X. Lana

Spatial and temporal characterization of annual extreme droughts in Catalonia (northeast Spain)

Several characteristics of annual extreme drought episodes for Catalonia (northeast Spain) are investigated from different points of view. Firstly, possible temporal trends on the length of these episodes are analysed, by means of Students t test and a Monte Carlo procedure, for each complete pluviometric series. Secondly, the spatial distribution of the seasonal predominance of the annual extreme droughts is analysed, showing the influence of the orographic factors. Thirdly, most of the annual extreme episodes are modelled correctly by the Gumbel distribution of extremes, and the expected lengths for return periods of 2, 5, 10, 25 and 50 years are computed. Nevertheless, some discordances were detected between this theoretical distribution and the empirical extremes. Thus, the return periods for some gauges were computed by means of the more general Jenkinson model. Finally, the set of pluviometric stations are characterized by the extreme drought lengths of the different return periods, and they are classified according to the average linkage algorithm into clusters of homogeneous gauges. In short, the results obtained suggest that most of the country is characterized by a typical Mediterranean behaviour. However, some differences can be established between littoral areas, some parts of the Central Basin and the pre-Pyrenean and Pyrenean ranges.

Daily dry–wet behaviour in Catalonia (NE Spain) from the viewpoint of Markov chains

The pluviometric regime of Catalonia (NE Spain) is analyzed from the point of view of empirical dry period lengths by considering 78 rain stations for an average of 34 years. Two possible statistical models are tested. The first model is the exponential distribution that offers reasonable results for moderate or long sequences of dry days. The other possibility comes from Markov chains of first or second order, with two or four states quantifying precipitation amounts, that are used to give better results for all the range of sequences. The Kolmogorov-Smirnov test has been applied with the aim of verifying the fit between empirical probabilities of the sequences and theoretical probabilities given by the exponential distribution and the Markov chains. It is noticeable that the Markov chain of second order is many times the distribution either satisfying the test criteria or the closest one to its fulfilment. The exponential distribution satisfies better the test criteria only for a few gauges and we have to keep in mind that for a remarkable number of pluviometric stations, none of the proposed models accomplish the test. In spite of these limited results, the Markov chains are employed to quantify important aspects which can not be studied by the exponential distribution. These aspects include return periods for a new dry or wet episode and the stationary probabilities for different precipitation amounts quantifying the states of the chains, among other aspects. As a global achievement and remembering the limited success of the test, the results depicted by Markov chains of first and second order are employed to show a differentiated behaviour among the Pyrenees and Pre-Pyrenees areas, the Central Basin, the Littoral and Pre-Littoral areas, the Transversal chain and the southern Mediterranean coast.

Return period maps of dry spells for Catalonia (northeastern Spain) based on the Weibull distribution / Périodes de retour des périodes sèches en Catalogne (nord-est de l'Espagne) à partir de la distribution de Weibull

Abstract A detailed description of the spatial distribution of dry spell lengths in Catalonia (northeastern Spain) for the years 1950–2000 is obtained from a statistical analysis. The database is derived from the daily records of 75 raingauges, and a dry spell is defined as a set of consecutive days with daily rainfall amounts below thresholds of 0.1, 1.0, 5.0 and 10.0 mm/d. The Weibull model fits the distribution of dry spell lengths well, whatever the raingauge and threshold. The Weibull parameters estimated by L-moments and the L-skewness—kurtosis diagrams allow quantification of the goodness of fit between the model and the empirical distribution. Dry spell lengths for return periods of 2, 5, 10, 25 and 50 years indicate the areas where drought phenomena might be more severe, as well as how often they might occur. A regional homogeneity method is used to test whether a single set of Weibull parameters could describe the series of dry spells for the whole region.

Catalonian precipitation patterns: Principal component analysis and automated regionalization

SummaryA principal component analysis (PCA) based on a network including 179 pluviometric gauges is attempted in order to describe the main patterns governing precipitation in Catalonia (NE Spain). This PCA procedure is applied to the interstation covariance matrix and component loadings are then deduced from it and extensively interpreted. PCA results are used immediately after in a Davis and Kalkstein clustering process, leading to a rainfall division. The homogeneity of the different groups obtained is tested by means of the analysis of their spatial and temporal within-group variances.

Inversion of Rayleigh wave phase and group velocities by simulated annealing

Abstract A joint inversion of regionalized phase and group velocities of fundamental mode Rayleigh waves is performed by means of a simulated annealing scheme which makes use of some thermodynamic analogies based on the Gibbs’ distribution. The correlations and the uncertainties on shear-wave velocities defining the elastic model are estimated by using a procedure based on the same thermodynamic analogies. As a relevant feature, the simulated annealing algorithm permits reducing the usual dependence of the solution on the sometimes arbitrary starting model. A shortcoming linked to the use of this procedure for solving an inversion problem is the necessity of a previous empirical determination of some parameters controlling the annealing process. With this aim, a simulation of a joint inversion of synthetic surface wave dispersion data has been performed. After that, path-averaged velocity dispersion curves corresponding to a Hercynian area of the Iberian Peninsula have been inverted to derive two possible crust and upper mantle elastic structures. Whereas, the first structure has been deduced by considering as unknown variables the shear velocities of the stratified Earth model, the second has been obtained also including the respective layer thicknesses. Both structures are compared with shear velocity and Qβ models derived by the stochastic inversion procedure in previous studies and from the same Rayleigh waves dataset. Finally, advantages and shortcomings of the inversion of phase and group velocity data by means of simulated annealing are discussed.

Winter and autumn daily precipitation patterns in Catalonia, Spain

SummaryA principal component analysis (PCA), based on a network consisting of 60 pluviometric gauges and their daily precipitation data, is attempted in order to describe the main winter and autumn patterns governing precipitation in Catalonia (NE Spain). This PCA procedure is applied to the interstation correlation matrix and rotated component loadings are then deduced and extensively interpreted. The PCA results are then used in a clustering process (Average Linkage), leading to two rainfall divisions, one for each season, which are then compared.

EXTREME WINTER MINIMUM TEMPERATURES IN CATALONIA (NORTH‐EAST SPAIN): EXPECTED VALUES AND THEIR SPATIAL DISTRIBUTION

The climatic diversity of Catalonia (north-east Spain) is studied from the point of view of extreme minimum temperatures recorded during the coldest months of the year (December, January, February and March). The data base was created from the Catalonian thermometric network by removing gauges affected by significant trends. Extreme analysis is made according to the general Jenkinson formulation. Geographical distribution of the expected extremes is investigated for several return periods and orographic factors governing this distribution are discussed. Synoptic situations causing extreme minimum temperature episodes are summarized.

Anelastic structure of the Iberian Peninsula obtained from an automated regionalization algorithm and stochastic inversion

Abstract An automated regionalization of a reduced domain of the Earths surface, without a-priori seismotectonic information, and the corresponding anelastic structures are achieved by means of a sequential application of a principal component analysis, a clustering procedure and the stochastic inversion algorithm to anelastic attenuation coefficients of Rayleigh waves. Our database is formed by path-averaged attenuation coefficients derived from fundamental mode Rayleigh waves crossing the Iberian Peninsula. The wavetrains were recorded at the broad-band stations installed some years ago in the Iberian Peninsula for the ILIHA project. Before the sequential application of the algorithms, the area covered by the seismic paths was characterized by 22 maps of local attenuation coefficients corresponding to the 10–120 s period range. All these maps were obtained by applying Yanovskayas formulation for laterally heterogeneous media to the set of path-averaged attenuation coefficients mentioned above. After that, we divided the Iberian Peninsula into six homogeneous regions in terms of the local attenuation coefficients for 22 different periods and by means of the principal component analysis and the clustering algorithm. We then obtained, by stochastic inversion, the respective anelastic structure down to a depth of 225 km for the six homogeneous regions. The coefficients vary from 1.0 to 2.0×10−3 km−1 and the Q−1β structures, ranging from 110×10−3 to 40×10−3, suggest for five of the regions an asthenosphere with its upper and lower boundaries close to 80 and 180 km depth, respectively. A shallow beginning of the asthenosphere is detected for the sixth homogeneous region. This range is very similar to the depths deduced from previous tomographic studies of the Iberian Peninsula based on Rayleigh wave phase and group velocities. Finally, correlations obtained between the regions and some seismotectonic characteristics of the Peninsula are discussed. It is noteworthy that we cannot associate each of the six homogeneous regions with a single Hercynian, Alpine or Neogene domain.

Local lithospheric stress distribution deduced by means of the total inversion algorithm and an objective classification method

Classification of populations of focal mechanisms are commonly used as a step in retrieving states of stress in the lithosphere. Although such classifications are usually made according to subjective criteria, an objective one based on a classification algorithm is presented. The procedure is based on two steps. First, each mechanism is characterized by the components of its principal tensional and compressional axes and then objectively classified accordingly. Second, each mechanism is characterized by its geographic coordinates (latitude and longitude) after which each group obtained in the first step is divided into new groups according to geographic coordinates. The classification algorithm, together with a standard inversion algorithm, is applied to data from aftershocks belonging to the Irpinia (southern Italy) Earthquake of November 23, 1980. A few incompatible groups of focal mechanisms obtained by the classification algorithm show a spatial overlap. Nevertheless, the new methodology allows us to present a more accurate description of a local state of stress for an aftershock area than the stress pattern obtained from subjective classifications. The new stress pattern is compared with principal stress axes deduced from the main event and two local states of stress deduced previously from the aftershock mechanisms.

Winter synoptic weather types in Catalonia (NE Spain) and their linkage with minimum temperature anomalies

An objective classification of winter synoptic weather types in Catalonia (NE Spain) is obtained by means of the principal component analysis of daily variables followed by cluster analysis based on Average Linkage and K-Means methods. The period analysed includes days belonging to December, January and February, from 1976 to 1980. The data list consists of thermal, pressure, cloud cover, humidity, wind and precipitation values recorded at four different stations in Catalonia. The classification includes 14 clusters which accurately explain the winter synoptic variety of the country. All cluster features are discussed in detail and a study concerning transitions between these clusters is presented. Three groups (1, 3 and 7) are related to high pressure weather types, clusters 4, 5 and 8 to cold sectors of mid-latitude depressions and clusters 2 and 9 can be associated with cyclonic conditions with western circulation. The weather types related to extended precipitation are included in clusters 6 and 10 which are linked to eastern surface circulation. The obtained classification is then applied in a study concerning daily minimum temperature anomalies. The spatial distribution of these anomalies are analysed for each significant weather type. The clusters related to widely extended frost risk are finally identified (groups 4, 8 and 14).