Uncertainty Estimation of Extreme Precipitations Under Climate Change: A Non-Parametric Approach

Assessment of climate change impacts on hydrology at watershed scale incorporates (a) downscaling of global scale climatic variables into local scale hydrologic variables and (b) assessment of future hydrologic extremes. Atmosphere-Ocean Global Climate Models (AOGCM) are designed to simulate time series of future climate responses accounting for human induced green house gas emissions. The present study addresses the following limitations of climate change impact research: (i) limited availability of observed historical information; (ii) limited research on the detection of changes in hydrologic extremes; and (iii) coarse spatio-temporal resolution of AOGCMs for use at regional or local scale. Downscaled output from a single AOGCM with a single emission scenario represents only a single trajectory of all possible future climate realizations and cannot be representative of the full extent of climate change. Present research, therefore addresses the following questions: (i) how should the AOGCM outputs be selected to assess the severity of extreme climate events?; (ii) should climate research adopt equal weights from AOGCM outputs to generate future climate?; and (iii) what is the probability of the future extreme events to be more severe? Assessment of regional reanalysis hydro-climatic data has shown promising potential as an addition to the observed data in data scarce regions. A new approach using statistical downscaling based nonparametric data-driven kernel estimator is developed for quantifying uncertainties from multiple AOGCMs and emission scenarios. The results are compared with a Bayesian reliability ensemble average method. The generated future climate scenarios represent the nature and progression of uncertainties from several global climate models and their emission scenarios. Treating the extreme precipitation indices as independent realization at every time step, the kernel estimator provides variable weights to the multi-model quantification of uncertainties. The probabilities of the extreme indices have added useful insight into future climate conditions. Finally, the current method of developing future rainfall intensity-duration-frequency curves is extended by introducing a probabilistic weighted curve to include AOGCM and emission scenario uncertainties using the plug-in kernel. Present research has thus expanded the existing knowledge of dealing with the uncertainties of extreme events

Assessment of Global and Regional Reanalyses Data for Hydro- Climatic Impact Studies in the Upper Thames River Basin

This study evaluates NCEP-NCAR reanalyses hydro-climatic data as an initial check for assessment of climate change studies and hydrologic modeling on the basin scale. Reanalysis data set for daily precipitation, and temperature from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) (a) global (NNGR) and (b) regional (NARR) reanalysis project are used as input into the semi-distributed hydrologic model (HEC-HMS) during the period of 1980-2005. First, the precipitation and temperature data are interpolated to selected stations to check for their trends and similarity in means and variances. Although NARR shows some over-estimated values, mainly in estimating temperature during the summer months, it has been able to capture the trends. NNGR, on the other hand, has produced inferior results in many cases, especially in generating precipitation when compared with the observed values. With its improved atmospheric analytical ability, NARR appears to have performed better than the NNGR, suggesting that with coarse resolution NNGR may not be applied in climate change studies for medium or small watersheds. Next, an extensive analysis is performed for assessing the performance of the reanalysis data generated flows by comparing it with the observed inputs during May-November. The stream flows generated from the NARR dataset show encouraging results for simulating summertime low flows with less variability and error. NNGR dataset, have proven to be less accurate and highly variable. This study suggests that NARR can be adequately used as either an additional source of data or as an alternative to observations in data scarce regions.https://ir.lib.uwo.ca/wrrr/1033/thumbnail.jp

Development of Probability Based Intensity- Duration-Frequency Curves under Climate Change

Hydrologic design of storm sewers, culverts, retention/detention basins and other components of storm water management systems are typically performed based on specified design storms derived from the rainfall intensity-duration-frequency (IDF) estimates and an assumed temporal distribution of rainfall. Use of inappropriate data or design storms could lead to malfunctions of the infrastructure systems: over-estimation may result in costly over-design or under-estimation may be associated with risk and human safety. One of the expected hydroclimatic impacts of climate change for London is the increase in the magnitude and frequency of extreme rainfalls which can have serious impact on the design, operation and maintenance of existing municipal water infrastructure. This study presents a methodology for updating the rainfall IDF curves for the City of London incorporating various uncertainties associated with the assessment of climate change impacts on a local scale. Overall, two objectives have been achieved: first, an extensive investigation of the possible realizations of future climate from 29 scenarios developed from Atmosphere-Ocean Global Climate Models (AOGCM) and scenarios are performed using a downscaling based disaggregation approach. Annual maximum series of rainfall are fitted to Gumbel distribution to develop IDF curves for 1, 2, 6, 12 and 24 hour durations for 2, 5, 10, 25, 50 and 100 years of return periods. Next, the associated uncertainties are estimated using nonparametric kernel estimation approach and the resultant IDF curve is developed based on a probabilistic approach. The results indicate that rainfall patterns in the City of London will most certainly change in future due to climate change. The use of the multi-model approach, rather than a single scenario is encouraged. Inherent uncertainties associated with different AOGCMs are quantified by a kernel based plug-in estimation approach. The resultant scenario indicates approximately 20- 40% changes in different duration rainfalls for all return periods. Use of a probability based intensity-duration-frequency curve is encouraged in order to apply the updated IDF information with higher level of confidence.https://ir.lib.uwo.ca/wrrr/1034/thumbnail.jp

INTEGRAL high energy behaviour of 4U 1812-12

The low mass X-ray binary system 4U 1812-12 was monitored with the INTEGRAL observatory in the period 2003-2004 and with BeppoSAX on April 20, 2000. We report here on the spectral and temporal analysis of both persistent and burst emission. The full data set confirms the persistent nature of this burster, and reveals the presence of emission up to 200 keV. The persistent spectrum is well described by a comptonization (CompTT) model plus a soft blackbody component. The source was observed in a hard spectral state with a 1-200 keV luminosity of 2*10^(36) ergs/s and L/LEdd~1% and no meaningful flux variation has been revealed, as also confirmed by a 2004 RXTE observation. We have also detected 4 bursts showing double peaked profiles and blackbody spectra with temperatures ranging from 1.9 to 3.1 keV.Comment: 6 pages, 4 figures. Accepted for publication by A&

The failed state transition of the ATOLL source GRS 1724-308

The 2004-2012 X-ray time history of the NS LMXB GRS 1724-308 shows, along with the episodic brightenings associated to the low-high state transitions typical of the ATOLL sources, a peculiar, long lasting (about 300 d) flaring event, observed in 2008. This rare episode, characterised by a high-flux hard state, has never been observed before for GRS 1724-308 , and in any case is not common among ATOLL sources. We discuss here different hypotheses on the origin of this peculiar event that displayed the spectral signatures of a failed transition, similar in shape and duration to those rarely observed in Black Hole binaries. We also suggest the possibility that the atypical flare occurred in coincidence with a new rising phase of the 12-years super-orbital modulation that has been previously reported by other authors. The analysed data also confirm for GRS 1724-308 the already reported orbital period of about 90 d.Comment: accepted for publication in MNRA

Quantifying Uncertainties in the Modelled Estimates of Extreme Precipitation Events at the Upper Thames River Basin

Assessment of climate change impact on hydrology at watershed scale incorporates downscaling of global scale climatic variables into local scale hydrologic variables and computations of risk of hydrologic extremes in future for water resources planning and management. Atmosphere-Ocean General Circulation (AOGCM) models are designed to simulate time series of future climate responses accounting for enthropogenically induced green house gas emissions. The climatological inputs obtained from several AOGCMs suffer the limitations due to incomplete knowledge arising from the inherent physical, chemical processes and the parameterization of the model structure. This study explores the methods available for quantifying uncertainties from the AOGCM outputs by considering fixed weights from different climate model means for the overall data lengths and provides an extensive investigation of the variable weight nonparametric kernel estimator based on the choice of bandwidths for investigating the severity of extreme precipitation events over the next century. The results of this study indicate that the variable width method is better equipped to provide more useful information of the uncertainties associated with different AOGCM scenarios. This study further indicates an increase of probabilities for higher intensities and frequencies of events. The applied methodology is flexible and can be adapted to any uncertainty estimation studies with unknown densities.https://ir.lib.uwo.ca/wrrr/1032/thumbnail.jp

INTEGRAL spectral variability study of the atoll 4U 1820-30: first detection of hard X-ray emission

We study the 4-200 keV spectral and temporal behaviour of the low mass X-ray binary 4U 1820-30 with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high energy tail, above 50 keV, in the hard state has been observed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power law component, with the photon index of ~2.4, on top of thermal Comptonization emission from a plasma with the electron temperature of kT_e~6 keV and optical depth of \tau~4. Alternatively, but at a lower goodness of the fit, the hard-state broad band spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During this monitoring the source spent most of the time in the soft state, usual for this source, and the >~4 keV spectra are represented by thermal Comptonization with kT_e~3 keV and \tau~6-7.Comment: 14 pages, 4 figures, accepted for publication by Ap

Spectral states evolution of 4U 1728-34 observed by INTEGRAL and RXTE: non-thermal component detection

We report results of a one-year monitoring of the low mass X-ray binary (LMXB) source (atoll type) 4U 1728-34 with INTEGRAL and RXTE. Three time intervals were covered by INTEGRAL, during which the source showed strong spectral evolution. We studied the broad-band X-ray spectra in detail by fitting several models in the different sections of the hardness-intensity diagram. The soft states are characterised by prominent blackbody emission plus a contribution from a Comptonized emission. The hard states are characterised by the presence of an excess flux with respect to the Comptonization model above 50 keV while the soft component is fainter. To obtain an acceptable fit to the data this excess is modeled either with a power law with photon index Gamma ~ 2 or a Comptonization (CompPS) spectrum implying the presence of hybrid thermal and non-thermal electrons in a corona. This makes 4U 1728-34 one of the few LMXBs of atoll type showing non-thermal emission at high energy. From our analysis, it is also apparent that the presence of the hard tail is more prominent as the overall spectrum becames harder. We discuss also alternative models which can discribe these hard states.Comment: Accepted for publication in MNRAS (accepted 2011 April 20. Received 2011 April 20; in original form 2010 December 07); 9 pages, 7 figure