Land-atmosphere interactions and their effect on Australian precipitation

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

Local and Remote Drivers of Southeast Australian Drought

Droughts are associated with large‐scale modes of variability, synoptic‐scale systems, and terrestrial processes. Quantifying their relative roles in influencing drought guides process understanding, helps identify weaknesses in climate models, and focuses model improvements. Using a Lagrangian back‐trajectory approach we provide the first quantification of the change in moisture supply during major droughts in southeast Australia, including the causes of the changes. Drought onset and intensification 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. During termination, strengthened moist easterly flows from the Tasman and Coral Seas promoted anomalously high rainfall. Our approach reveals terrestrial moisture sources played a secondary role, amplifying rainfall anomalies by less than 6%. Simulating droughts therefore requires deeper understanding of the relationship between moisture advection and synoptic‐scale circulation and how large‐scale climate variability and terrestrial processes modify these relationships.This work was possible thanks to an Australian National University AGRT Scholarship (C.M.H.

Australian Precipitation Recycling and Evaporative Source Regions

The relative importance of atmospheric advection and local land–atmosphere coupling to Australian precipitation is uncertain. Identifying the evaporative source regions and level of precipitation recycling can help quantify the importance of local and remote marine and terrestrial moisture to precipitation within the different hydroclimates across Australia. Using a three-dimensional Lagrangian back-trajectory approach, moisture from precipitation events across Australia during 1979–2013 was tracked to determine the source of moisture (the evaporative origin) and level of precipitation recycling. We show that source regions vary markedly for pre- cipitation falling in different regions. Advected marine moisture was relatively more important than terrestrial contributions for precipitation in all regions and seasons. For Australia as a whole, contributions from precip- itation recycling varied from ;11% in winter up to ;21% in summer. The strongest land–atmosphere coupling was in the northwest and southeast where recycled local land evapotranspiration accounted for an average of 9% of warm-season precipitation. Marine contributions to precipitation in the northwest of Australia increased in spring and, coupled with positive evaporation trends in the key source regions, suggest that the observed pre- cipitation increase is the result of intensified evaporation in the Maritime Continent and Indian and Pacific Oceans. Less clear were the processes behind an observed shift in moisture contribution from winter to summer in southeastern Australia. Establishing the climatological source regions and the magnitude of moisture re- cycling enables future investigation of anomalous precipitation during extreme periods and provides further insight into the processes driving Australia’s variable precipitation.This work was made possible by an Australian National University Australian Government Research Training Scholarship for author Holgate and support from the ARC Centre of Excellence for Climate System Science (CE110001028). Holgate and author van Dijk were supported through the ARC Discovery Projects funding scheme (project DP40103679). Authors Evans and Pitman were supported through the ARC Centre of Excellence for Climate Extremes (CE170100023). The authors thank the NCI and its staff for computational support and Jessica Keune and two anonymous reviewers for their constructive feedback. CORDEX-Australasia climate simulations are publicly available online (https:// climatechange.environment.nsw.gov.au/Climate-projections- for-NSW/About-NARCliM

Using alternative soil moisture estimates in the McArthur Forest Fire Danger Index

McArthur’s Forest Fire Danger Index (FFDI) incorporates the Keetch–Byram Drought Index (KBDI) estimate of soil dryness. Improved approaches for estimating soil moisture now exist, with potential for informing the calculation of FFDI. We evaluated the effect, compared with KBDI, of two alternative methods of estimating soil moisture: the rainfall-based Antecedent Precipitation Index and soil moisture from the Soil Moisture Ocean Salinity satellite mission. These methods were used to calculate FFDI over a sample period of 5 years (2010–14) at seven locations around Australia. The effect of substituting the alternatives for KBDI, and of entirely replacing the Drought Factor (DF) (a measure of fuel availability in FFDI) with the alternatives was explored by studying the effect on magnitude, distribution and timing of FFDI and associated Fire Danger Rating (FDR). Both approaches predicted drier soil conditions than KBDI, resulting in fewer Low–Moderate FDR days and more days of High FDR and above. The alternative methods replacing KBDI had little effect on seasonal patterns of FDR. Of all approaches, replacing DF entirely with the soil moisture alternatives most closely mimicked McArthur’s FFDI. Overall, if alternative measures of soil moisture are adopted for FFDI, the entire replacement of the DF term should be considered

Comparison of remotely sensed and modelled soil moisture data sets across Australia

This study compared surface soil moisture from 11 separate remote sensing and modelled products across Australia in acommonframework. The comparison was based on a correlation analysis between soil moisture products and in situ data collated from three separate ground-based networks: OzFlux, OzNet and CosmOz. The correlation analysis was performed using both original data sets and temporal anomalies, and was supported by examination of the time series plots. The interrelationships between the products were also explored using cluster analyses. The products considered in this study include: Soil Moisture Ocean Salinity (SMOS; both Land Parameter Retrieval Model (LPRM) and L-band Microwave Emission of the Biosphere (LMEB) algorithms), Advanced Microwave Scanning Radiometer 2 (AMSR2; both LPRM and Japan Aerospace Exploration Agency (JAXA) algorithms) and Advanced Scatterometer (ASCAT) satellite-based products, and WaterDyn, Australian Water Resource Assessment Landscape (AWRA-L), Antecedent Precipitation Index (API), Keetch-Byram Drought Index (KBDI), Mount’s Soil Dryness Index (MSDI) and CABLE/BIOS2 modelbased products. The comparison of the satellite and model data sets showed variation in their ability to reflect in situ soil moisture conditions across Australia owing to individual product characteristics. The comparison showed the satellite products yielded similar ranges of correlation coefficients, with the possible exception of AMSR2 JAXA. SMOS (both algorithms) achieved slightly better agreement with in situ measurements than the alternative satellite products overall. Among the models, WaterDyn yielded the highest correlation most consistently across the different locations and climate zones considered. All products displayed a weaker performance in estimating soil moisture anomalies than the original data sets (i.e. the absolute values), showing all products to be more effective in detecting interannual and seasonal soil moisture dynamics rather than individual events. Using cluster analysis we found satellite products generally grouped together, whereas modelsweremoresimilar to other models.SMOS(basedonLMEBalgorithm and ascending overpass) and ASCAT (descending overpass) were found to be very similar to each other in terms of their temporal soil moisture dynamics, whereas AMSR2 (based on LPRM algorithm and descending overpass) and AMSR2 (based on JAXA algorithm and ascending overpass) were dissimilar. Of the model products, WaterDyn and CABLE were similar to each other, as were the API/AWRA-L and KBDI/MSDI pairs. The clustering suggests systematic commonalities in error structure and duplication of information may exist between products. This evaluation has highlighted relative strengths, weaknesses, and complementarities between products, so the drawbacks of each may be minimised through a more informed assessment of fitness for purpose by end users.Vanessa Haverd, Cathy Trudinger and Peter Briggs thank the support of the Australian Climate Change Science Program. The contribution of Richard De Jeu and RobinVan Der Schalie was funded by the European Space Agency through the Climate Change Initiative for Soil Moisture (Contract 4000104814/11/I-NB)