F. J. Acero

Peaks-over-Threshold Study of Trends in Extreme Rainfall over the Iberian Peninsula

A peaks-over-threshold (POT) approach is used to study trends in extreme rainfall over the Iberian Peninsula (IP) at a daily scale. Records from 52 observatories regularly distributed over Iberia with no missing data were available for the common period from 1958 to 2004. The POT approach was used because it is particularly effective at extracting information concerning true extreme events. A generalized Pareto distribution fit was made to the data involving time-dependent parameters to account for possible temporal changes in the frequency distribution. These parameters were analyzed for trends in the return-level period, of importance for engineering purposes. A time-varying threshold was defined and an automatic declustering scheme was used to select independent extreme events exceeding the threshold. The results indicate a high variability of extreme events over the coastline of the IP, greater over the Mediterranean coast than over the Atlantic coast. The calculation of the trends for the 2-yr return level yielded a large proportion of negative trends for all three seasons considered: 58% for winter, 63% for spring, and 69% for autumn. The parametric approach also revealed an increase in the area with a positive trend of the 20-yr return level relative to the 2-yr return period, especially in autumn in the east of the IP.

Liquid bridge equilibrium contours between non-circular supports

The equilibrium shape of a liquid bridge confined between two parallel solid supports of arbitrary shape is analyzed theoretically. The influence of the shape of the solid supports, the volume of the liquid bridge and the axial, lateral, and centrifugal forces acting on it are considered. The theoretical study is performed using three different techniques: (i) a numerical process for the minimization of the free energy associated with the fluid configuration, (ii) a second-order analytical expansion around the cylindrical solution to the Young-Laplace equation, and (iii) a finite-difference scheme to numerically integrate the Young-Laplace equation. The two numerical methods are in excellent agreement, indicating the degree of accuracy of the calculations, and there is also good agreement between the analytical expansion and the numerical solutions for moderately large values of the perturbation parameters.

Influence of isorotation on the linear dynamics of liquid bridges

The influence of isorotation on the axisymmetric and nonaxisymmetric oscillation modes of inviscid liquid bridges is analyzed theoretically. The dependence of the natural frequencies of the surface wave modes on the rotation speed is obtained. The study is completed by calculating the natural frequencies corresponding to the transition from surface to internal waves. The results are obtained analytically for cylindrical liquid bridges and numerically for the axisymmetric equilibrium shapes, including the influence of the outer bath in both cases.

Non-stationary future return levels for extreme rainfall over Extremadura (southwestern Iberian Peninsula)

ABSTRACT Based on a previous study for temperature, a new method for the calculation of non-stationary return levels for extreme rainfall is described and applied to Extremadura, a region of southwestern Spain, using the peaks-over-threshold approach. Both all-days and rainy-days-only datasets were considered and the 20-year return levels expected in 2020 were estimated taking different trends into account: first, for all days, considering a time-dependent threshold and the trend in the scale parameter of the generalized Pareto distribution; and second, for rainy days only, considering how the mean, variance, and number of rainy days evolve. Generally, the changes in mean, variance and number of rainy days can explain the observed trends in extremes, and their extrapolation gives more robust estimations. The results point to a decrease of future return levels in 2020 for spring and winter, but an increase for autumn.

Extreme Value Theory Applied to the Millennial Sunspot Number Series

In this work, we use two decadal sunspot number series reconstructed from cosmogenic radionuclide data (14C in tree trunks, SN-14C and 10Be in polar ice, SN-10Be) and the Extreme Value Theory to study variability of solar activity during the last 9 millennia. The peaks-over-threshold technique was used to compute, in particular, the shape parameter of the generalized Pareto distribution for different thresholds. Its negative value implies an upper bound of the extreme SN-10Be and SN-14C time series. The return level for 1000 and 10000 years were estimated leading to values lower than the maximum observed values, expected for the 1000-year, but not for the 10000-year return levels, for both series. A comparison of these results with those obtained using the observed sunspot numbers from telescopic observations during the last four centuries suggest that the main characteristics of solar activity have already been recorded in the telescopic period (from 1610 to nowadays) which covers the full range of solar variability from a Grand minimum to a Grand maximum.

Changes in heat wave characteristics over Extremadura (SW Spain)

Heat wave (HW) events are becoming more frequent, and they have important consequences because of the negative effects they can have not only on the human population in health terms but also on biodiversity and agriculture. This motivated a study of the trends in HW events over Extremadura, a region in the southwest of Spain, with much of its area in summer devoted to the production of irrigated crops such as maize and tomatoes. Heat waves were defined for the study as two consecutive days with temperatures above the 95th percentile of the summer (June–August) maximum temperature (Tmax) time series. Two datasets were used: One consisted of 13 daily temperature records uniformly distributed over the Region, and the other was the SPAIN02 gridded observational dataset, extracting just the points corresponding to Extremadura. The trends studied were in the duration, intensity and frequency of HW events, and in other parameters such as the mean, low (25th percentile) and high (75th percentile) values. In general terms, the results showed significant positive trends in those parameters over the east, the northwest and a small area in the south of the region. In order to study changes in HW characteristics (duration, frequency and intensity) considering different subperiods, a stochastic model was used to generate 1000 time series equivalent to the observed ones. The results showed that there were no significant changes in HW duration in the last 10-year subperiod in comparison with the first. But, the results were different for warm events (WE), defined with a lower threshold (the 75th percentile), which are also important for agriculture. For several sites, there were significant changes in WE duration, frequency and intensity.

A Bayesian hierarchical spatio-temporal model for extreme rainfall in Extremadura (Spain)

ABSTRACT A statistical study was made of the temporal trend in extreme rainfall in the region of Extremadura (Spain) during the period 1961–2009. A hierarchical spatio-temporal Bayesian model with a GEV parameterization of the extreme data was employed. The Bayesian model was implemented in a Markov chain Monte Carlo framework that allows the posterior distribution of the parameters that intervene in the model to be estimated. The results show a decrease of extreme rainfall in winter and spring and a slight increase in autumn. The uncertainty in the trend parameters obtained with the hierarchical approach is much smaller than the uncertainties obtained from the GEV model applied locally. Also found was a negative relationship between the NAO index and the extreme rainfall in Extremadura during winter. An increase was observed in the intensity of the NAO index in winter and spring, and a slight decrease in autumn.

Numerical Calculation of a Liquid Bridge Equilibrium Contour between Noncircular Supports

Abstract The equilibrium shape of a liquid bridge confined between two parallel plates of arbitrary shape is numerically calculated. Two alternative procedures are used: (i) an algorithm that minimizes the sum of the potential and surface energies, and (ii) a finite-difference scheme to integrate the Young-Laplace equation. Both methods preserve the imposed value for the liquid bridge volume. The comparison between the results obtained from the two methods shows an excellent agreement, which indicates the degree of accuracy of the calculations.

Equilibrium contour of liquid bridges connected by pressure

The equilibria and stability in zero gravity of fixed boundary dual liquid bridges have been analyzed recently [B.J. Lowry, Phys. Fluids, vol. 12, p. 1005 (2000)]. Dual liquid bridges consist of two fixed-length sub-bridges connected by a free liquid passage so that the two sub-bridges are coupled by pressure. In the fixed pressure case, this fluid configuration is found to be more stable than a single bridge of equal length. This result lends a certain relevance to dual liquid bridges, especially in applications which involve pumping through capillaries. We here provide explicit formulas to predict the equilibrium contour of an arbitrary number of connected liquid bridges under microgravity conditions. The results are obtained for configurations close to cylindrical by using a perturbation expansion. The analytical predictions are compared with numerical data obtained by means of a finite difference scheme. The agreement is quite good for moderate axisymmetric and non-axisymmetric perturbations. Both analytical and numerical results are compared with experiments. The comparison confirms the validity of the theoretical approaches.