School of Earth Sciences - Research Publications
Permanent URI for this collection
Search Results
1 - 10 of 18
-
ItemPlio-Pleistocene climate change and the onset of aridity in southeastern Australia(ELSEVIER, 2010-03-01)
-
ItemNo Preview Available
-
ItemNon-stationarity in daily and sub-daily intense rainfall – Part 2: regional assessment for sites in south-east Australia(Copernicus Publications on behalf of the European Geoscience Union, 2011)Using data for a common period (1976–2005) for a set of 31 sites located in south-east Australia, variations in frequency and magnitude of intense rainfall events across durations from 6 min to 72 h were assessed. This study was driven by a need to clarify how variations in climate might affect intense rainfall and the potential for flooding. Sub-daily durations are of particular interest for urban applications. Worldwide, few such observation-based studies exist, which is mainly due to limitations in data. Analysis of seasonality in frequency and magnitude of events revealed considerable variation across the set of sites, implying different dominating rainfall-producing mechanisms and/or interactions with local topography. Both these factors are relevant when assessing the potential effects of climate variations on intense rainfall events. The set of sites was therefore split into groups ("north cluster" and "south cluster") according to the characteristics of intense rainfall events. There is a strong polarisation in the nature of changes found for the north cluster and south cluster. While sites in the north cluster typically exhibit decrease in frequency of events, particularly in autumn and at durations of 1 h and longer; sites in the south cluster experience an increase in frequency of events, particularly for summer and sub-hourly durations. Non-stationarity found in historical records has the potential to significantly affect design rainfall estimates. An assessment of quantile estimates derived using a standard regionalisation technique and periods representative of record lengths available for practical applications show that such estimates may not be representative of long-term conditions, so alternative approaches need to be considered, particularly where short records are concerned. Additional rainfall information, in particular radar data, could be used for an in-depth spatial analysis of intense rainfall events.
-
ItemNon-stationarity in daily and sub-daily intense rainfall – Part 1: Sydney, Australia(Copernicus Publications on behalf of the European Geoscience Union, 2011)This study was driven by a need to clarify how variations in climate might affect intense rainfall and the potential for flooding. Sub-daily durations are of particular interest for urban applications. Worldwide, few such observation-based studies exist, which is mainly due to limitations in data. While there are still large discrepancies between precipitation data sets from observations and models, both show that there is a tendency for moist regions to become wetter and for dry regions to become drier. However, changes in extreme conditions may show the opposite sign to those in average conditions. Where changes in observed intense precipitation have been studied, this has typically been for daily durations or longer. The purpose of this two-part study is to examine daily and sub-daily rainfall extremes for evidence of non-stationarity. Here the problem was addressed by supplementing one long record (Part 1) by a set of shorter records for a 30-yr concurrent period (Part 2). Variations in frequency and magnitude of rainfall extremes across durations from 6 min to 72 h were assessed using data from sites in the south-east of Australia. For the analyses presented in this paper, a peaks-over-threshold approach was chosen since it allows investigating changes in frequency as well as magnitude. Non-parametric approaches were used to assess changes in frequency, magnitude, and quantile estimates as well as the statistical significance of changes for one station (Sydney Observatory Hill) for the period 1921 to 2005. Deviations from the long-term average vary with season, duration, and threshold. The effects of climate variations are most readily detected for the highest thresholds. Deviations from the long-term average tend to be larger for frequencies than for magnitudes, and changes in frequency and magnitude may have opposite signs. Investigations presented in this paper show that variations in frequency and magnitude of events at daily durations are a poor indicator of changes at sub-daily durations. Studies like the one presented here should be undertaken for other regions to allow the identification of regions with significant increase/decrease in intense rainfall, whether there are common features with regards to duration and season exhibiting most significant changes (which in turn could lead to establishing a theoretical framework), and assist in validation of projections of rainfall extremes.
-
ItemA combined climate extremes index for the Australian Region(American Meteorological Society, 2010)Changes in the area of Australia experiencing concurrent temperature and rainfall extremes are investigated through the use of two combined indices. The indices describe variations between the fraction of land area experiencing extreme cold and dry or hot and wet conditions. There is a high level of agreement between the variations and trends of the indices from 1957 to 2008 when computed using (i) a spatially complete gridded dataset without rigorous quality control checks and (ii) spatially incomplete high-quality station datasets with rigorous quality control checks. Australian extremes are examined starting from 1911, which is the first time a broad-scale assessment of Australian temperature extremes has been performed prior to 1957. Over the whole country, the results show an increase in the extent of hot and wet extremes and a decrease in the extent of cold and dry extremes annually and during all seasons from 1911 to 2008 at a rate of between 1% and 2% decade−1. These trends mostly stem from changes in tropical regions during summer and spring. There are relationships between the extent of extreme maximum temperatures, precipitation, and soil moisture on interannual and decadal time scales that are similar to the relationships exhibited by variations of the means. However, the trends from 1911 to 2008 and from 1957 to 2008 are not consistent with these relationships, providing evidence that the processes causing the interannual variations and those causing the longer-term trends are different.
-
ItemThe Greenland Ice Sheet Response to Transient Climate Change(AMER METEOROLOGICAL SOC, 2011-07-01)Abstract This study applies a multiphase, multiple-rheology, scalable, and extensible geofluid model to the Greenland Ice Sheet (GrIS). The model is driven by monthly atmospheric forcing from global climate model simulations. Novel features of the model, referred to as the scalable and extensible geofluid modeling system (SEGMENT-Ice), include using the full Navier–Stokes equations to account for nonlocal dynamic balance and its influence on ice flow, and a granular sliding layer between the bottom ice layer and the lithosphere layer to provide a mechanism for possible large-scale surges in a warmer future climate (granular basal layer is for certain specific regions, though). Monthly climate of SEGMENT-Ice allows an investigation of detailed features such as seasonal melt area extent (SME) over Greenland. The model reproduced reasonably well the annual maximum SME and total ice mass lost rate when compared observations from the Special Sensing Microwave Imager (SSM/I) and Gravity Recovery and Climate Experiment (GRACE) over the past few decades. The SEGMENT-Ice simulations are driven by projections from two relatively high-resolution climate models, the NCAR Community Climate System Model, version 3 (CCSM3) and the Model for Interdisciplinary Research on Climate 3.2, high-resolution version [MIROC3.2(hires)], under a realistic twenty-first-century greenhouse gas emission scenario. They suggest that the surface flow would be enhanced over the entire GrIS owing to a reduction of ice viscosity as the temperature increases, despite the small change in the ice surface topography over the interior of Greenland. With increased surface flow speed, strain heating induces more rapid heating in the ice at levels deeper than due to diffusion alone. Basal sliding, especially for granular sediments, provides an efficient mechanism for fast-glacier acceleration and enhanced mass loss. This mechanism, absent from other models, provides a rapid dynamic response to climate change. Net mass loss estimates from the new model should reach ~220 km3 yr−1 by 2100, significantly higher than estimates by the Intergovernmental Panel on Climate Change (IPCC) Assessment Report 4 (AR4) of ~50–100 km3 yr−1. By 2100, the perennial frozen surface area decreases up to ~60%, to ~7 × 105 km2, indicating a massive expansion of the ablation zone. Ice mass change patterns, particularly along the periphery, are very similar between the two climate models.
-
ItemA regional modeling study of climate change impacts on warm-season precipitation in the Central United States(American Meteorological Society, 2011)In this study, the Weather Research and Forecasting (WRF) model is employed as a nested regional climate model to dynamically downscale output from the National Center for Atmospheric Research’s (NCAR’s) Community Climate SystemModel (CCSM) version 3 and the National Centers for Environmental Prediction (NCEP)–NCARglobal reanalysis (NNRP). The latter is used for verification of late-twentieth-century climate simulations from the WRF.
-
ItemA new global gridded data set of CO2 emissions from fossil fuel combustion: methodology and evaluation(American Geophysical Union, 2010)We describe a system for constraining the spatial distribution of fossil fuel emissions of CO2. The system is based on a modified Kaya identity which expresses emissions as a product of areal population density, per capita economic activity, energy intensity of the economy, and carbon intensity of energy. We apply the methodology of data assimilation to constrain such a model with various observations, notably, the statistics of national emissions and data on the distribution of nightlights and population. We hence produce a global, annual emission field at 0.25° resolution. Our distribution of emissions is smoother than that of the population downscaling traditionally used to describe emissions. Comparison with the Vulcan inventory suggests that the assimilated product performs better than downscaling for distributions of either population or nightlights alone for describing the spatial structure of emissions over the United States. We describe the complex structure of uncertainty that arises from combining pointwise and area-integrated constraints. Uncertainties can be as high as 50% at the pixel level and are not spatially independent. We describe the use of 14CO2 measurements to further constrain national emissions. Their value is greatest over large countries with heterogeneous emissions. Generated fields may be found online (http://ffdas.org/).
-
ItemA three-dimensional synthesis inversion of the molecular hydrogen cycle: sources and sinks budget and implications for the soil uptake(American Geophysical Union, 2011)Our understanding of the global budget of atmospheric hydrogen (H2) contains large uncertainties. An atmospheric Bayesian inversion of H2 sources and sinks is presented for the period 1991-2004, based on a two networks of flask measurement stations. The types of fluxes and the spatial scales potentially resolvable by the inversion are first estimated from an analysis of the correlations of errors between the different processes and regions emitting or absorbing H2. Then, the estimated budget of H2 and its uncertainties is presented and discussed, for five groups of fluxes and three groups of large regions, in terms of mean fluxes, seasonal and interannual variations, and long-term trends. One main focus of the study is the improvement of the estimate of H2 soil uptake, which is the largest sink of H2. Various sensitivity tests are performed defining an ensemble of more than 20 inversions. We show that inferring a robust estimate of the H2 soil uptake requires to prescribe the prior magnitude of some other sources and sinks with a small uncertainty. Doing so an estimate of the H2 soil uptake of -62 ± 3 Tg y−1 is inferred for the period 1991-2004 (the uncertainty is the residual error after inversion). The inferred soil H2 sink presents a negative long-term trend that is qualitatively consistent with a bottom-up process-based model.
-
ItemA severe thunderstorm climatology for Australia and associated thunderstorm environments(Australian Meteorological and Oceanographic Society, 2011)Severe thunderstorms can present a significant threat to life and property in Australia. A unique and broad database of severe thunderstorm reports has been constructed for the Australian region for 2003–2010 from observer reports of hailstones, winds in excess of 90 km h–1 and, less frequently, tornadoes. Based on this database, a climatology of atmospheric environments associated with the occurrence of severe thunderstorms in Australia was developed using pseudo-proximity soundings from the MesoLAPS numerical weather prediction model simulations. Observed soundings have been used to verify derived soundings from MesoLAPS simulations, with a reasonable performance over much of the continent. Proximity rawinsonde soundings from the MesoLAPS simulations were identified for each of the severe thunderstorm reports to develop the climatology of environments. This climatology was then used to derive discriminants between environments with an increased likelihood of severe thunderstorm occurrence and other thunderstorm environments. This appears to be the best way to produce a long-term climatology of severe thunderstorm environment occurrence in a sparsely populated continent without considering the complex problem of initiation.