Browsing by Author "McCabe, MF"

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    A comparison between direct and pan-derived measurements of the isotopic composition of atmospheric waters
    (The Modelling and Simulation Society of Australia and New Zealand Inc., 2011-12-12) Azcurra, CS; Hughes, CE; Parkes, SD; Hollins, SE; Gibson, JJ; McCabe, MF; Evans, JP
    The stable isotopes of water can be used to examine and quantify the contribution to atmospheric moisture from evaporation, transpiration and surface-waters. However, obtaining extensive and ongoing time series data of the isotopic composition of atmospheric moisture has been difficult. Presented here is an alternate method using an isotope mass balance approach to estimate the isotopic composition of atmospheric moisture using water samples collected from class A evaporation pans. While this evaporation pan method does not provide the high-resolution time series data that can be obtained from an isotope analyser taking in-situ measurements of atmospheric moisture, the method is relatively simple and inexpensive to set-up and maintain. In this preliminary investigation, a comparison between the isotopic composition of atmospheric moisture estimated from the evaporation pan method and in-situ measurements of the isotopic composition of water vapour using a Fourier Transform Infrared (FTIR) spectrometer deployed at the Lucas Heights weather station in New South Wales is undertaken. Through comparison of the two series of hydrogen isotope data, an assessment of the evaporation pan method can be made. Although there was some agreement between the isotopic composition of vapour measured by the FTIR spectrometer and the estimation for the atmospheric moisture (R2 = 0.49), the comparison is sensitive to climatic parameters that vary significantly within a 24-hour period such as the relative humidity of air and the air and pan temperatures. Inverting the model to use the FTIR spectrometer measurements at an hourly resolution improved the performance of the model (R2 =0.57). However, this also revealed that the model produced more depleted values of the evaporation pan water isotopes than those observed. In contrast, there was a variable relationship between the modelled and observed isotope values of atmospheric moisture. These conflicting results will need to be resolved before the evaporation pan method is broadly applied in isotope hydrology. © 2011 The Modelling and Simulation Society of Australia and New Zealand Inc.
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    Comparison of in-situ water vapour isotope analysers
    (American Geophysical Union, 2011-12-05) Element, A; Parkes, SD; Griffith, DWT; Wang, L; McCabe, MF
    Recently there have been a number of spectroscopic based in-situ water vapour isotope analysers developed. These analysers are capable of providing datasets that are useful for a range studies including interpreting rapid fluctuations associated with land atmosphere exchange processes, and validation of process based models that work on different spatial and temporal scales. Here we present a comparison of three spectroscopic analysers that provide in-situ analysis of water vapour isotopes. These include a Fourier Transform InfraRed (FTIR) spectroscopy based system (broad band technique), a system based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS), and a Wavelength-Scanned Cavity Ring-Down Spectroscopy (WS-CRDS) system. Using a continuous flow calibration system, liquid isotopic standards were vaporised and used to determine the response of the three analysers to changes isotopic composition and the water vapour mixing ratio. Based on these experiments it was shown that all three analysers required significant corrections to move the raw data onto the Standard Mean Ocean Water (SMOW) scale. We then deployed the three analysers at a coastal location near Sydney, Australia where diurnal temperature variations are relatively small and high mixing ratios are generally observed. The WS-CRDS and OA-ICOS were then deployed in central NSW, Australia where large diurnal temperature variations and relatively low mixing ratios were observed. The in-situ isotope data collected from the three analysers is then compared between these two locations.
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    Continuous monitoring of mixing depth with radon-222 and lidar
    (Australian Meterological & Oceanographic Society, 2012-01-31) Griffiths, AD; Chambers, SD; Parkes, SD; McCabe, MF
    We report on the development of a combined approach using both lidar and radon-222 measurements to obtain a near-continuous record of mixing depth which can, in turn, be used to assist in the interpretation of simultaneous trace gas measurements. Interactions between the land surface and the atmosphere above are moderated by the strength and depth of mixing in the lower atmosphere which ranges diurnally between several meters at night to over one kilometer during the day. Elastic backscatter lidar can be used to measure the depth of mixing during the day, i.e. the height of the planetary boundary layer (PBL), by employing the change in aerosol concentration, and hence lidar signal, at the boundary between the PBL and the free atmosphere. These measurements are only possible when the mixing depth is large. A complimentary approach, based on radon measurements, works well from the time turbulence decays in the afternoon through till mid morning when mixing depths are too small to be observed using lidar. Radon- 222 is chemically inert and is released from the surface at a relatively constant rate and as such is a natural passive tracer. Since it is radioactive, with a half-life of 3.8 days, it does not accumulate in the atmosphere. At horizontally homogeneous inland sites, vertical mixing is the main process affecting near-surface concentration. An estimate can therefore be obtained of an “equivalent mixing depth” from time-series of radon concentration measurements, which can themselves be obtained with robust and low-maintenance instrumentation. Using two measurement techniques sidesteps the limitations of each to make a combined dataset a useful component of field studies which seek to understand the exchanges of trace gases between the land surface and atmosphere.
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    Daily observations of rainfall, vapour and pan water δ2H for improved quantification of atmospheric and terrestrial water interactions
    (American Geophysical Union, 2012-12-03) Hughes, CE; Azcurra, CS; Parkes, SD; Hollins, SE; McCabe, MF; Evans, JP; Pickering, D; Gibson, JJ; Edwards, K
    The stable isotopes of water (δ2H, δ18O) have been used extensively to track the movement of water through the hydrological cycle. As water moves through the hydrological cycle, its isotopic composition changes (fractionation) as a result of phase changes and interactions with other waters. When combined with other measurements of water fluxes and volumes, the stable isotopes can provide information about sources, processing, and transport of water suitable for determining water and energy balances. While the isotopic composition of atmospheric moisture influences the isotopic composition of terrestrial waters, obtaining ongoing time series data of this has been difficult. One method for estimating the isotopic composition of atmospheric moisture combines the Craig-Gordon isotopic evaporation model with a mass balance approach, using water samples collected from Class A evaporation pans at weekly intervals. The method assumes steady-state conditions and thus does not represent the highly variable meteorological conditions that can strongly influence fractionation. Although water-sampling regimes cannot match the frequency of the observed variability in meteorological conditions, increasing the sampling resolution to a sub-weekly resolution will more closely align isotopic measurements with meteorological conditions and allow the assessment of established water isotope relationships at a greater temporal frequency. An alternate method for estimating the isotopic composition of atmospheric moisture is based on local precipitation being in equilibrium with atmospheric moisture. As per the evaporation pan method, it is assumed that steady-state conditions apply. At the Lucas Heights weather station in south-eastern Australia, atmospheric moisture water isotopes (δA) have been directly measured at sub-hourly intervals using a Fourier Transform Infrared (FTIR) spectrometer. This hydrogen isotope time series was evaluated against daily observations of isotopes in water from a Class A evaporation pan (δL) and in precipitation (δP) collected from the same site. This study assesses the validity of established (steady-state) relationships between liquid and vapor water isotopes for interactions that have been measured at the medium-resolution temporal scale, and provides a basis for improving flux estimates based on an isotope mass balance approach. ©Author(s) 2012
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    Improved mixing height monitoring through a combination of lidar and radon measurements
    (European Geosciences Union, 2013-02-01) Griffiths, AD; Parkes, SD; Chambers, SD; McCabe, MF; Williams, AG
    Surface-based radon (222Rn) measurements can be combined with lidar backscatter to obtain a higher quality time series of mixing height within the planetary boundary layer (PBL) than is possible from lidar alone, and a more quantitative measure of mixing height than is possible from only radon. The reason why lidar measurements are improved is that there are times when lidar signals are ambiguous, and reliably attributing the mixing height to the correct aerosol layer presents a challenge. By combining lidar with a mixing length scale derived from a time series of radon concentration, automated and robust attribution is possible during the morning transition. Radon measurements provide mixing information during the night, but concentrations also depend on the strength of surface emissions. After processing radon in combination with lidar, we obtain nightly measurements of radon emissions and are able to normalise the mixing length scale for changing emissions. After calibration with lidar, the radonderived equivalent mixing height agrees with other measures of mixing on daily and hourly timescales and is a potential method for studying intermittent mixing in nocturnal boundary layers.© 2013, Copernicus Publications.
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    In-situ measurements of the stable isotopic composition of atmospheric water vapour using FTIR spectroscopy
    (Université Pierre et Marie Curie, 2010-04-27) Parkes, SD; Griffith, DWT; Williams, AG; Element, A; Chambers, SD; McCabe, MF
    The stable isotopic composition of atmospheric water vapour is related to the hydrological processes that occur along the back trajectory of an air mass, including evaporation at the moisture source, atmospheric mixing and precipitation. Thus, by collecting continuous measurements of the stable isotopes in water vapour a record of the hydrological history of air passing a site can be compiled. To collect such a record a FTIR instrument capable of making real‐time in‐situ measurements of the stable isotopes in water vapour has been developed. The instrument has been deployed at a site near Sydney, Australia for approximately 18 months. During this time we have shown that the FTIR instrument compares well with laser based analysers that are capable of making similar real‐time measurements. In addition to the comparison between the different analysers, we have been investigating some of the large signals that are observed in the time series of isotopic measurements. The analysis of the dataset indicates that the lowest isotope values are generally associated with cold fronts that pass over the South East of the Australian Continent and then over the Sydney region. When a cold front passes over or near the measurement site, the deuterium isotope value can be observed to change by up to 100 per mille within the space of a few hours. In addition, cold frontal passages with contrasting moisture source and precipitation histories exhibit systematic differences in water vapour stable isotope signals as they pass over Sydney. On the other hand, higher and more slowly changing isotope values are generally associated with anticyclonic conditions. The study shows that for our site the variations in the stable isotope values are strongly influenced by the hydrological history of air parcels at a synoptic scale.
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    Stable water isotope and surface heat flux simulation using ISOLSM: evaluation against in-situ measurements
    (Elsevier, 2015-04) Cai, MY; Wang, L; Parkes, SD; Strauss, J; McCabe, MF; Evans, JP; Griffiths, AD
    The stable isotopes of water are useful tracers of water sources and hydrological processes. Stable water isotope-enabled land surface modeling is a relatively new approach for characterizing the hydrological cycle, providing spatial and temporal variability for a number of hydrological processes. At the land surface, the integration of stable water isotopes with other meteorological measurements can assist in constraining surface heat flux estimates and discriminate between evaporation (E) and transpiration (T). However, research in this area has traditionally been limited by a lack of continuous in-situ isotopic observations. Here, the National Centre for Atmospheric Research stable isotope-enabled Land Surface Model (ISOLSM) is used to simulate the water and energy fluxes and stable water isotope variations. The model was run for a period of one month with meteorological data collected from a coastal sub-tropical site near Sydney, Australia. The modeled energy fluxes (latent heat and sensible heat) agreed reasonably well with eddy covariance observations, indicating that ISOLSM has the capacity to reproduce observed flux behavior. Comparison of modeled isotopic compositions of evapotranspiration (ET) against in-situ Fourier Transform Infrared spectroscopy (FTIR) measured bulk water vapor isotopic data (10 m above the ground), however, showed differences in magnitude and temporal patterns. The disparity is due to a small contribution from local ET fluxes to atmospheric boundary layer water vapor (∼1% based on calculations using ideal gas law) relative to that advected from the ocean for this particular site. Using ISOLSM simulation, the ET was partitioned into E and T with 70% being T. We also identified that soil water from different soil layers affected T and E differently based on the simulated soil isotopic patterns, which reflects the internal working of ISOLSM. These results highlighted the capacity of using the isotope-enabled models to discriminate between different hydrological components and add insight into expected hydrological behavior. © 2015, Elsevier B.V.