School of Earth Sciences - Theses
Permanent URI for this collection
Search Results
1 - 10 of 156
-
ItemNo Preview AvailableAntarctic sea ice and its interactions with high latitude weather and climate( 1998)Antarctic sea ice plays a major role in the earth system by greatly influencing the high latitude exchanges of heat, moisture and momentum between the ocean and atmosphere, as well as profoundly effecting the salt budget of the ocean, and thus the production of Antarctic Bottom Water, one of the driving mechanisms of worldwide oceanic circulation. With such considerable and far reaching impact, it is important to document its climatology, understand its variability and quantify its influence. Climatologies and trends of the Southern Ocean sea ice pack are presented using the most recent satellite observations available from the Defense Meteorological Program’s (DMSP) Special Sensor Microwave Imager (SSM/I). The analysis of these data show that Antarctic sea ice is highly variable in both time and space. Statistically significant increases in the sea ice extent, open water and ice areas have been determined from the SSM/I data for the 9 year period 1987 to 1996, a result which differs from the Scanning Multichannel Microwave Radiometer (SMMR) observations (1978-1987). The increasing trend in the SSM/I observations can be attributed to the large increases in sea ice observed in 1994-1995, as confirmed by an analysis of data from the ERS-1 satellite. The mean season length during these years has remained relatively unchanged. Regional trends, both in the sea ice concentration and in season length, showed vast spatial inhomogeneity. SSM/I data displayed increasing season length in the central Weddell Sea, Bellingshausen Sea and Balleny Islands regions, with decreasing length in the Amundsen Sea, eastern Ross Sea and in the coastal areas off Wilkes Land. Similar trends are observed in the seasonal sea ice concentration. In most cases, these trends are opposite to those observed in the SMMR data, which may be linked to the shift observed in the Amundsen Sea low after 1990. Comparisons with historical data would suggest that no large scale anomalous change has occurred in the Antarctic sea ice limits over the course of human observation. Furthermore, the degree of variability suggests great care is needed in interpreting large scale changes in sea ice conditions, and hence atmospheric or oceanic change, from locally observed anomalies. Case studies of the effect of individual cyclones upon the sea ice concentration show small but definite modification of the ice conditions. To further diagnose aspects of the thermodynamic and dynamic forcing upon the Antarctic pack, detailed analysis of the sea ice concentration variability has been conducted using spectral techniques, and the spectra have been compared to those of the European Centre for Medium Range Weather Forecasts (ECMWF) temperature and wind data. In all cases, and with the seasonal cycle removed, the sea ice concentration shows a bias towards longer timescales of variability than either the wind stress or surface air temperature. This “red shift” in its frequency spectrum is strongest with the wind stress, and weakest with the temperature. For longer period waves, this may be due to the formation of new ice by surface cooling or the moderation of melting by the cold surface water, whereas for shorter period waves, where wind stress dominates temperature and ice concentration respectively, time is required for winds to draw in warmer or cooler air, as well as to overcome the ice masses inertia and keel friction to open or close leads. Strong intraseasonal variability of the sea ice concentration is observed in the 20-25 day period, reflecting similar timescales of the temperature variability, as well as that of the energetic eddies of the Antarctic circumpolar current. Examination of the latitudinal variation of the sea ice concentration, temperature and wind stress spectra showed not only the importance of the north-south temperature gradient in influencing the variability, but also the seasonal changes in the semi annual oscillation of the circumpolar trough. Regional spectra showed clear differences between location, and reflected the influences of the atmosphere and ocean upon the sea ice pack. This is clearly shown in the Weddell Polynya region and off East Antarctica, with high variability in the synoptic timescales, and in the western Ross Sea where changes occur in timescales of greater than 20 days. In order to determine if satellite derived, real time sea ice concentration and distribution would be of benefit to operational numerical weather prediction (NWP) schemes, the effect of sea ice concentration change upon the atmosphere in synoptic timescales was examined using a general circulation model in conjunction with the Australian Bureau of Meteorology’s GASP analyses. Experiments were conducted with a typical July sea ice concentration and distribution, as well as slab concentrations of 0, 10, 25, 50, 80 and 100%. Results from 5-day numerical weather forecasts show that the central pressure, structure and tracks of individual cyclones are sensitive to the ‘switch on’ of different sea ice conditions. Composites of all forecasts made with each concentration showed considerable, and mostly statistically significant, anomalies in the surface temperatures and turbulent heat fluxes over the sea ice. The magnitudes of these changes varied monotonically with the area of open water. The largest changes were simulated closest to the coast for all concentrations except for the typical July sea ice run, which displayed maxima over the outer pack. Significant westerly anomalies were induced over the ice in all cases, as were reductions in mean sea level pressure. The July sea ice runs displayed a distribution of the mean sea level pressure anomaly different from all others, with maxima occurring in the central to outer pack. All other forecasts displayed maxima at the coast. The results suggest that sea ice concentration does induce anomalies in the atmospheric parameters in timescales of less than five days. Further, the use of a realistic distribution of sea ice concentration produces results distinct from the constant concentration forecasts. Hence it is suggested that real time Antarctic sea ice data may be of considerable benefit to numerical weather prediction models.
-
ItemThe impact of climate extreme events on global agricultural yields( 2018)Climate extreme events, such as droughts, heat waves, cold spells or heavy precipitation events, pose significant risks to food production and the livelihoods of farmers. Understanding the effect of such events on crop yields is crucial to predict the response of agricultural production to climate change and inform adaptation processes. This PhD research investigated the effect of temperature and precipitation extremes on the yields of four major crops — maize, rice, soybeans, and wheat — over the years 1961–2008, using a global, high-resolution yield dataset and global, gridded data on past weather conditions and climate extremes. First, a bibliographic network-type literature review was conducted to provide an overview of the research landscape. The number of publications focusing on climate and agriculture significantly increased after 2005, reflecting an increased awareness of the challenges related to climate change impacts as well as effects of extreme events on food production. An overview of the most influential publications and research clusters is presented and the main challenges faced by the agricultural sector, as identified in the literature, are summarised. The second part of this thesis analysed the effects of growing season climate variability and extremes (warm and cold temperature extremes, drought and heavy precipitation) on yield anomalies of maize, rice, soybeans, and spring wheat at the global scale, using random forests, a machine learning algorithm. The findings suggest that globally, 20–49% (range over all crops) of the variability of yield anomalies is explained by variations in growing season climate, including climate extremes. Excluding climate extreme indicators from the statistical models decreases the explained variance by 18–43% (range over all crops). This decrease represents more than half of the explained variance for maize, rice, and soybeans, highlighting the importance of climate extremes for understanding yield fluctuations. Yield anomalies are more strongly associated with temperature-related climate variables than with precipitation-related predictors. However, irrigation modulates the impacts of high temperatures on yield anomalies, reflecting the convoluted effects of water and temperature stress. By developing a composite indicator, regions were identified that are most strongly influenced by climate extremes and at the same time major contributors to global crop production, and hence may be the focus of adaptation efforts. The final analysis presented in this study focused on the effects of temperature extremes on maize yields more specifically. The first part assessed the effect of growing degree days above 30°C (GDD30+) — a commonly used, plant-critical temperature threshold — on maize yields across major producing regions. Most detrimental effects are found for rainfed yields in Europe, North America and Oceania, i.e. regions with highly industrialised agricultural systems. The strength of the negative relationship between GDD30+ and maize yield depends on a region’s mean climate and climate variability, with most negative yield effects found in colder and drier regions and regions with low temperature and precipitation variability. The second part determined region- and irrigation-specific temperature thresholds above which adverse yield effects accumulate. These thresholds range from 20°C in Europe to 26°C in Asia and North America. In summary, this study shows that the effects of high temperatures on maize production differ considerably between producing regions, with important implications for the adaptation of maize production systems to climate change and temperature extremes.
-
ItemAustralian rainfall and El Niño diversity: past variability and context for recent changes( 2018)The climate system integrates internally and externally induced variability at various time scales as a result of interactions between the ocean and atmosphere. The influence of external forcing on the climate system and with it the structural changes of climate variability, in particular on seasonal and longer time-scales, is difficult to examine due to high natural variability and short observational records. The interplay between high and low-frequency variability restricts our understanding of the full range of climate variability and our ability to contextualise changes. This thesis explores and evaluates the potential to use seasonal paleoclimate information to advance our knowledge of natural climate variability and the multi-century context of recent changes in the Australasian and tropical Pacific region. Climate modes of variability including the El Niño -Southern Oscillation influence Australian rainfall and make Australian rainfall highly variable at interannual timescales. Multi-century reconstructions of past climate variability are developed for Australian rainfall at bi-seasonal resolution. The rainfall reconstruction is based on local paleoclimate proxies and teleconnected links between remote paleoclimate proxies, climate modes of variability and Australian rainfall. In a multi-century context, the recent drying trends in parts of southern Australia, as well as the tendency towards wetter conditions in northern Australia, are found to be unusual. The cool and warm season rainfall reconstructions allow the documentation of distinct characteristics of past major droughts in terms of their spatial extent, duration, intensity, and seasonality. Using coral data at seasonal resolution, two El Niño index reconstructions illustrate the sequence of diversity of past eastern and central Pacific El Niño events for the last 400 years. The distinct spatio-temporal signatures of both types of El Niño are exploited, and together with a novel machine learning approach, the diversity of past El Niño events is reconstructed and compared to recent changes. The recent increase in the frequency of central Pacific El Niño events relative to eastern Pacific El Niño events during the late 20th century appears unusual. The most recent 30-year period includes more intense eastern Pacific events compared to the past four centuries. To further investigate the changes and interactions between Australian rainfall and El Niño diversity, observations and climate model simulations are compared to the multi-century reconstructions. A number of climate models taking part in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are identified that simulate spatially distinct El Niño behaviour. Identification of El Niño events reveals a lack of model agreement about projected changes of El Niño diversity. The probability of infrequent El Niño characteristics is evaluated and point towards an under-representation of central Pacific events that are followed by eastern Pacific events in the observational records. Future simulations in climate models indicate that this El Niño transition as observed most recently in 2014-2016, could become less common. Based on the rainfall and El Niño reconstructions, the general drying impacts of El Niño is consistent for both types. Despite the strength asymmetry between eastern and central Pacific El Niño events, the impact on Australian rainfall is of a similar order of magnitude but also highlights a strong variable nature of the different types of El Niño and Australian hydroclimate. The context of recent changes provided by the reconstructions in this thesis advances our knowledge of natural climate variability in the Australasian and tropical Pacific region and offers new insights into the future climate of the region.
-
ItemThe energy business system: transformation, social innovation & disruptive smart grid business models( 2018)A radical shift from a high- to a low-carbon energy system is not occurring at the speed required to address climate change. One reason for this, is that conventional energy firms are locked into producing shareholder profit with fossil-intensive business models that are still operable in current markets. This PhD thesis employs systems thinking to analyse lock-in of the energy business system (EBS) and then adopts design thinking to propose disruptive innovations that can accelerate low-carbon transitions. Dynamics of the EBS transition are evaluated in an interdisciplinary way and across systems scales, from the Earth system to the distribution edge of the electrical power system. Conceptual frameworks that combine complex system and transition theory are developed to evaluate the global EBS in an Earth system context and to analyse the role of business models in the decarbonisation of the electricity sector. Previous research suggests that strategies for escaping EBS lock-in include incorporation of environmental liability to shareholders, carbon taxation and other economic measurements to alter market conditions. The systems analysis presented here suggests that to ensure environmental resilience the EBS’s fundamental purpose and governance need revision. I theorise that more social innovation in business models could influence business trajectory in the energy sector. Instead of maximising shareholder profit, social innovations can shift business purpose towards long-term social and environmental value-creation using emerging market-based tools. A business model analysis of the electricity sector is used to identify opportunities for EBS disruption from social innovations. I find that, in general, social and environmentally driven low-carbon projects often struggle to achieve scale and commercial advantage. However, recent business model innovations in smart grids can provide such projects with the required competitive position. Specifically, Virtual Power Plant technology has emerged as an energy management system that allows aggregation and coordination of multiple distributed energy resources. Aggregation can include diverse resources such as photovoltaics, batteries, electric vehicles and windfarms. Coordination can achieve improved physical and market performance as a functional unit within the electricity market. Using theoretical model development and illustrative examples, I highlight how emerging opportunities such as peer-to-peer Internet platforms and blockchain technology, also have significant potential as tools to enable disruptive business models, through decentralised value creation using assets from online participants. Building from the systems thinking analysis of the EBS lock-in, the second part of this PhD thesis uses design thinking to propose and further develop a new business model termed a ‘social virtual energy network’ (SVEN). As an urban social electricity-trading network, a SVEN is designed to help accelerate the decarbonisation of the power system and influence paradigm shifts in EBS governance. Two iterations in the design of the SVEN concept are presented and critiqued based on insights derived from the first part of the thesis. The first iteration focuses on the role of virtual power plants and tariff design for business feasibility, and the second on blockchain and user interfaces for mainstream market adoption. Through systems analysis, this study argues that an adequate response to climate change requires a paradigm shift in the EBS. Using a systems design approach, the thesis provides a vision for the architecture of a democratic open energy economy where users and their distributed energy resources have an active role in the value chain of the EBS. The findings and proposals of this work are relevant to debates about the most effective ways of accelerating targeted innovations to achieve a low-carbon energy system.
-
ItemGoverning geoengineering sustainably: a scenario exercise to inform Australian geoengineering policy development( 2018)Geoengineering, the technofix to climate change, is a complex, contentious and high-stakes proposal. Yet in the absence of credible long-term global emissions reduction, the idea cannot be overlooked. To ensure that geoengineering contributes to sustainable environmental and social outcomes, a forward-looking, inclusive and reflexive decision-making framework is needed. Instead, Australia’s history on climate policy reflects a short- sighted, impulsive and polarised approach. The motivation of this project is therefore to inform sustainable geoengineering governance in Australia. Within a sustainable governance context, scenarios are often used in the management of long-term, complex, and uncertain issues. This thesis investigates how a scenario exercise can inform sustainable geoengineering governance in Australia. The sustainability focus justifies an interdisciplinary mixed-methods approach. The thesis begins by surveying the present before interrogating the future and is presented as a compilation of five papers. First, the thesis explores from an Earth System Governance perspective what global geoengineering governance exists and how this might evolve. The analysis characterises geoengineering governance as governance-by-default where there is no purposive regulation—decisions are guided by existing norms and driven by the motivations of engaged academics. Given the influence of these actors, the research examines, through a systematic quantitative review, the types of geoengineering governance frameworks proposed in academic and non-academic literature. The study finds that the challenges of geoengineering governance can be likened to issues in other policy domains but suggests that normalising the debate thusly could obscure major threats and novel opportunities. Next, a meta-analysis of geoengineering scenarios is undertaken. It finds that the treatment of geoengineering within these scenarios does not align with sustainability concepts. An emphasis on technological solutions ignores the interdependence of nature and society and conceals alternative options; a focus on global effects and actions disregards local or regional issues; and scenarios portray only a narrow range of perspectives. Finally, an inductive and deductive scenario design method is proposed and demonstrated, producing four scenarios that are analysed in several ways: their key determinants are compared to those of scenarios in the geoengineering literature; they are studied individually and collectively to identify causal relationships and early warning signals; shared learning throughout scenario process is explored; and finally, they are used to stress-test climate policies and inform robust strategies. Proposed Australian climate strategies are found not to be robust. Policies are based on the expectation of enduring government legitimacy and that technological solutions obviate the need for behavioural change. The geoengineering strategy proposed for Australia is engagement nationally and internationally on geoengineering issues in a technologically and ideologically neutral manner and investment in transparent and inclusive research. The contributions of this thesis are several. It establishes that geoengineering governance is not tracking on a sustainable trajectory globally. It suggests that the role of scenarios, already central to geoengineering scholarship and governance design, can be expanded. It proposes and demonstrates a successful scenario development and analysis method. It begins a cross-sectoral Australian geoengineering conversation. It makes specific policy recommendations; and in doing so, it opens up the scope of policy options.
-
ItemCenozoic floral and climate record from brown coals of southeastern Australia( 2018)The brown coals of the Latrobe Group, Gippsland Basin, Australia, record an exceptional terrestrial floral and climate record from the Eocene to Miocene. Detailed palynological analysis reveals that the peatland floral communities evolved in response to global climate change through the Cenozoic. The new floral record extracted from the Latrobe Group coals reveals subtropical (Middle Eocene), warm-temperate (Late Eocene) and cool-temperate (Oligocene and Early Miocene) rainforest communities and conditions for southeastern Australia through the Eocene-Oligocene climate transition. In the onshore Latrobe Group coals the floral record highlights the return of warm-temperate conditions during the Middle Miocene Climatic Optimum. Cooling in the Gippsland Basin therefore appears to have begun in the Middle Eocene, in agreement with benthic marine δ18O records. A detailed examination of the brown coal facies preserved in the Latrobe Group coals reveals that the development of lightening-upwards lithotype cycles, observed in brown coals globally, is controlled by the peat-forming palaeoenvironment and associated hydrological regime. Each environment produces distinct lithotypes and lightening-upwards cycles are interpreted as terrestrialization cycles. The observed colour change, from darker to lighter lithotypes, results from the environment evolving from anaerobic/inundated to less anaerobic/less moist settings via peatland aggradation. The floral successions documented in modern peatlands, from inundated wetland assemblages to more elevated and well-drained forest, after investigation, also result from changes in substrate wetness during peatland aggradation in high rainfall settings. The distribution of charcoal and fire-prone flora in the Latrobe Group brown coals is likewise entirely controlled by the palaeoenvironments within the peatland and does not result from drier climates as has been previously suggested. Instead, charcoal and fire-prone floras are associated with emergent and meadow marsh environments that produce darker coal lithotypes. It is proposed that the low-nutrient and fire-prone environments that fringed the ever-wet rainforests of the Latrobe Group peatlands represented an ideal setting for southeastern Australia’s modern fire-adapted and sclerophyllous flora to evolve in. The chronological basis of the Eocene, Oligocene and Miocene palynological biostratigraphy of southeastern Australia has been improved using new and revised palynological, strontium-isotope and foraminiferal analysis from the Otway, Torquay and Gippsland basins. The improvements to and recalibration of the biostratigraphic schemes of southeastern Australia suggests that a near continuous Early Miocene neritic sequence is present in the Torquay Basin. In contrast, coal seam deposition in the Latrobe Valley was episodic rather than continuous as has been previously suggested.
-
ItemCryogenian iron formations: glaciation and oxygenation( 2018)The Cryogenian Period (720–635 Ma) experienced extreme glaciations broadly coincident with a transformation of the Earth’s surface oxidation state, supercontinent breakup, and the evolution of complex animal multicellularity. However, the cause-and-effect relationships of these events are unresolved. The Cryogenian ice ages, known as ‘Snowball Earth' events, would have placed important constraints on the biosphere, and it remains unclear what role global refrigeration played in setting the stage for eukaryotic diversification and the origin of animals. The Cryogenian also experienced the deposition of iron-rich marine chemical sediments (iron formations), representing the first episode of global iron formation deposition in over one billion years. This shift in iron cycling highlights complexities in seawater chemistry and oxidation state during this time, and these iron formations offer valuable insights into Cryogenian palaeoenvironments. Iron formations from Cryogenian glacial successions in Namibia, USA and Australia were studied in order to investigate Cryogenian iron formation genesis and elucidate the relationships between glaciation, ocean chemistry, oxygenation and biotic evolution. In-depth sedimentology, stratigraphy and petrography reveals that these iron formations are intimately associated with Sturtian glacial sediments and are interpreted have been deposited in a range of glaciomarine environments. Geochemical analysis of these chemical sediments permits the reconstruction of Cryogenian ocean chemistry and the synglacial palaeoredox landscape. Multiple geochemical proxies, including rare earth element and iron isotope systematics, indicate widespread marine anoxia with increasing seawater oxidation with proximity to the ice shelf grounding line. A genetic model is proposed whereby the mixing of oxygenated glacial fluids with ferruginous seawater led to the deposition of iron formations in glacial successions during the Cryogenian. Atmospheric oxygen trapped in glacial ice was likely an important oxidant source, delivered to Cryogenian glaciomarine environments via subglacial meltwater outwash. This meltwater supply may have been crucial in establishing oxygenated marine habitats for eukaryotes, including early animals, during Snowball Earth. Multi-million-year oxidation of the oceans via this mechanism may have also set the stage for a Neoproterozoic marine oxygenation event.
-
ItemEarly Earth crustal evolution: insights from the mafic rock record of the Pilbara Craton( 2018)Archaean greenstone belts are Earth’s oldest known volcanic rock records of an early, less differentiated crust. This study targets the Palaeoarchaean volcanic rock record of the East Pilbara Craton in Western Australia in order to address the following issues (1) compositional and temporal links between greenstone belts in the eastern Pilbara (2) existence of an older, potentially assimilated crust (3) inferences on early Archaean mantle composition (4) potential constraints on geodynamic setting (5) trace element remobilisation in metavolcanic rocks (6) identification of mineral phases that retained most of the trace element budget. New petrographic, chemical and isotopic (Nd, Hf) data are presented for 3.53–3.30 Ga Warrawoona and Kelly Group greenstones of the Pilbara Supergroup. The majority of Pilbara greenstones are Fe- or Mg-rich tholeiitic basalts, with subordinate contributions of komatiite and calc-alkaline volcanism. Most metabasaltic rocks show MORB-like flat to mildly fractionated trace element patterns, with a smaller subset of rocks with variable major element composition displaying chemical affinities similar to crustal melts (LREE-enriched, low Nb/Th, high La/Nb and Zr/Y, pronounced negative Nb-Ta and Ti anomalies). The difference in trace element chemistry for the latter group is not reflected by their Hf-Nd isotopic compositions. Initial isotope ratios are fairly homogenous for Warrawoona Group rocks peaking at eNd = 1.1 ± 0.5 and eHf = 1.3 ± 1.0. Isotopic signatures across all samples are consistent with melt derivation from magma sources in the depleted mantle and show no evidence for an extensive Hadaean (>4.1 Ga) basement. Trace element data resulting from acid-leaching experiments of Pilbara metavolcanic rocks demonstrate effective mobilisation of trace elements including lanthanides (preferentially the LREE) that are hosted in carbonate and phosphate minerals. Previous studies have shown that carbonisation occurred during interaction with hydrothermal seawater (shortly after emplacement) and metamorphic recrystallisation of primary plagioclase and pyroxene. Results from the leaching experiment imply that the trace element chemistry of greenstones might not be as well-preserved as commonly assumed and caution is warranted regarding petrogenetic interpretations that heavily rely on these elements.
-
ItemRemote sensing tropical cyclone rainfall over the southwest Pacific region( 2018)The south-west Pacific (SWP) region is susceptible to the catastrophic effects of tropical cyclones (TCs). The region, therefore, has received adequate attention in terms of scientific research pertaining to TC genesis, tracks and intensity. It (especially the island countries), nonetheless, has not received much attention, in comparison to studies elsewhere (e.g. the Atlantic and western North Pacific region), on additional important aspects of TCs such as rainfall estimation and characteristics of the TC-related rainfall drop size distribution (DSD). The latter has implications for radar rainfall estimation and cloud modelling studies. In this thesis, we first validate the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) 3B42 quantitative precipitation estimates (QPEs) during the passage of TCs over New Caledonia and Fiji. It is shown that TMPA has skill in representing the rainfall during the passage of TCs over New Caledonia and Fiji. TMPA overestimates light rainfall and underestimates moderate to heavy daily rainfall. The skill deteriorates with increasing elevation, as underestimation is greater at large altitudes. The ability of TMPA also varies with TC intensity and distance from the TC centre, whereby it is more skilful for less intense TCs (category 1-2) and near the TC centre than in the outer rainbands. The rainfall DSD during the passage of TCs over Darwin, Australia, is evaluated next and this is compared with the DSD associated with non-tropical cyclone (non-TC) events. It is shown that the TC-related DSD is statistically different from the non-TC related DSD, the former encompassing a larger concentration of small to moderate size drops. The TC related drop diameter is lower than the non-TC values at all rain rates and also for the different precipitation types (convective, transition and stratiform). The TC DSD also varies with distance from the TC centre, as rainfall near the TC centre comprises of relatively smaller drops which are strongly evident at small to moderate rainfall rates (< 30 mm hr-1). These variations in the DSD have implications for the parameters used in the algorithm that converts radar reflectivity to rainfall rate, as well as for the analytical expressions used in describing the observed DSD employed in cloud modelling parameterizations. Finally, the feasibility of estimating the DSD parameters using (i) the TRMM precipitation radar (PR) and (ii) a combined PR and TRMM Microwave Imager (TMI) algorithm (COM) is investigated using the Darwin C-band dual polarised (C-POL) ground radar (GR) as the reference. The correspondence of the TRMM instruments with the GR is generally dependent on the precipitation type: the PR and the COM usually overestimate (underestimate) the reflectivity and the rainfall rate from events that are highly stratiform (convective) whereas they mostly overestimate the median volume diameter (Do) of all rainfall types whereby the overestimation increases with an increase in the percentage of convective fraction. Also, the COM reflectivity estimates are similar to the PR estimates but it has a smaller bias in the Do for most of the greater stratiform events. This suggests that combining the TMI with the PR adjusts the Do towards the “correct” direction if the GR is taken as the reference. Moreover, for the TC events considered in this study, the association of the TRMM estimates with the GR is similar to the highly stratiform non-TC events (there is no significant difference) but it differs largely from the majority of the highly convective non-TC events.
-
ItemVerification of satellite estimates and NWP model forecasts for tropical cyclone rainfall( 2018)This study evaluates the TRMM 3B42 version 7 (V7) estimates of daily rainfall in tropical cyclones (TCs) using the Comprehensive Pacific Rainfall Database (PACRAIN) of 24h rain gauge observations. The evaluation is performed on two different terrain types: low-lying atoll sites (assumed to represent open-ocean conditions) and coastal and island sites (over land). The results show that TRMM 3B42 has good skill at detecting intense TC rainfall, with good correlation and pattern matching with PACRAIN observations. However, it tends to overestimate heavy rain frequency on atoll sites, but tends to underestimate heavy rain frequency on coastal and island sites. Overall, TRMM 3B42 is better able to estimate the intensity of TC heavy rain over ocean than over land. It is least skillful at coastal and island sites with high elevation, where it significantly underestimates TC heavy rainfall, suggesting that TRMM 3B42 is unable to capture orographic enhancement during TC landfall. Finally, results from V7 were compared with results from its predecessor, Version 6, showing that Version 7 of TRMM 3B42 has higher values on average for TC rain. TRMM 3B42 rainfall estimates in landfalling TCs are then evaluated over Australia. A high quality gauge–based gridded rainfall product from the Australian Water Availability Project (AWAP) is utilized as reference data. The comparisons show good correspondence over space and time between TRMM 3B42 and AWAP analysis for rainfall at TC landfall over Australia. The results also show that TRMM 3B42 generally overestimates TC rain for low rain rate but underestimates TC rain at high rain rate. TC intensity, location, terrain, and TC seasons all have impacts on TRMM 3B42’s detection skill. For TC heavy rain, TRMM 3B42 shows better agreement with AWAP during more intense TCs (category 3–5), in the eyewall as opposed to the rain bands, in the tropics as op posed to the subtropics, and late in the TC season as opposed to early and peak of the TC season. The last part of this study is to demonstrate the useful information that can be derived from Contiguous Rain Area (CRA) evaluation, such as systematic errors in predicted tropical cyclone (TC) rainfall location and components of rainfall error due to incorrect predictions of location, rain volume, and rain pattern. CRA verification was applied to evaluate ACCESS-TC, the tropical cyclone version of the Australian Community Climate and Earth System Simulator (ACCESS), daily rainfall forecasts over 15 TCs in the north west Pacific ocean during 2012-2013, by comparing with TRMM 3B42 satellite estimates. The results showed that pattern error was the major contributor to the total TC rainfall forecast error, followed by displacement and volume. ACCESS-TC forecasts tended to predict more rainfall closer to the TC center compared to TRMM 3B42 estimates. Furthermore, rain event verification showed that for short lead time (24h) forecasts, overestimation of rain volume was a major problem for ACCESS-TC forecasts, while displacement error was more significant in longer lead time (72h) forecasts. Finally, probability distribution functions and radial probability distributions of rainfall in the forecasts and observations are used to further characterise the rain volume error. This confirmed that ACCESS-TC tended to produce more extreme rain in the locations closer to the TC center (eyewall).