Etude de la couche limite de surface sahelienne — Experience Yantala

This paper presents data concerning the energy budget in the surface layer in the Sahel region (a semi-desert area). The results are drawn from a measurement campaign made in the Niamey region in the Niger, in April–May 1984 (the Yantala Campaign). The sensible heat flux is computed with the profile method, the ground heat flux is deduced from measurement of the temperature field, and the radiative net flux is measured directly with a balancemeter. The latent heat flux, which is deduced from the energy budget balance is very weak and within the accuracy limit of the method. The diurnal variation of the net flux is symmetrical, with a maximum at noon. On the other hand, the sensible heat flux variation is asymmetrical, with an afternoon decrease much slower than the morning increase. After 3.30 pm, it becomes higher than the net flux. This is compensated for by the sign change of the ground heat flux, whose maximum is found in the morning at 11 am. The second part of this paper shows the importance of one term in the surface-layer energy budget: the long-wave radiative divergence between the ground and the top of the surface layer in high superadiabatic conditions. We show, with a radiative model on the one hand and direct measurement of the radiative divergence on the other hand, that this term reaches several tens of W m^-2 in the superadiabatic conditions found in the Sahel region.      

A modal pushover analysis procedure to estimate seismic demands for unsymmetric‐plan buildings

An Erratum has been published for this article in Earthquake Engng. Struct. Dyn. 2004; 33:1429. Based on structural dynamics theory, the modal pushover analysis (MPA) procedure retains the conceptual simplicity of current procedures with invariant force distribution, now common in structural engineering practice. The MPA procedure for estimating seismic demands is extended to unsymmetric‐plan buildings. In the MPA procedure, the seismic demand due to individual terms in the modal expansion of the effective earthquake forces is determined by non‐linear static analysis using the inertia force distribution for each mode, which for unsymmetric buildings includes two lateral forces and torque at each floor level. These ‘modal’ demands due to the first few terms of the modal expansion are then combined by the CQC rule to obtain an estimate of the total seismic demand for inelastic systems. When applied to elastic systems, the MPA procedure is equivalent to standard response spectrum analysis (RSA). The MPA estimates of seismic demand for torsionally‐stiff and torsionally‐flexible unsymmetric systems are shown to be similarly accurate as they are for the symmetric building; however, the results deteriorate for a torsionally‐similarly‐stiff unsymmetric‐plan system and the ground motion considered because (a) elastic modes are strongly coupled, and (b) roof displacement is underestimated by the CQC modal combination rule (which would also limit accuracy of RSA for linearly elastic systems). Copyright © 2004 John Wiley & Sons, Ltd.     

The effects of hazard zone information on judgements about earthquake damage

This study investigates the extent to which people's views on the causes and preventability of earthquake damage might be influenced by their degree of exposure to hazard as well as what information they have been given about the hazard. The results show that the provision of hazard zoning information influences judgements on preventability and causes of damage, but this effect depends on the degree of hazard faced by residents. In low hazard zones, information leads to the view that causes are manageable, whereas in high hazard zones information may induce a degree of fatalism. The use of public information in risk management needs to take into account the degree of risk faced by the recipients.     

SMOS reveals the signature of Indian Ocean Dipole events

The tropical Indian Ocean experiences an interannual mode of climatic variability, known as the Indian Ocean Dipole (IOD). The signature of this variability in ocean salinity is hypothesized based on modeling and assimilation studies, on account of scanty observations. Soil Moisture and Ocean Salinity (SMOS) satellite has been designed to take up the challenge of sea surface salinity remote sensing. We show that SMOS data can be used to infer the pattern of salinity variability linked with the IOD events. The core of maximum variability is located in the central tropical basin, south of the equator. This region is anomalously salty during the 2010 negative IOD event, and anomalously fresh during the 2011 positive IOD event. The peak-to-peak anomaly exceeds one salinity unit, between late 2010 and late 2011. In conjunction with other observational datasets, SMOS data allow us to draw the salt budget of the area. It turns out that the horizontal advection is the main driver of salinity anomalies. This finding is confirmed by the analysis of the outputs of a numerical model. This study shows that the advent of SMOS makes it feasible the quantitative assessment of the mechanisms of ocean surface salinity variability in the tropical basins, at interannual timescales.

     

Fault zone geometry of a mature active normal fault: A potential high permeability channel (Pirgaki fault, Corinth rift, Greece)

We present results from petrophysical analysis of a normal fault zone with the aim of defining the flow pathways and their behavior during seismic and interseismic periods. Data are obtained on porosity geometry, strain structure and mineralogy of different domains of a normal fault zone in the Corinth rift. Data point out a close relationship between mineralogy of the clayey minerals, porosity network and strain structures and allow definition of a macroscopic anisotropy of the flow parameters with a strong control by microscopic ultracataclasite structures. The Pirgaki fault zone, developed within pelagic limestone, has a sharp asymmetric porosity profile, with a high porosity volume in the fault core and in the damage zone of the hanging wall. From porosity volumes and threshold measurements, a matrix permeability variation of 6 orders of magnitude could be expected between the protolith and the fault core. Modifications of this pathway during seismic and interseismic phases are depicted. Healing of cracks formed during seismic slip events occurred in the fault core zone and the porous network in the damage zone is sealed in a second step. The lens geometry of the fault core zone is associated with dissolution surfaces and open conduits where dissolved matter could move out of the fault core zone. These elementary processes are developed in particular along Riedel's structures and depend on the orientation of the strain surfaces relative to the local stress and depend also on the roughness of each surface type. P-surfaces are smooth and control shearing process. R-surfaces are rough and present two wavelengths of roughness. The long one controls localization of dissolution surfaces and conduits; the short one is characteristic of dissolution surfaces. The dissolved matter can precipitate in the open structures of the hanging wall damage zone, decreasing the connectivity of the macroscopic conduit developed within this part of the fault zone.     

Stable carbon and oxygen isotope study on benthic foraminifera: Implication for microhabitat preferences and interspecies correlation

Stable isotopes of benthic foraminifera have widely been applied in micropalaeontological research to understand vital effects in foraminifera. Isotopic fractionations are mainly controlled by ontogeny, bottom/pore water chemistry, habitat preference, kinetic effect and respiration. Discontinuous abundance of a species for isotopic analysis has forced us to select multiple species from down-core samples. Thus standardisation factors are required to convert isotopic values of one species with respect to other species. The present study is pursued on isotopic values of different pairs of benthic foraminifera from the Krishna–Godavari basin and Peru offshore to understand habitat-wise isotopic variation and estimation of isotopic correction factors for the paired species (Cibicides wuellerstorfi–Bulimina marginata, Ammonia spp.–Loxostomum amygdalaeformis and Bolivina seminuda–Nonionella auris). Infaunal species (B. marginata, Ammonia spp. and N. auris) show a lighter carbon isotopic excursion with respect to the epifaunal to shallow infaunal forms (C. wuellerstorfi, L. amygdalaeformis and B. seminuda). These lighter \(\updelta ^{13}\) \(\hbox {C}\) values are related to utilisation of \(\hbox {CO}_{2}\) produced by anaerobic remineralisation of organic matter. However, enrichment of \(\updelta ^{18}\) \(\hbox {O}\) for the deeper microhabitat (bearing lower pH and decreased \({\hbox {CO}_{3}}^{2-})\) is only recorded in case of B. marginata. It is reverse in case of N. auris and related to utilisation of respiratory \(\hbox {CO}_{2}\) and internal dissolve inorganic carbon pool. Estimation of interspecies isotopic correction factors for the species pairs (\(\updelta ^{13}\) \(\hbox {C}\) of C. wuellerstorfi–B. marginata, L. amygdalaeformis–Ammonia spp., N. auris–B. seminuda) and \(\updelta ^{18}\) \(\hbox {O}\) of C. wuellerstorfi–B. marginata are statistically reliable and may be used in palaeoecological studies.     

Atmospheric boundary-layer structure observed during a haze event due to forest-fire smoke

During a haze event in Baltimore, U.S.A. from July 6 to 8, 2002, smoke from forest fires in the Québec region (Canada), degraded air quality and impacted upon local climate, decreasing solar radiation and air temperature. The smoke particles in and above the atmospheric boundary layer (ABL) served as a tracer and provided a unique opportunity to investigate the ABL structure, especially entrainment. Elastic backscatter lidar measurements taken during the haze event distinctly reveal the downward sweeps (or wisps) of smoke-laden air from the free atmosphere into the ABL. Visualisations of mechanisms such as dry convection, the entrainment process, detrainment, coherent entrainment structures, and mixing inside the ABL, are presented. Thermals overshooting at the ABL top are shown to create disturbances in the form of gravity waves in the free atmosphere aloft, as evidenced by a corresponding ripple structure at the bottom of the smoke layer. Lidar data, aerosol ground-based measurements and supporting meteorological data are used to link free atmosphere, mixed-layer and ground-level aerosols. During the peak period of the haze event (July 7, 2002), the correlation between time series of elastic backscatter lidar data within the mixed layer and the scattering coefficient from a nephelometer at ground level was found to be high (R=0.96 for z =324 m, and R=0.89 for z=504 m). Ground-level aerosol concentration was at a maximum about 2 h after the smoke layer intersected with the growing ABL, confirming that the wisps do not initially reach the ground.