Observations Of The Morning Transition Of The Convective Boundary Layer

The morning transition between the stable nocturnal situation and the daytime convective boundary layer (CBL) is of interest both for basic understanding and for initializing prognostic models. While the morning growth phase of the CBL has been studied in detail, relatively little has been published on the transition itself. In this paper, conventional observations of surface temperature, humidity, and turbulent fluxes,and data from a meteorological tower, are combined with measurements of the onset of convection by boundary-layer wind profilers to explore the timing and behaviour of the transition period. The transition is defined here as the period between sunrise and the time at which the depth ofconvection reaches about 200 m AGL. Diagnostic relationships based on surface heat flux, the temperature difference between 2 m and 200 m, and bulk Richardson number are explored. The transition is foundto be enabled by surface heating relaxing the surface stability, while the warming of the layerbetween 2 m and 200 m is in large part due to shear-driven entrainment.     

Post-colonial nature conservation in Southern Africa: same emperors, new clothes?

This paper analyses and evaluates the bordering and othering impacts of environmental geopolitical discourse on land conservation in Southern Africa. Through a theoretical in-depth analysis of the use and contents of the term conservation, this paper examines how conservation is determined, instrumentalized and interpreted by the state, international governmental and non-governmental institutions, and specific interest groups including neo-liberal capitalists and local communities especially in the developing world context. In particular, we discuss the impact of current transboundary park-like conservation practices in Southern Africa and how these feed into the continuous attempts to colonise Southern Africa's nature. Native Americans were not mistaken when they accused the Whites of having forked tongues. By separating the relations of political power from the relations of scientific reasoning while continuing to shore up power with reason and reason with power, the moderns have always had two irons in the fire. They have become invincible. (Latour, 1993, p. 38) Since most of us live in a hierarchical society, any discourse on wildlife tends to be about social relationships. Whom can we exclude from our Garden of Eden, and how can we keep `others' from trespassing on valuables that help sustain our life and livelihoods, if not our identities. Marks (1994, p. 120) ...it could be argued that binary divisions are deeply etched into social space and it is a deeper understanding of boundary erection and distancing that is required if we are to provide alternatives to exclusion and conflict (Sibley, 2001, p. 240).     

Insufficiency of compaction disequilibrium as the sole cause of high pore fluid pressures in pre-Cenozoic sediments

The method of indirect demonstration is used to investigate if compaction disequilibrium can account for high overpressures that occur in Mesozoic and older basin formations. First the equations governing compaction disequilibrium are analysed for the factors controlling overpressure levels. Then limiting values of these control parameters are sought which favour high fluid pressures. The analysis shows why 'close-to-lithostatic fluid pressures' in pre-Cenozoic basin units are difficult to attain by compaction disequilibrium alone. Subsequently, the limiting favourable conditions are used in a series of generic numerical model experiments. The experiments serve as templates to construct the upper bounds of overpressures due to sediment loading for most geological settings including those where shale seals have developed. Two regional examples are studied in some detail. It is shown that observed overpressures in Mesozoic strata on the Scotian Shelf can be explained by compaction disequilibrium, but require the limiting values assigned to the properties of shale. For the Central North Sea Graben these limiting conditions are not sufficient, providing evidence for an active role of other pressure-generating mechanisms.
     

Subsidenz und tektonik des Saar-Nahe-Beckens (SW-Deutschland)

The Saar-Nahe-Basin in SW-Germany is one of the largest Permo-Carboniferous basins in the internal zone of the Variscides. Its evolution is closely related to movements along the Hunsrück Boundary Fault, which separates the Rhenohercynian and the Saxothuringian zones. Recent deep seismic surveys indicate that the Saar-Nahe-Basin formed in the hanging wall of a major detachment which soles out at lower crustal levels at about 16 km depth. Oblique extension along an inverted Variscan thrust resulted in the formation of a half-graben, within more than 8 km of entirely continental strata accumulated. The structural style within the basin is characterized by normal faults parallel to the basin axis and orthogonal transfer fault zones. Balanced cross-section construction and subsidence analysis indicate extension of the orogenically thickened lithosphere by 35%. Subsidence modeling shows discontinuous depth-dependent extension with laterally varying extension factors for crust and mantle lithosphere. Thus, the offset between maximum rift and thermal subsidence can be explained by a zone of mantle extension shifted laterally with respect to the zone of maximum crustal extension.

     

Stress and strain during fault-controlled lithospheric extension—insights from numerical experiments

Two-dimensional finite element techniques are used to study the temporal evolution and spatial distribution of stress and strain during lithospheric extension. The thermomechanical model includes a pre-existing fault in the upper crust to account for the reactivation of older tectonic elements. The fault is described using contact elements which allow for independent meshing of hanging wall and foot wall as well as simulation of large differential displacements between the fault blocks. Numerical models are run for three different initial temperature distributions representing extension of weak, moderately strong and strong lithosphere and three different extension velocities. In spite of the simple geodynamic boundary conditions selected, i.e., wholesale extension at a constant rate, stress and strain vary substantially throughout the lithosphere. In particular, in case of the weak lithosphere model, lower crustal flow towards the locus of maximum upper crustal extension results in the formation of a lower crustal dome while maintaining a subhorizontal Moho relief. The core of the dome experiences hardly any internal deformation, although it is the part of the lower crust which is exhumed the most. Stress fields in the lower crustal dome vary significantly from the regional trend underlining mechanical decoupling of the lower crust from the rest of the lithosphere. These differences diminish if cooler temperatures and, hence, stronger rheologies are considered. Lithospheric strength also exerts a profound control on the basin architecture and the surface expressions of extension, i.e., rift flank uplift and basin subsidence. If the lower crust is sufficiently weak, its flow towards the region of extended upper crust can provide a threshold value for the maximum subsidence which can be achieved during the syn-rift stage. In spite of continuous regional extension, corresponding burial history plots show exponentially decreasing subsidence rates which would traditionally be interpreted in terms of lithospheric cooling during the post-rift stage. The models provide templates to genetically link the surface and sub-surface expressions of lithospheric extension, for which usually no contemporaneous observations are possible. In particular, they help to decipher the information on the physical state of the lithosphere at the time of extension which is stored in the architecture and subsidence record of sedimentary basins.     

Determination of porosity and saturation using seismic waveform inversion

Characterization of a reservoir model requires determination of its petrophysical parameters, such as porosity and saturation. We propose a new method to determine these parameters directly from seismic data. The method consists of the computation and inversion of seismic waveforms. A high frequency method is presented to model wave propagation through an attenuative and dispersive poroelastic medium. The high frequency approximation makes it possible to efficiently compute sensitivity functions. This enables the inversion of seismic waveforms for porosity and saturation. The waveform inversion algorithm is applied to two laboratory crosswell datasets of a water saturated sand. The starting models were obtained using travel time tomography. The first dataset is inverted for porosity. The misfit reduction for this dataset is approximately 50%. The second dataset was obtained after injection of a nonaqueous-phase liquid (NAPL), possibly with some air, which made the medium more heterogeneous. This dataset was inverted for NAPL and air saturation using the porosity model obtained from the first inversion. The misfit reduction of the second experiment was 70%. Regions of high NAPL and high air saturation were found at the same location. These areas correlate well with the position of one of the injection points as well as regions of higher NAPL concentrations found after excavation of the sand. It is therefore possible to directly invert waveforms for pore fluid saturation by taking into account the attenuation and dispersion caused by the poroelasticity.     

A Doubly Weighted Approach to Urban Data Disaggregation in GIS: A Case Study of Wuhan, China

Urban models are evolving to require more and more detailed data that in many cases have to be spatially disaggregated from larger zones. This paper deals with the disaggregation of statistical data in an urban context in which land use data are available at a less detailed level. With the availability of land use data, the traditional approach of areal weighting is improved with an areal and land use weighted approach. This weighted approach is further elaborated to include homogeneous weight zones (HWZ) so as to reflect general geographical variations among the same land use type. A case study in Wuhan, China has demonstrated the effectiveness of the doubly weighted approach within the specified context.     

Recruitment variability in dab (Limanda limanda) in the southeastern North Sea

Data from three annual surveys, covering inshore and offshore waters of the southeastern North Sea, were analysed to study recruitment variability in dab (Limanda limanda) over the period 1978–1997. Geometric mean abundance of 0- to 5-group dab was estimated using general linear models. Juvenile dab (0- and 1-group) were found over the entire area, from inside the estuaries to 50 m depth offshore. Environmental conditions (water temperature, wind stress, turbidity) affected the catch rates. The potential errors in the estimates of year-class strength, caused by differences in catchability, are discussed. The inter-annual pattern of year-class strength appeared to be established between ages 1 and 2, suggesting that factors determining recruitment are not restricted to the pelagic early life phase only, but also operate during the demersal juvenile phase. Recruitment variability at age 2 was in the order of 50–60% and appears to be equal to, or lower than, recruitment variability in plaice and sole. These results contradict expectations based on the concentration hypothesis, which states that the degree of variation in recruitment is inversely related to the degree of concentration during early life phases.