School of Earth Sciences - Theses
Permanent URI for this collection
Search Results
1 - 2 of 2
-
ItemThe Impacts of Atmospheric Rivers in Australia and New Zealand( 2022)Atmospheric Rivers (ARs) are narrow filaments of strong water vapour transport in the lower troposphere. They are responsible for about 90% of the meridional moisture transport over the globe and are comparable in magnitude to water transport in the largest terrestrial rivers on Earth. Globally, ARs can be associated with numerous hazards including heavy rainfall, floods, blizzards, landslides, strong winds and polar heatwaves. For these reasons, it is imperative for scientists to understand and ultimately be able to predict AR behaviour in the present and future climate. Atmospheric River research has been concentrated in the Northern Hemisphere particularly over North America and Europe. This is the first thesis to explore AR impacts over Australia. We developed a global AR identification algorithm and tested the sensitivity of the AR frequency results to the input dataset parameters including resolution, regridding method and moisture transport threshold. The results showed that the combination of low moisture thresholds and restrictive geometric requirements can lead identification algorithms to miss the strongest ARs especially in the Pacific Ocean. Additionally, the resolution and regridding method of the input data, and the order of regridding and calculating moisture flux, can all impact the final AR frequency results. We applied this identification algorithm to the European Centre for Medium-Range Weather Forecasts reanalysis (ERA5) and used this new dataset to analyse AR impacts over Australia and New Zealand (NZ). It was found that nine of the ten most expensive floods in NZ between 2007-2017 were associated with AR events and seven to ten of the ten wettest rainfall days at eleven different stations occurred at the same time as an AR over that station. In Australia, ARs contribute about 10-20% of annual rainfall except in the Murray-Darling Basin region, in the southeast, where ARs are associated with approximately one-third of the mean annual rainfall. Similarly, 30-40% of the heaviest rainfall days over southeast Australia occur during the passage of an AR. Using composites of the vertical structure of ARs in ERA5, we showed that the intensity of ARs that form at tropical and subtropical latitudes is driven by the wind component of the moisture flux, while ARs that form in the extratropics are strengthened by an increase in the specific humidity component of the moisture flux. Following widespread flooding over eastern Australia associated with persistent and high integrated water vapour transport (IVT), we evaluated and assessed future IVT changes in the latest generation of the Coupled Model Intercomparison Project (CMIP6) global climate models. We found that daily IVT would likely become more extreme over Sydney – Australia’s most populated city. This thesis fills a key regional gap in AR science and furthers understanding of extreme rainfall over Australia. It also contributes to our understanding of AR structure and potential response to climate change, which is valuable globally. This thesis provides the foundation for future AR work in the areas of climate change impacts and forecasting extreme rainfall over Australia and New Zealand.
-
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.