A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview,damage survey and radar analysis

This study presents an analysis of a severe weather case that took place during the early morning of the 2nd of November 2008, when intense convective activity associated with a rapidly evolving low pressure system affected the southern coast of Catalonia (NE Spain). The synoptic framework was dominated by an upper level trough and an associated cold front extending from Gibraltar along the Mediterranean coast of the Iberian Peninsula to SE France, which moved north-eastward. South easterly winds in the north of the Balearic Islands and the coast of Catalonia favoured high values of 0–3 km storm relative helicity which combined with moderate MLCAPE values and high shear favoured the conditions for organized convection. A number of multicell storms and others exhibiting supercell features, as indicated by Doppler radar observations, clustered later in a mesoscale convective system, and moved north-eastwards across Catalonia. They produced ground-level strong damaging wind gusts, an F2 tornado, hail and heavy rainfall. Total lightning activity (intra-cloud and cloud to ground flashes) was also relevant, exhibiting several classical features such as a sudden increased rate before ground level severe damage, as discussed in a companion study. Remarkable surface observations of this event include 24 h precipitation accumulations exceeding 100 mm in four different observatories and 30 minute rainfall amounts up to 40 mm which caused local flash floods. As the convective system evolved northward later that day it also affected SE France causing large hail, ground level damaging wind gusts and heavy rainfall.     

Optimal extraction of cosmological information from supernova data in the presence of calibration uncertainties

We present a new technique to extract the cosmological information from high-redshift supernova data in the presence of calibration errors and extinction due to dust. While in the traditional technique the distance modulus of each supernova is determined separately, in our approach we determine all distance moduli at once, in a process that achieves a significant degree of self-calibration. The result is a much reduced sensitivity of the cosmological parameters to the calibration uncertainties. As an example, for a strawman mission similar to that outlined in the SNAP satellite proposal, the increased precision obtained with the new approach is roughly equivalent to a factor of five decrease in the calibration uncertainty.     

An inspection of the long-term behaviour of the range of the daily geomagnetic field variation from comprehensive modelling

This paper attempts to reveal whether long-term trends in the ionosphere are reflected in the amplitude range of the geomagnetic daily variation recorded at ground level. The smooth and regular variation observed in the magnetograms on magnetically quiet days is induced by the ionospheric currents flowing in the dynamo region. So it is likely that trends in the conductivity or in the dynamics of this region could produce changes in the current densities, and consequently in the range of the geomagnetic variation. The crucial aspect is how to separate the changes produced by the geomagnetic activity itself, or by secular changes of the Earth's magnetic field, from the part of the variation produced by factors affecting trends in the ionosphere, which could have an anthropogenic origin. To investigate this, we synthesized for several geomagnetic observatories the daily ranges of the geomagnetic field components with a comprehensive model of the quiet-time, near-Earth magnetic field, and finally we removed the synthetic values from the observed ranges at those observatories. This comprehensive model accounts for contributions from Earth's core, lithosphere, ionosphere, magnetosphere and coupling currents, and, additionally, accounts for influences of main field and solar activity variations on the ionosphere. Therefore, any trend remaining in the residuals, assuming that all the contributions mentioned above are properly described and thus removed by the comprehensive model, should reflect the influence of other sources. Results, based on series of magnetic data from observatories worldwide distributed, are presented. Trends in the X and Z components are misleading, since the current system changes in form as well as in intensity, producing changes of the focus latitude in the course of a solar cycle and from one cycle to another. Some differences exist between the long-term trends in the Y component between the real and modelled ranges, suggesting that other non-direct solar causes to the amplitude changes of the solar quiet geomagnetic variation should not be ruled out. Nevertheless, the results also reflect some short-comings in the way that the comprehensive modelling accounts for the influence of the solar activity on the range of the daily geomagnetic variation.     

A Regional Archaeomagnetic Model for the Palaeointensity in Europe for the last 2000 Years and its Implications for Climatic Change

The SCHA.DI.00 directional model for the geomagnetic field in Europe for the last 2000 years (Pavón–Carrasco et al., 2008) has been updated by modelling the palaeointensity. This model, SCHA.DI.00, was developed from available Bayesian European Palaeosecular Variation Curves using the regional Spherical Cap Harmonic Analysis technique. The comparison of the palaeosecular variation curves, given by the regional model, with available archaeomagnetic data not used in its development showed an improvement with respect to the fit obtained by global archaeomagnetic models. In this paper advantage is taken of recently published palaeointensity databases to develop a complete (direction and intensity) regional archaeomagnetic model for the last 2000 years valid for the European region: the SCHA.DI.00–F model. Not only does this complete model provide an improvement for example for archaeomagnetic data studies, but it is also shown that this new regional model can be used to study the recently proposed link between the centennial secular variation of the geomagnetic field and climate change. The pattern of the archaeosecular variation of the field intensity obtained by SCHA.DI.00–F seems to verify the hypothesis presented by Gallet et al. (2005) about a possible (causal) connection between changes in the geomagnetic field intensity and in climate parameters, opening the door for more discussions on this challenging subject.     

High downward flux of mucilaginous aggregates in the Ligurian Sea during summer 2002: similarities with the mucilage phenomenon in the Adriatic Sea

The DYFAMED sediment trap station in the Ligurian Sea (NW Mediterranean) has been active since 1986 and today comprises the longest time‐series of downward particle flux in the Mediterranean Sea. As such, it provides valuable information on the interannual variability of the particle flux, and also documents possible recent changes in the NW Mediterranean pelagic ecosystems. We report an unprecedented episode of downward flux of mucilaginous material at the DYFAMED station during summer 2002 in association with singular hydrometeorological conditions. The rain of mucilaginous aggregates clogged a PPS5 sediment trap at 260 m depth and was also clearly detected at 1080 m depth. The possible factors governing the development and sinking of the mucilaginous material are discussed. A very sharp increase of sea surface temperature during June and the presence of freshened waters in the surface the following month resulted in a stronger than usual stratification of the upper water column throughout the summer season. We suggest that the steepness of the vertical density gradient was responsible for the unusual accumulation of mucous aggregates. Additionally, a diatom bloom took place during the nutrient‐depleted conditions typical of summer, a factor which may have contributed to feed the pycnocline with transparent exopolymer substances. A storm occurring in the beginning of August relaxed the stratification and promoted the deposition of the mucilaginous aggregates accumulated in the upper water column during the preceding months. Important similarities of ambient conditions preceding the apparition of mucilaginous material in our open‐sea site and those reported in the Adriatic Sea during major mucilage events, suggest that general climatic conditions, rather than local factors, drive the occurrence of major accumulations of mucilaginous material in the water column at both sub‐basins of the Mediterranean Sea. In this regard, the strength of the air temperature increase during the onset of the stratified season is proposed as a major controlling factor.     

Intra-clinothem variability in sedimentary texture and process regime recorded down slope profiles

Shelf-margin clinothem successions can archive process interactions at the shelf to slope transition, and their architecture provides constraints on the interplay of factors that control basin-margin evolution. However, detailed textural analysis and facies distributions from shelf to slope transitions remain poorly documented. This study uses quantitative grain-size and sorting data from coeval shelf and slope deposits of a single clinothem that crops out along a 5 km long, dip-parallel transect of the Eocene Sobrarbe Deltaic Complex (Ainsa Basin, south-central Pyrenees, Spain). Systematic sampling of sandstone beds tied to measured sections has captured vertical and basinward changes in sedimentary texture and facies distributions at an intra-clinothem scale. Two types of hyperpycnal flow-related slope deposits, both rich in mica and terrestrial organic matter, are differentiated according to grain size, sorting and bed geometry: (i) sustained hyperpycnal flow deposits, which are physically linked to coarse channelized sediments in the shelf setting and which deposit sand down the complete slope profile; (ii) episodic hyperpycnal flow deposits, which are disconnected from, and incise into, shelf sands and which are associated with sediment bypass of the proximal slope and coarse-grained sand deposition on the medial and distal slope. Both types of hyperpycnites are interbedded with relatively homogenous, organic-free and mica-free, well-sorted, very fine-grained sandstones, which are interpreted to be remobilized from wave-dominated shelf environments; these wave-dominated deposits are found only on the proximal and medial slope. Coarse-grained sediment bypass into the deeper-water slope settings is therefore dominated by episodic hyperpycnal flows, whilst sustained hyperpycnal flows and turbidity currents remobilizing wave-dominated shelf deposits are responsible for the full range of grain sizes in the proximal and medial slope, thus facilitating clinoform progradation. This novel dataset highlights previously undocumented intra-clinothem variability related to updip changes in the shelf process-regime, which is therefore a key factor controlling downdip architecture and resulting sedimentary texture.     

A single vs. double spike approach to improve the accuracy of Th measurements in small-volume seawater samples

²³⁴Th is a particle-reactive radionuclide widely used to trace biogeochemical oceanic processes occurring over short timescales. During the last few years, small-volume techniques based on the co-precipitation of ²³⁴Th with MnO₂ coupled with beta-counting have been developed as an alternative to large volume gamma-spectrometric techniques. Here a procedure has been developed to enhance quantitative measurement of ²³⁴Th in MnO₂ precipitates. The main objectives were to obtain a purified Th fraction for beta-counting and to determine the chemical recovery of ²³⁴Th using Th spikes and alpha-spectrometry as an alternative to ICP-MS based methods. Two variations of the procedure are presented. In the first “1 spike” method a ²³⁰Th tracer is added to the sample prior to precipitation of MnO₂, and UTEVA® extraction chromatography is used to obtain a NdF₃(Th) purified source that can be used for both beta-counting of ²³⁴Th and alpha-spectrometry of ²³⁰Th. In the “2 spike” method a ²³⁰Th spike is added and the MnO₂(Th) precipitate is directly beta-counted for ²³⁴Th and subsequently spiked with ²²⁸Th or ²²⁹Th prior to UTEVA® purification and alpha-spectrometry. The results confirm the need to process small-volume seawater samples for ²³⁴Th measurement in presence of a yield tracer, and show that both the 1 spike and 2 spike methods allow an accurate and precise determination of ²³⁴Th (relative percent difference, RPD, between expected and mean measured value < 1%; CV between replicate samples < 3%). Our work also suggests that, although the combined analytical uncertainty on total ²³⁴Th measurements accomplished with both versions of the NdF₃ procedure is promising (6% for 2-L samples), the precision of the ²³⁴Th flux estimation will ultimately depend on the degree of disequilibrium between ²³⁴Th and ²³⁸U.