School of Earth Sciences - Research Publications
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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.
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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.
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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.
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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.
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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.
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ItemFuture climate change in the Southern Hemisphere: competing effects of ozone and greenhouse gases(American Geophysical Union, 2011)Future anthropogenic climate change in the Southern Hemisphere is likely to be driven by two opposing effects, stratospheric ozone recovery and increasing greenhouse gases. We examine simulations from two coupled climate models in which the details of these two forcings are known. While both models suggest that recent positive summertime trends in the Southern Annular Mode (SAM) will reverse sign over the coming decades as the ozone hole recovers, climate sensitivity appears to play a large role in modifying the strength of their SAM response. Similar relationships are found between climate sensitivity and SAM trends when the analysis is extended to transient CO(2) simulations from other coupled models. Tropical upper tropospheric warming is found to be more relevant than polar stratospheric cooling to the intermodel variation in the SAM trends in CO(2)-only simulations.
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ItemA multirheology ice model: formation and application to the Greenland ice sheet(American Geophysical Union, 2011)Accurate prediction of future sea level rises requires models which can reproduce recent observed change in ice sheet behavior. This study describes a new multiphase, multiple‐rheology ice dynamics model (SEGMENT‐ice), which is used to examine Greenland ice sheet (GrIS) responses both to past and to possible future warming climate conditions. When applied to the GrIS, SEGMENT‐ice exhibits skill in reproducing the mass loss rate derived from the Gravity Recovery and Climate Experiment (GRACE), the interferometric synthetic aperture radar (InSAR) measured surface flow speed, and the microwave remotely sensed surface melt area over the past decade. When forced by the NCEP/NCAR reanalysis atmospheric parameters, the ice model simulates closely the GrIS mass loss rate obtained from GRACE. An increase of summer maximum melt area extent (SME) is indicative of an expansion of the ablation zone. The modeled SME from 1979 to 2006 also simulates well the observed interannual variability of SME, with a high correlation of 0.88 between the two time series. The geographical distributions of the modeled and observed SME also agree well. Comparison of modeled and observed velocity over three regions, covering the west, northeast, and north sides of the GrIS, respectively, indicates a satisfactory model performance in delineating flow direction and magnitudes for regions with flow speed less than 500 m/y, with no region‐specific systematic errors. However, the model cannot simulate extremes in the observations, mainly because it is limited by spatial resolution. The SEGMENT‐ice simulations are sufficiently close to the observations to employ the model to project the future behavior of the GrIS. By the end of this century, if the moderate A1B scenario is realized, the total mass loss rate will reach ∼220 km3/y. The ice divergence contribution will be about 60%, outweighing the contribution from surface processes.