Browsing by Author "Loh, ZM"

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    A global transport model comparison for methane: results for two Australian sites
    (International Union of Geodesy and Geophysics, 2011-07-06) Law, RM; Loh, ZM; Corbin, KD; Krummel, PB; Steele, LP; Fraser, PJ; Etheridge, DM; Zahorowski, W
    Methane (CH4) is an important greenhouse gas. Using atmospheric CH4 measurements to estimate CH4 emissions requires a good understanding of how CH4 is transported in the atmosphere. Hence, simulations of atmospheric CH4 concentration have been made with two atmospheric models, namely ACCESS and CCAM, as part of the Transport Model Intercomparison project, TransCom-CH4. The simulations ran for the period 1990-2008 and used six different sets of surface CH4 emissions, while the chemical CH4 sink was modelled using prescribed OH and stratospheric loss fields. Radon, sulphur hexafluoride and methyl chloroform tracers were also simulated. Model output has been analysed for two Australian sites with in-situ CH4 measurements: Cape Grim, Tasmania (AGAGE in-situ data) and the CO2CRC Otway project, Victoria. Cape Grim is a coastal site, observing periods of clean (baseline) air from the Southern Ocean and periods of non-baseline air, influenced by emissions from South Eastern Australia including Melbourne. Otway is a rural location, 4 km from the coast, where the land use is predominantly dairy farming, resulting in a large local CH4 signal from enteric fermentation (diurnal amplitudes up to 250 ppb). Nevertheless, during well mixed periods, measured CH4 concentrations at Otway may be similar to the baseline CH4 concentrations measured at Cape Grim or represent broader regional South Eastern Australian emissions. Preliminary findings indicate that CH4 at Otway and non-baseline CH4 at Cape Grim are sensitive to the choice of wetland emissions. There is also some indication that Melbourne emissions may be underestimated in these simulations.
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    Methane simulations at Cape Grim to assess methane flux estimates for South East Australia
    (Centre for Australian Weather and Climate Research, 2011-11-15) Loh, ZM; Law, RM; Corbin, KD; Steele, LP; Krummel, PB; Fraser, PJ; Zahorowski, W
    A transport model intercomparison for methane (TransCom-CH4) has been run involving twelve models (Patra et al., 2011). We contributed simulations using two climate models, CCAM and ACCESS. The CCAM simulations were nudged to NCEP analysed meteorology, which allows simulated atmospheric concentrations to be compared to observations on synoptic timescales. The ACCESS simulations were forced only with observed sea surface temperatures and are consequently not expected to match observed synoptic variations. The TransCom experiment involved simulating six CH4 tracers (with different prescribed fluxes) along with SF6, radon and methyl chloroform. We have analysed hourly model output for Cape Grim and find that the magnitude of the non-baseline signal differs, especially in winter, dependent on the CH4 flux scenario used. The magnitude of the non-baseline signal also varies between models, although these differences can be reconciled when methane is scaled by model-simulated radon concentration. Comparison with observed CH4, also scaled using radon, suggests that the CH4 flux scenario with little or no wetland emissions in winter matches the observations. The observations also indicate an apparent extra source of CH4 in October-November not seen in the model simulations. However this appears to be an artefact of this analysis method which assumes that radon emissions are known (and in this case constant in space and time). We have found that the discrepancy between model and observations in spring appears to be due to a poor simulation of radon, rather than CH4. Observed radon shows a larger seasonality than modelled radon, which suggests that temporal and spatial variations in radon flux need to be considered. It would also be helpful to understand why the simulated CCAM and ACCESS radon (and non-baseline CH4) concentrations differ in magnitude. Comparisons with Cape Grim output from the other participating TransCom-CH4 models may provide some insight.
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    A new pilot Australian tropical atmospheric research station (ATARS)
    (CSIRO Marine and Atmospheric Research, 2014-01-01) van der Schoot, MV; Fraser, PJ; Krummel, PB; Spencer, DA; Loh, ZM; Langenfelds, RL; Steele, LP; Gregory, RL; Meyer, CP; Keywood, MD; Lawson, S; Fedele, R; Atkinson, B; Klau, D; Zahorowski, W
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    Speculation on the origin of sub-baseline excursions of CH4 at Cape Grim
    (NOAA Earth System Research Laboratory, 2016-01-01) Loh, ZM; Krummel, PB; Gregory, RL; Steele, LP; Stavert, AR; Schoot, MVVD; Spencer, DA; Mitrevski, B; Thornton, DP; Galbally, IE; Ward, JZ; Somerville, NT; Chambers, SD; Williams, AG
    The Advanced Global Atmospheric Gases Experiment (AGAGE) program has historically measured in situ methane (CH4 ) at Cape Grim via gas chromatography with flame ionization detection (GC-FID) in 40 minutely grab samples. By adding continuous, high precision in situ measurements of CH4 (Picarro cavity ring-down spectroscopy [CRDS]) at both Cape Grim, Tasmania, and Casey, Antarctica, a new feature has become apparent in the Cape Grim CH4 record. During the austral summer (December to February), the Cape Grim CH4 record periodically drops below baseline. For example, in Figure 1, a number of sustained episodes of depressed CH4 concentration can be seen below the baseline selected data shown in red. Notably, these episodes are also seen in the GC-FID record. In this presentation, we examine these sub-baseline excursions of CH4 . In conjunction with meteorology and a variety of other chemical species measured at Cape Grim, including radon, ozone, hydrogen and ethane, we speculate on a number of possible mechanisms that might be responsible for these dips in CH4 mixing ratio.
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    Testing atmospheric monitoring techniques for geological storage of CO2
    (Centre for Australian Weather and Climate Research, 2011-11-15) Etheridge, DM; Loh, ZM; Luhar, A; Leuning, R; Steele, LP; Allison, CE; Smith, AM; Hibberd, MF; Feitz, A; Berko, H
    The success of CO2 geological storage in mitigating climate change will depend on its ability to withhold large amounts of CO2 from the atmosphere over centuries or more. Atmospheric techniques have been used to monitor Australia’s first geosequestration project, the CO2CRC Otway Project, since its inception (Etheridge et al. 2011; Jenkins et al. 2011). These techniques have been developed to be sensitive (detecting small potential leakage signals against large and variable background CO2 concentrations and fluxes), specific (attributing variations to sources using chemical and isotopic fingerprints and dispersion modelling) and practical (continuous remote operation) (Leuning et al. 2008; Luhar et al. 2009). A recent stage of the Otway project involved periods of controlled releases of injected gas at the surface that could mimic leakage. This provided a test of the original atmospheric scheme, complemented by additional measurements of CO2 and CH4 concentrations and carbon isotopes of CO2. Based on the experience at Otway and recent results from the new Arcturus baseline atmospheric station in Queensland, this presentation will consider the potential merits of atmospheric techniques for monitoring greenhouse gas emissions from emerging energy technologies such as geosequestration and coal seam methane. © 2011 CSIRO and the Bureau of Meteorology.
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    Transport modelling and inversions for the interpretation of greenhouse gas measurements
    (Bureau of Meteorology and CSIRO Oceans and Atmosphere Flagship, 2014-11-12) Law, RM; Loh, ZM; Ziehn, T; Haynes, KD; Krummel, PB; Steele, LP; Chambers, SD; Williams, AG
    The interpretation of greenhouse gas measurements can be aided by forward transport modelling while greenhouse gas fluxes can be estimated using atmospheric inversions. Here we (a) provide an update on a study of methane model simulations at Cape Grim and their use for determining methane fluxes from SE Australia and (b) show results from some recent CO2 inversions. Observed and model simulated non-baseline methane concentrations at Cape Grim have been compared (Loh et al., 2014). Two atmospheric models (CCAM and ACCESS) and six different methane emission scenarios are used. To minimise the influence of transport model errors on the analysis, deviations of Cape Grim methane concentration above baseline have been compared to coincident radon measurements. This methane to radon ratio shows a clear seasonal signal implying seasonal variations in methane emissions from SE Australia relative to a more temporally uniform radon flux. The ability of the model simulations to match the observed seasonality is dependent on the choice of methane emission scenario but all scenarios underestimate the observed methane to radon ratio in spring. We find that the most likely explanation for the discrepancy is wetland emissions that are too small in some emission scenarios or at the wrong time of year in other scenarios. CO2 inversions have been run recently for two purposes. The first is an international comparison of greenhouse gas inversions focussed on South, East and South East Asia. We have submitted a CCAM inversion for 1993-2012 using a fixed year of winds and expect to submit a second inversion with interannually varying winds. The second purpose is to use a CO2 inversion to estimate the magnitude of regional fluxes that are required to fit the larger difference in annual mean CO2 concentration between Mauna Loa and Cape Grim over recent years.