Multimodel combination techniques for analysis of hydrological simulations: Application to distributed model intercomparison project results

This paper examines several multimodel combination techniques that are used for streamflow forecasting: the simple model average (SMA), the multimodel superensemble (MMSE), modified multimodel superensemble (M3SE), and the weighted average method (WAM). These model combination techniques were evaluated using the results from the Distributed Model Intercomparison Project (DMIP), an international project sponsored by the National Weather Service (NWS) Office of Hydrologic Development (OHD). All of the multimodel combination results were obtained using uncalibrated DMIP model simulations and were compared against the best-uncalibrated as well as the best-calibrated individual model results. The purpose of this study is to understand how different combination techniques affect the accuracy levels of the multimodel simulations. This study revealed that the multimodel simulations obtained from uncalibrated single-model simulations are generally better than any single-member model simulations, even the best-calibrated single-model simulations. Furthermore, more sophisticated multimodel combination techniques that incorporated bias correction step work better than simple multimodel average simulations or multimodel simulations without bias correction. © 2006 American Meteorological Society

Multi-model ensemble hydrologic prediction using Bayesian model averaging

Multi-model ensemble strategy is a means to exploit the diversity of skillful predictions from different models. This paper studies the use of Bayesian model averaging (BMA) scheme to develop more skillful and reliable probabilistic hydrologic predictions from multiple competing predictions made by several hydrologic models. BMA is a statistical procedure that infers consensus predictions by weighing individual predictions based on their probabilistic likelihood measures, with the better performing predictions receiving higher weights than the worse performing ones. Furthermore, BMA provides a more reliable description of the total predictive uncertainty than the original ensemble, leading to a sharper and better calibrated probability density function (PDF) for the probabilistic predictions. In this study, a nine-member ensemble of hydrologic predictions was used to test and evaluate the BMA scheme. This ensemble was generated by calibrating three different hydrologic models using three distinct objective functions. These objective functions were chosen in a way that forces the models to capture certain aspects of the hydrograph well (e.g., peaks, mid-flows and low flows). Two sets of numerical experiments were carried out on three test basins in the US to explore the best way of using the BMA scheme. In the first set, a single set of BMA weights was computed to obtain BMA predictions, while the second set employed multiple sets of weights, with distinct sets corresponding to different flow intervals. In both sets, the streamflow values were transformed using Box-Cox transformation to ensure that the probability distribution of the prediction errors is approximately Gaussian. A split sample approach was used to obtain and validate the BMA predictions. The test results showed that BMA scheme has the advantage of generating more skillful and equally reliable probabilistic predictions than original ensemble. The performance of the expected BMA predictions in terms of daily root mean square error (DRMS) and daily absolute mean error (DABS) is generally superior to that of the best individual predictions. Furthermore, the BMA predictions employing multiple sets of weights are generally better than those using single set of weights. © 2006 Elsevier Ltd. All rights reserved

A hybrid stabilization technique for simulating water wave - Structure interaction by incompressible Smoothed Particle Hydrodynamics (ISPH) method

The Smoothed Particle Hydrodynamics (SPH) method is emerging as a potential tool for studying water wave related problems, especially for violent free surface flow and large deformation problems. The incompressible SPH (ISPH) computations have been found not to be able to maintain the stability in certain situations and there exist some spurious oscillations in the pressure time history, which is similar to the weakly compressible SPH (WCSPH). One main cause of this problem is related to the non-uniform and clustered distribution of the moving particles. In order to improve the model performance, the paper proposed an efficient hybrid numerical technique aiming to correct the ill particle distributions. The correction approach is realized through the combination of particle shifting and pressure gradient improvement. The advantages of the proposed hybrid technique in improving ISPH calculations are demonstrated through several applications that include solitary wave impact on a slope or overtopping a seawall, and regular wave slamming on the subface of open-piled structure

Some problems in hot rolling of al-alloys solved by the finite element method

This thesis is focused on employing the finite element method (FEM) to simulate hot flat rolling process. The relevant work involves selecting a suitable constitutive equation, predicting the rolling load and roll torque, computing temperature changes and lateral deformation, simulating the evolution of substructure, modelling static recrystallisation and designing the rolling pass schedule. A practical pass schedule supplied by an aluminium company and containing reliable measured data of roll load and torque is analysed by a commercial 3-D thermornechanical coupled FEM program FORGE3 V5.3. The inverse analysis method is adopted to obtain the friction coefficient and heat transfer coefficient. The distribution of pressure, equivalent strain, the stress and damage in the roll gap in breakdown rolling are discussed. The changes of temperature and lateral profile under both laboratory and industrial rolling conditions are computed and compared with experimental measurements, the differences are then investigated. Through applying the Taguchi experimental design method, the influence of each rolling parameter on the spread, i. e. the ratio of width to thickness, the roll radius to thickness, the thickness reduction, and the deformation temperature, the relative contribution of each control parameter is quantitatively estimated and expressed as a percentage. A new spread formula is built up based on a large amount of FE analyses. The new formula is able to deal with both laboratory and industrial rolling conditions with high accuracy. Critical reviews are presented for the previous work in the modelling of subgrain size and static recrystallisation. Both empirical and physical models are applied to investigate the evolution of subgrain size, dislocation density, misorientation and the flow stress in the roll gap. The predicted subgrain size agrees very well with the experimental measurement. The difference between the use of two models are illustrated and analysed. Studies on modelling of static recrystallisation are carried out by incorporating the plastomechanical parameters, i. e. strain, strain rate and temperature, into empirical model. Various approaches are proposed to reduce the predicted volume fraction recrystallised at the surface and are verified by the comparison with measurement. Simulation results show that some of the previous work reported in the literature are erroneous. Further work in the modelling of static recrystallisation and texture evolution is detailed. The Taguchi experimental method is also applied to study the influence of the rolling parameters on the fraction recrystallised (Xv ). The study shows that rolling temperature has the greatest influence on the Xv, followed by the parameter H. 1L. The roll temperature and roll speed have little influence on the Xv. Designing a rational rolling pass schedule is critical for the control of strip profile and product quality. In the present thesis, the procedure of designing a rolling pass schedule is illustrated. The formulae used in scheduling are listed and explained. The scheduling program is then performed to check with two existing industrial schedules. The comparison shows that the rolling load, temperature and power model is reliable and shows high accuracy. A multipass simulation by the use of finite element method is also carried out and the results are compared with various model predictions. The problems in the simulation are illustrated and explained