Use of very high resolution climate model data for hydrological modelling: baseline performance and future flood changes

Increasingly, data from Regional Climate Models (RCMs) are used to drive hydrological models, to investigate the potential water-related impacts of climate change, particularly for flood and droughts. Generally, some form of further downscaling of RCM data has been required, but recently the first decadal-length runs of very high resolution RCMs (with convection-permitting scales) have been performed. Here, a set of such runs for southern Britain has been used to drive a gridded hydrological model. Results using a 1.5km RCM nested in a 12km RCM driven by European-reanalysis boundary conditions show that the 1.5km RCM generally performs worse than the 12km RCM for simulating river flows in 32 example catchments. The clear spatial patterns of bias are consistent with bias patterns shown in the RCM precipitation data. Results using 1.5km and 12km RCM runs for the current climate and a potential future climate (driven by GCM boundary conditions) show clear differences in projected changes in flood peaks. The 1.5km RCM tends towards larger increases than the 12km RCM, particularly in spring and winter. If robust, this could have important consequences for adaptation planning under climate change, but further research is required, particularly given the greater biases in the baseline flow simulations driven by 1.5km RCM data, and the use of only a single short future climate projection

Framework for setting up a Hydro-JULES perturbed parameter ensemble (PPE)

Land surface and hydrological processes and feedbacks that act at sub-grid scales need to be parameterised within models. These parameterisation schemes are an important source of uncertainty in model simulations. One of the aims of Hydro- JULES (HJ) is to quantify uncertainties in river flows induced by land surface and hydrological model parameterisations

Climate change impacts on peak river flows: combining national-scale hydrological modelling and probabilistic projections

Potential future increases in flooding due to climate change need to be taken into consideration when designing flood defences or planning new infrastructure or housing developments. Existing guidance on climate change allowances in Great Britain was based on research that developed a sensitivity-based approach to estimating the impacts of climate change on flood peaks, which was applied with catchment-based hydrological models. Here, the sensitivity-based approach is applied with a national-scale grid-based hydrological model, producing modelled flood response surfaces for every river cell on a 1km grid. This provides a nationally consistent assessment of the sensitivity of flood peaks across Britain to climatic changes. The flood response surfaces are then combined with the most recent climate change projections, UK Climate Projections 2018 (UKCP18), to provide location-specific information on the potential range of impacts on floods across the country, for three flood return periods, three future time-slices and four emissions scenarios. An accompanying web-tool provides a convenient way to explore the large amount of data produced. Consideration is now being given to how to use the latest work to update guidance on climate change and flood peaks, including a workshop held to gather stakeholder views

National-scale analysis of low flow frequency: historical trends and potential future changes

The potential impact of climate change on hydrological extremes is of increasing concern across the globe. Here, a national-scale grid-based hydrological model is used to investigate historical trends and potential future changes in low flow frequency across Great Britain. The historical analyses use both observational data (1891–2015) and ensemble data from a regional climate model (1900–2006). The results show relatively few significant trends in historical low flows (2- or 20-year return period), whether based on 7- or 30-day annual minima. Significant negative trends seen in some limited parts of the country when using observational data are generally not seen when using climate model data. The future analyses use climate model ensemble data for both near future and far future time periods (2020–2049 and 2070–2099 respectively), which are compared to a baseline sub-period from the historical ensemble (1975–2004). The results show future reductions in low flows, which are generally larger in the south of the country, at the higher (20-year) return period, and for the later time period. Reductions are more limited if the estimates of future potential evaporation include the effect of increased carbon dioxide concentrations on stomatal resistance. Such reductions in river flow could have significant impacts on the aquatic environment and on agriculture, and present a challenge for water managers, especially as reductions in water supply are likely to occur alongside increases in demand

Effect of stress-triaxiality on void growth in dynamic fracture of metals: a molecular dynamics study

The effect of stress-triaxiality on growth of a void in a three dimensional single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular dynamics (MD) simulations using an embedded-atom (EAM) potential for copper have been performed at room temperature and using strain controlling with high strain rates ranging from 10^7/sec to 10^10/sec. Strain-rates of these magnitudes can be studied experimentally, e.g. using shock waves induced by laser ablation. Void growth has been simulated in three different conditions, namely uniaxial, biaxial, and triaxial expansion. The response of the system in the three cases have been compared in terms of the void growth rate, the detailed void shape evolution, and the stress-strain behavior including the development of plastic strain. Also macroscopic observables as plastic work and porosity have been computed from the atomistic level. The stress thresholds for void growth are found to be comparable with spall strength values determined by dynamic fracture experiments. The conventional macroscopic assumption that the mean plastic strain results from the growth of the void is validated. The evolution of the system in the uniaxial case is found to exhibit four different regimes: elastic expansion; plastic yielding, when the mean stress is nearly constant, but the stress-triaxiality increases rapidly together with exponential growth of the void; saturation of the stress-triaxiality; and finally the failure.Comment: 35 figures, which are small (and blurry) due to the space limitations; submitted (with original figures) to Physical Review B. Final versio

CCRA3 flooding projections, task 2a: high resolution climate change projections — fluvial. Technical note

As part of the CCRA3 flooding projections project, this task provides: 1. estimates of percentage changes in flood peaks for locations across the UK, using UKCP18 probabilistic projections applied for a set of global mean surface temperature (GMST) changes (ranging from 1.0°C to 4.5°C in increments of 0.5°C); and 2. estimates of change in return period corresponding to a range of peak flow uplifts, as look-up tables, for locations across the UK. The data are provided for use within the Future Flood Explorer (FFE) to investigate potential future flood risks under climate change, under a range of adaptation options. This technical report details the methodology used to produce the flood peak and return period data, including differences in the methods used for Great Britain and Nothern Ireland

General Requirements on Matter Power Spectrum Predictions for Cosmology with Weak Lensing Tomography

Forthcoming projects such as DES, LSST, WFIRST, and Euclid aim to measure weak lensing shear correlations with unprecedented precision, constraining the dark energy equation of state at the percent level. Reliance on photometrically-determined redshifts constitutes a major source of uncertainty for these surveys. Additionally, interpreting the weak lensing signal requires a detailed understanding of the nonlinear physics of gravitational collapse. We present a new analysis of the stringent calibration requirements for weak lensing analyses of future imaging surveys that addresses both photo-z uncertainty and errors in the calibration of the matter power spectrum. We find that when photo-z uncertainty is taken into account the requirements on the level of precision in the prediction for the matter power spectrum are more stringent than previously thought. Including degree-scale galaxy clustering statistics in a joint analysis with weak lensing not only strengthens the survey's constraining power by ~20%, but can also have a profound impact on the calibration demands, decreasing the degradation in dark energy constraints with matter power spectrum uncertainty by a factor of 2-5. Similarly, using galaxy clustering information significantly relaxes the demands on photo-z calibration. We compare these calibration requirements to the contemporary state-of-the-art in photometric redshift estimation and predictions of the power spectrum and suggest strategies to utilize forthcoming data optimally.Comment: 3 new figures; new section added on multipole-dependence of calibration requirements; references added; version accepted by JCA

Serum magnesium and calcium levels in relation to ischemic stroke : Mendelian randomization study

ObjectiveTo determine whether serum magnesium and calcium concentrations are causally associated with ischemic stroke or any of its subtypes using the mendelian randomization approach.MethodsAnalyses were conducted using summary statistics data for 13 single-nucleotide polymorphisms robustly associated with serum magnesium (n = 6) or serum calcium (n = 7) concentrations. The corresponding data for ischemic stroke were obtained from the MEGASTROKE consortium (34,217 cases and 404,630 noncases).ResultsIn standard mendelian randomization analysis, the odds ratios for each 0.1 mmol/L (about 1 SD) increase in genetically predicted serum magnesium concentrations were 0.78 (95% confidence interval [CI] 0.69-0.89; p = 1.3 7 10-4) for all ischemic stroke, 0.63 (95% CI 0.50-0.80; p = 1.6 7 10-4) for cardioembolic stroke, and 0.60 (95% CI 0.44-0.82; p = 0.001) for large artery stroke; there was no association with small vessel stroke (odds ratio 0.90, 95% CI 0.67-1.20; p = 0.46). Only the association with cardioembolic stroke was robust in sensitivity analyses. There was no association of genetically predicted serum calcium concentrations with all ischemic stroke (per 0.5 mg/dL [about 1 SD] increase in serum calcium: odds ratio 1.03, 95% CI 0.88-1.21) or with any subtype.ConclusionsThis study found that genetically higher serum magnesium concentrations are associated with a reduced risk of cardioembolic stroke but found no significant association of genetically higher serum calcium concentrations with any ischemic stroke subtype