ANSTO :: Browsing by Author "Law, RM"

Browsing by Author "Law, RM"

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Constraining annual and seasonal radon-222 flux density from the Southern Ocean using radon-222 concentrations in the boundary layer at Cape Grim
(Taylor & Francis Group, 2013-02-14) Zahorowski, W; Griffiths, AD; Chambers, SD; Williams, AG; Law, RM; Crawford, J; Werczynski, S
Radon concentrations measured between 2001 and 2008 in marine air at Cape Grim, a baseline site in northwestern Tasmania, are used to constrain the radon flux density from the Southern Ocean. A method is described for selecting hourly radon concentrations that are least perturbed by land emissions and dilution by the free troposphere. The distribution of subsequent radon flux density estimates is representative of a large area of the Southern Ocean, an important fetch region for Southern Hemisphere climate and air pollution studies. The annual mean flux density (0.27 mBq m 2 s 1) compares well with the mean of the limited number of spot measurements previously conducted in the Southern Ocean (0.24 mBq m 2 s 1), and to some spot measurements made in other oceanic regions. However, a number of spot measurements in other oceanic regions, as well as most oceanic radon flux density values assumed for modelling studies and intercomparisons, are considerably lower than the mean reported here. The reported radon flux varies with seasons and, in summer, with latitude. It also shows a quadratic dependence on wind speed and significant wave height, as postulated and measured by others, which seems to support our assumption that the selected least perturbed radon concentrations were in equilibrium with the oceanic radon source. By comparing the least perturbed radon observations in 2002 2003 with corresponding ‘TransCom’ model intercomparison results, the best agreement is found when assuming a normally distributed radon flux density with s 0.075 mBq m 2 s 1. © 2013, W. Zahorowski et al.
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.
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.
Simulations of atmospheric methane for Cape Grim, Tasmania, to constrain southeastern Australian methane emissions
(Copernicus Publications, 2014-01-13) Loh, ZH; Law, RM; Haynes, KD; Krummel, PB; Steele, LP; Fraser, PJ; Chambers, SD; Williams, AG
This study uses two climate models and six scenarios of prescribed methane emissions to compare modelled and observed atmospheric methane between 1994 and 2007, for Cape Grim, Australia (40.7° S, 144.7° E). The model simulations follow the TransCom-CH4 protocol and use the Australian Community Climate and Earth System Simulator (ACCESS) and the CSIRO Conformal-Cubic Atmospheric Model (CCAM). Radon is also simulated and used to reduce the impact of transport differences between the models and observations. Comparisons are made for air samples that have traversed the Australian continent. All six emission scenarios give modelled concentrations that are broadly consistent with those observed. There are three notable mismatches, however. Firstly, scenarios that incorporate interannually varying biomass burning emissions produce anomalously high methane concentrations at Cape Grim at times of large fire events in southeastern Australia, most likely due to the fire methane emissions being unrealistically input into the lowest model level. Secondly, scenarios with wetland methane emissions in the austral winter overestimate methane concentrations at Cape Grim during wintertime while scenarios without winter wetland emissions perform better. Finally, all scenarios fail to represent a~methane source in austral spring implied by the observations. It is possible that the timing of wetland emissions in the scenarios is incorrect with recent satellite measurements suggesting an austral spring (September–October–November), rather than winter, maximum for wetland emissions. © Author(s) 2015. Creative Commons Attribution 3.0 Licence
TransCom continuous experiment: comparison of 222Rn transport at hourly time scales at three stations in Germany
(Copernicus, 2011) Taguchi, S; Law, RM; Rödenbeck, C; Patra, PK; Maksyutov, S; Zahorowski, W; Sartorious, H; Levin, I
Fourteen global atmospheric transport models were evaluated by comparing the simulation of 222Rn against measurements at three continental stations in Germany: Heidelberg, Freiburg and Schauinsland. Hourly concentrations simulated by the models using a common 222Rn-flux without temporal variations were investigated for 2002 and 2003. We found that the mean simulated concentrations in Heidelberg are related to the diurnal amplitude of boundary layer height in each model. Summer mean concentrations simulated by individual models were negatively correlated with the seasonal mean of diurnal amplitude of boundary layer height, while in winter the correlation was positive. We also found that the correlations between simulated and measured concentrations at Schauinsland were higher when the simulated concentrations were interpolated to the station altitude in most models. Temporal variations of the mismatch between simulated and measured concentrations suggest that there are significant interannual variations in the 222Rn exhalation rate in this region. We found that the local inversion layer during daytime in summer in Freiburg has a significant effect on 222Rn concentrations. We recommend Freiburg concentrations for validation of models that resolve local stable layers and those at Heidelberg for models without this capability. © 2011, © Author(s) 2011.
TransCom continuous experiments: comparison of radon concentrations at inland stations
(Global Carbon Project, 2008-06) Taguchi, S; Law, RM; Peters, W; Rodenbeck, C; Patra, PK; Maksyutov, S; Zahorowski, W; Sartorius, H; Levin, I
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.

Browsing by Author "Law, RM"

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