 2010 and a Bruker IFS 125HR from February 2010 to present. The bias between the two instruments is assessed to be 0.068 % for CO2. Since measurements using the IFS 125HR began, the SD about the hourly mean has been better than 0.1 % for 96.81 % of CO2 column retrievals. The retrievals have been calibrated against in situ airborne measurements to correct for biases and provide traceability to the World Meteorological Organization (WMO) scales with an accuracy of 0.1 % for CO2. The Lauder TCCON time series of retrieved dry-air mole fractions of CO2, CH4, N2O, HF, H2O, HDO and CO are available from the TCCON data archive. The DOIs arehttps://doi.org/10.14291/tccon.ggg2014.lauder01.R0/1149293 for the IFS 120HR datahttps://doi.org/10.14291/tccon.ggg2014.lauder02.R0/1149298 for the IFS 125HR data.

Description: Several researchers have suggested that perturbed stress fields associated with active faults significantly affect flow properties in the subsurface. To examine this insight quantitatively, we combine an elastic dislocation model of active faults and a stress-compliant function for fracture apertures to evaluate flow properties in a fractured basement reservoir in the Yufutsu field, Japan. The most productive well in the reservoir shows about 90° of rotation of the axes of horizontal stresses along the well trajectory, whereas the directions of horizontal stresses at nonproductive wells are commonly oriented consistently with those of the regional stresses. Additionally, results of step-rate hydraulic injection tests at the nonproductive well indicate that fracture apertures fluctuate significantly in response to the magnitude of stresses normal to the fracture surfaces. These observations support the important effect of perturbed stress fields on flow properties. The observed stress orientation profiles are correlated to those from a geomechanical model. Nonproductive wells were drilled in the area where the regional stress state prevails, and the most productive well penetrates the area near the tips of active faults, where the stress concentration leads to enhancements in fracture permeability by opening or shearing. The modeled perturbed stress fields combined with a stress-compliant function for opening of fracture apertures can explain the observed difference of well performances to the first order. The model demonstrates that areas near the tips of active faults have high potentials for hydrocarbon production in fractured basement reservoirs. Tetsuya Tamagawa is a research manager of the reservoir characterization and monitoring group of Japan Petroleum Exploration Co., Ltd. (JAPEX) Research Center. In 1991, he received his M.S. degree of engineering from Kyushu University in mining engineering. In 2005–2006, he worked as a visiting researcher at the Rock Fracture Project held by Stanford University. His research interests include modeling of fracture network and predictions of fault-related fractures. David Pollard is Morris Professor of Earth Sciences at Stanford University and co-director of the Rock Fracture Project, an industrial affiliates program. He received a B.A. degree from Pomona College, a Ph.D. from Stanford University, and a Diploma of the Imperial College from Imperial College, all in structural geology. His research focuses on rock fracturing and faulting with applications to fluid flow in heterogeneous reservoirs.

Description: To unravel the four-dimensional evolution of the Elk Hills oil field, Kern County, California, we integrated seismically interpreted fault surfaces, stratigraphic units, and stratigraphic features with mechanical models. Correspondence of synthetic stratigraphic surfaces, deformed by modeled vertical-displacement fields, to seismically interpreted stratigraphic surfaces represented on structure contour maps suggests that the tectonic history described here is structurally, stratigraphically, and mechanically consistent, placing constraints on the regional deformation mechanism and local structure. During the period investigated, middle Miocene to present, the eastern and the western parts of the Elk Hills anticline developed in response to a regional horizontal shortening oriented at about 35°. The apparent bend in the trend of the anticline, from northwest-southeast in the western part of the field to east-west in the eastern part of the field, is generated by the intersection of two distinct fault systems. In both fault systems, north-dipping fault surfaces are backthrusts of older south-dipping faults. These results have direct implications for the migration and emplacement of hydrocarbons at Elk Hills, suggesting that the upper Miocene Stevens turbidite oil pools were derived from sources to the south. Additionally, this study indicates that the method by which stratigraphic and structural interpretations are incorporated into a sequence of forward mechanical models represents an effective means of constraining the structural evolution of a fault network that developed within a syndepositional tectonic setting. Patricia Fiore is a graduate student in the Structural Geology and Geomechanics Program at Stanford University. Her Ph.D. research uses both subsurface and outcrop data to investigate the mechanics of brittle deformation and folding. She received a B.A. degree in earth and planetary sciences from Harvard University in 2000. David Pollard is the Morris Professor of Earth Sciences at Stanford University and codirector of the Rock Fracture Project, an industrial affiliates program. He received a B.A. degree from Pomona College, an M.Sc. degree from Imperial College, and a Ph.D. from Stanford University in structural geology. His research focuses on rock fracturing and faulting, with applications to fluid flow in heterogeneous reservoirs. Bill Currin earned a B.S. degree in geology from the Slippery Rock State University. He has been an exploration and exploitation geophysicist for 25 years with Occidental Oil and Gas Corporation. He is currently a chief geophysicist of Oxy's Libya exploration program. David M. Miner earned a B.S. degree in geology from the University of New Hampshire and an M.S. degree in mineral economics from the Colorado School of Mines. He was an exploration geologist for 27 years with Occidental Oil and Gas Corporation. Currently, he works as an exploitation geologist for Aera Energy LLC.

Description: Column-averaged dry-air mole fraction of CO (XCO) measurements are obtained from two ground-based Fourier transform infrared (FTIR) spectrometer networks: the Total Carbon Column Observing Network (TCCON) and the Network for the Detection of Atmospheric Composition Change (NDACC). In this study, the differences between the TCCON and NDACC XCO measurements are investigated and discussed based on six NDACC–TCCON sites using data over the period 2007–2017. A direct comparison shows that the NDACC XCO measurements are about 5.5 % larger than the TCCON data at Ny-Ålesund, Bremen, and Izaña (Northern Hemisphere), and the absolute bias between the NDACC and TCCON data is within 2 % at Saint-Denis, Wollongong and Lauder (Southern Hemisphere). The hemispheric dependence of the bias is mainly attributed to their smoothing errors. The systematic smoothing error of the TCCON XCO data varies in the range between 0.2 % (Bremen) and 7.9 % (Lauder), and the random smoothing error varies in the range between 2.0 % and 3.6 %. The systematic smoothing error of NDACC data is between 0.1 % and 0.8 %, and the random smoothing error of NDACC data is about 0.3 %. For TCCON data, the smoothing error is significant because it is higher than the reported uncertainty, particularly at Southern Hemisphere sites. To reduce the influence from the a priori profiles and different vertical sensitivities, the scaled NDACC a priori profiles are used as the common a priori profiles for comparing TCCON and NDACC retrievals. As a result, the biases between TCCON and NDACC XCO measurements become more consistent (5.6 %–8.5 %) with a mean value of 6.8 % at these sites. To determine the sources of the remaining bias, regular AirCore measurements at Orléans and Sodankylä are compared to co-located TCCON measurements. It is found that TCCON XCO measurements are 6.1 ± 1.6 % and 8.0 ± 3.2 % smaller than the AirCore measurements at Orléans and Sodankylä, respectively, indicating that the scaling factor of TCCON XCO data should be around 1.0000 instead of 1.0672. Further investigations should be carried out in the TCCON community to determine the correct scaling factor to be applied to the TCCON XCO data. This paper also demonstrates that the smoothing error must be taken into account when comparing FTIR XCO data, and especially TCCON XCO data, with model or satellite data.