Land-atmosphere interactions and their effect on Australian precipitation
Creators: name: Holgate, Chiara
Publisher: name: The Australian National University
Year: 2021

description: The aim of the research presented in this thesis is to determine the influence of land-atmosphere interactions on Australian precipitation, both under average conditions and during drought. This aim is addressed using a combination of statistical and numerical atmospheric water accounting techniques. The first part of the research examines soil moisture, a key variable that underpins the analysis of land-atmosphere interactions. Due to the range of estimation techniques and variety of applications utilising soil moisture information, numerous data sets are available. This thesis evaluates soil moisture from ground, satellite and model estimates across Australia and identifies data sets suitable to the study of land-atmosphere interactions and other applications. Soil moisture information was then combined with observations of precipitation to identify where land-atmosphere interactions have a detectable influence on Australian precipitation. Analysing the statistical relationship between soil moisture and subsequent precipitation, the results showed detectable relationships in north and southeast Australia and the importance of scale in interpreting physical relationships with a statistical metric. With regions of land-atmosphere interaction identified, the next stage of the research quantified the interaction with the precipitation recycling ratio - a measure of how much of a region's precipitation is derived from evaporation in that same region. Precipitation recycling was quantified using a "back-trajectory" model that identified the evaporative moisture sources of Australia's precipitation. Strongest land-atmosphere interactions and recycling were found in the north and southeast of the continent in spring and summer, along with long term trends in regional moisture sources. The importance of land-atmosphere interaction during drought was the subject of the final stage of the research. Focusing on the Murray-Darling Basin in southeast Australia, the research analysed the sources of moisture supplying precipitation and the degree to which the land surface amplified precipitation anomalies during drought onset, persistence and termination. The results indicate that major droughts were driven by reduced moisture supply from the ocean, as moisture was circulated away from the region, combined with an absence of precipitation-generating mechanisms over land. Droughts terminated when moist easterly flows from the Tasman and Coral Seas strengthened, promoting high precipitation. Terrestrial moisture sources played a secondary role, amplifying precipitation anomalies by less than 6%. In summary, the research presented in this thesis has determined the influence of land-atmosphere interactions on Australian precipitation, both under average conditions and during drought. The analysis demonstrates that Australian precipitation is predominantly driven by large scale processes transporting marine moisture to the continent for precipitation, with terrestrial moisture sources forming an important contribution to precipitation in the north and southeast of the continent. In the southeast, drought is driven by atmospheric circulation anomalies redirecting ocean moisture away from the region, with land-atmosphere interactions playing a secondary role.