School of Earth Sciences - Theses
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ItemVertical structure Of atmospheric trace gases over Southeast Australia( 2000-01)Trace gas (CO2 and its carbon and oxygen isotopes, CH4, CO, H2 and N2O) vertical profile data above Cape Grim, Tasmania for the period April 1992 to February 1997 are investigated. A climatology of the distribution of each trace gas has been compiled from statistical treatment of the raw data. These climatologies are useful for verification of transport model outputs. Here, the CO2 climatology is compared to simulation results from two transport models (Melbourne University Transport Model and TM2Z) using three different sets of CO2 fluxes separately (compiled with different methods by different authors). Large discrepancies are found between simulations and observations, especially in the free troposphere (4-6 km). By considering emission ratios, trajectories, satellite fire counts and simulation with biomass burning fluxes, the influence of tropical biomass burning plumes on the southeastern Australian region in the austral winter/spring is studied and quantified. This identification process requires a multiple-species approach where the large CO anomalies and the unexpected behaviour of H2 are most revealing. The frequent presence of burning plumes in the mid troposphere complicates one of the original motivations for the Cape Grim Overflight Program, which is to estimate the air-sea exchange of CO2 in this region. A suggestion arising from analysis of pre-1992 aircraft sampling in this region was that the regional CO2 air-sea flux south of Australia is exceptionally large.
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ItemThe impacts of climate variability and change on severe thunderstorm environments in Australia( 2012)Severe thunderstorms present a relatively infrequent but significant threat to property and life in Australia during the spring and summer. These thunderstorms can produce hailstones over 2cm in diameter, winds in excess of 90kmh-1 and less frequently tornadoes. Any of these phenomena can result in localised high impact severe events. Recent examples of this potential are illustrated by damage caused by the 1999 Sydney Hailstorm, 2008 Gap Microburst and the 2010 and 2011 Melbourne Hailstorms. This risk makes the implications of a changing and variable climate on severe thunderstorms important to understand. Recent studies into the impacts of anthropogenic climate change on severe weather events, including thunderstorms, suggest a potential increasing trend in both frequency and intensity for Australia. While current convective parameterisations in both global and regional climate models limit direct assessments of future convection, the use of environmental parameters to estimate changes in severe thunderstorm environments has been successful in other geographical regions. This study seeks an answer to the question “Is the frequency and distribution of severe thunderstorm environments in Australia likely to change in the future?” A database of 1550 independent severe thunderstorm reports in Australia has been developed for the period March 2003 to April 2010. Severe thunderstorm reports are then used to identify relationships with their associated environments estimated using proximal soundings from a mesoscale numerical weather assimilation and prediction model (MesoLAPS). This proximity climatology of known severe thunderstorm environments has been successfully used to derive covariate discriminants that identify the potential of an environment to produce severe thunderstorms. These covariates use variables describing the potential for organised convection (deep-layer wind shear), and the potential for instability over the depth of the atmosphere (convective available potential energy). Applying these discriminants to a reanalysis dataset (ERA-Interim), a climatology of the frequency and spatial distribution of environments favourable to the development of severe and significant severe thunderstorms for Australia has been developed for warm seasons during the period 1979-2011. This climatology demonstrates that inter-annual variability in terms of both the frequency and spatial distribution of environments is influenced by El Niño- Southern Oscillation. La Niña conditions are typically associated with an increased frequency and an inland shift of favourable environments over eastern Australia, while El Niño typically results in fewer environments, particularly along the coastal fringe. Applying this climatology, the environments simulated by two climate models (CSIRO Mk3.6 and CCAM) for the 20-season period 1980-2000 are examined over Australia and tested against the reanalysis climatology. In particular, the ability of the models to resolve the intra-annual variability and both quantify and simulate the spatial distribution of convective variables are analysed, and are found to perform reasonably well, especially in the case of the higher resolution CCAM. Finally, future simulations of severe thunderstorm environments from high emissions projections for the period 2079-2099 are presented for both models. Comparing these simulations to the 20th century, a potential small increase in the frequency of severe thunderstorm environments appears likely for southeast and eastern Australia under a warming climate.
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ItemExtreme temperature events in Australia( 2001)A high-quality set of historical daily temperature data has been developed for Australia. This data set includes 103 stations, most of which have data from the period between 1957 and 1996, and some for longer periods. A new technique, involving the matching of frequency distributions, is presented for the adjustment of temperature records for inhomogeneities at the daily timescale, and this technique is used in the development of the data set. A number of additional findings are presented on the impact of changing times of observation and accumulation of observations over periods longer than one day on the Australian temperature record. This data set was used for an extensive study of extreme temperature events in Australia. Widespread changes in the frequency of extreme temperature events in Australia were found over the 1957-1996 period. These changes were found both by an analysis or trends at individual stations and by analysis of spatial averages of indices of extreme temperature. In general, increases were found in the frequency of high maximum and high minimum temperatures, and decreases in the frequency of low maximum and low minimum temperatures. The changes were greatest for low minimum temperatures and least for high maximum temperatures, and were generally greatest in winter. The greatest decreases in the frequency of extreme low minima were found in Queensland. The trends were not universal, with trends opposite to those for Australia as a whole being found in some regions in some seasons. It was found, after examination of several possible models, that the frequency distribution of Australian daily maximum and minimum temperatures was best represented by a composite of two or three Gaussian distributions with different parameters. Using this model, it was found that the observed changes in temperature primarily resulted from changes in the means of the component distributions, indicating that the changes resulted principally from overall warming of the atmosphere rather than changes in circulation or air-mass incidence. The relationship between the frequency of extreme temperatures and the Southern Oscillation Index (SOl) was examined, with strong relationships being found in some seasons in many parts of Australia for most extreme variables, particularly high maximum temperatures. The weakest relationships were found for low minimum temperatures. Many of these relationships, except in winter, were as strong (or stronger) with the value of the SOl one season previously as they were with the SOl of the current season, indicating potential useful skill in the forecasting of seasonal frequencies of extreme temperatures in many cases.