Radiation balance at the surface in the city of S?o Paulo, Brazil: diurnal and seasonal variations

The main goal of this work is to describe the diurnal and seasonal variations of the radiation balance components at the surface in the city of S?o Paulo based on observations carried out during 2004. Monthly average hourly values indicate that the amplitudes of the diurnal cycles of net radiation (Q*), downwelling and upwelling shortwave radiation (SW_DW, SW_UP), and longwave radiations (LW_DW, LW_UP) in February were, respectively, 37%, 14%, 19%, 11%, and 5% larger than they were in August. The monthly average daily values indicate a variation of 60% for Q*, with a minimum in June and a maximum in December; 45% for SW_DW, with a minimum in May and a maximum in September; 50% for SW_UP, with a minimum in June and a maximum in September; 13% for LW_DW, with a minimum in July and a maximum in January; and 9% for LW_UP, with a minimum in July and a maximum in February. It was verified that the atmospheric broadband transmissivity varied from 0.36 to 0.57; the effective albedo of the surface varied from 0.08 to 0.10; and the atmospheric effective emissivity varied from 0.79 to 0.92. The surface effective emissivity remained approximately constant and equal to 0.96. The albedo and surface effective emissivity for S?o Paulo agreed with those reported for urban areas in Europe and North America cities. This indicates that material and geometric effects on albedo and surface emissivity in S?o Paulo are similar to ones observed in typical middle latitudes cities. On the other hand, it was found that S?o Paulo city induces an urban heat island with daytime maximum intensity varying from 2.6°C in July (16:00 LT) to 5.5°C in September (15:00 LT). The analysis of the radiometric properties carried out here indicate that this daytime maximum is a primary response to the seasonal variation of daily values of net solar radiation at the surface.     

Correction to: Observational Investigation of the Statistical Properties of Surface-Layer Turbulence in a Suburban Area of São Paulo,Brazil: Objective Analysis of Scaling-Parameter Accuracy and Uncertainties

Boundary-Layer Meteorology -     

A Variable Mesh Spacing for Large-Eddy Simulation Models in the Convective Boundary Layer

A variable vertical mesh spacing for large-eddy simulation (LES) models in a convective boundary layer (CBL) is proposed. The argument is based on the fact that in the vertical direction the turbulence near the surface in a CBL is inhomogeneous and therefore the subfilter-scale effects depend on the relative location between the spectral peak of the vertical velocity and the filter cut-off wavelength. From the physical point of view, this lack of homogeneity makes the vertical mesh spacing the principal length scale and, as a consequence, the LES filter cut-off wavenumber is expressed in terms of this characteristic length scale. Assuming that the inertial subrange initial frequency is equal to the LES filter cut-off frequency and employing fitting expressions that describe the observed convective turbulent energy one-dimensional spectra, it is feasible to derive a relation to calculate the variable vertical mesh spacing. The incorporation of this variable vertical grid within a LES model shows that both the mean quantities (and their gradients) and the turbulent statistics quantities are well described near to the ground level, where the LES predictions are known to be a challenging task.     

A Geographic Information System (GIS)-based approach for relating anthropogenic impact and environmental stress: a case study within the African Albertine Rift

This study introduces a potentially cost-effective methodology to assess habitat quality remotely using anthropogenic variables derived from both Geographic Information System (GIS) and remote sensing. We related anthropogenic impact to organism-level response across the Albertine Rift of East-Africa, which was measured using body condition (BC) in Lophuromys aquilus, the dark-coloured brush-furred rat. We chose seven variables to measure anthropogenic impact, ranging from land cover/land use to nighttime light radiance. Using a principal component analysis, we extracted four components that describe human impact and correlated each with BC. We documented that BC tended to be higher in more disturbed areas. We recommend this GIS-based methodology to relate anthropogenic impact to organismal-level response in the Albertine Rift region, and we provide broad guidelines for its application to assess habitat quality for species of conservation concern.     

Lagrange multiplier-based domain decomposition methods for a non-linear sedimentary basin problem

We present two Lagrange multiplier-based domain decomposition methods for solving iteratively a non-linear sedimentary basin problem. Both methods are Uzawa-type algorithms derived from saddle point equations of Lagrangian and augmented Lagrangian functionals. Numerical experiments show that both methods are scalable with respect to the mesh size.     

Submarine landslides caused by seamounts entering accretionary wedge systems

Seamounts entering active subduction zone trenches initially collide with the frontal sedimentary accretionary wedges resulting in severe deformation of the overriding plate. A typical feature of this deformation is the occurrence of submarine landslides due to gravitational instabilities. Such landslides have been reported from the Middle America and Hikurangi trenches and potentially generate tsunami waves. Yet, the dynamics of accretionary wedges during seamount indentation, and landsliding as a mechanical response in particular, have not been investigated qualitatively. Here, I use 3D high‐resolution numerical experiments to model the collision of conical and flat‐topped seamounts into accreting sedimentary sequences. Results show that the topographical evolution of an accretionary wedge mainly depends on the volume of the entering seamount and not on its height. Submarine landslides occur only if seamounts are not completely buried by the sedimentary sequence, and the volume of the avalanche is roughly correlated with the seamount volume overtopping the incoming sediments.     

Allogenic and autogenic processes combined in the formation of shallow-water carbonate sequences (Middle Berriasian,Swiss and French Jura Mountains)

Sediment production and accumulation on shallow carbonate platforms are controlled by allogenic, externally controlled processes (such as sea level, climate, and/or platform-wide subsidence patterns) as well as by autogenic factors that are inherent to the sedimentary system (such as lateral migration of sediment bodies). The challenge is to determine how and in which proportion these processes interacted to create the observed sedimentary record. Here, a case study of Middle Berriasian, shallow-marine carbonates of the Swiss and French Jura Mountains is presented. Based on vertical facies evolution and bedding surfaces, different orders of depositional sequences (elementary, small-scale, medium-scale) have been identified in the studied sections. The hierarchical stacking pattern of these sequences and the time span represented by the investigated interval imply that eustatic sea-level fluctuations in the Milankovitch frequency band were an important controlling factor. The small-scale and medium-scale sequences relate to the 100 and 400-kyr orbital eccentricity cycles, respectively. The elementary sequences are attributed to the 20-kyr precession cycle. Differential subsidence additionally produced accommodation changes. The present study focuses on one specific small-scale sequence situated at the base of the transgressive systems tract of large-scale sequence Be4, which is identified also in other European basins. This small-scale sequence has been logged in detail at eight different outcrops in the Jura Mountains. Detailed facies analysis reveals that different depositional environments (tidal flats, internal lagoons, open lagoons, carbonate sand shoals) were juxtaposed and evolved through time, often shifting position on the platform. The boundaries of the small-scale (100-kyr) sequence can be followed over the entire study area and thus must have formed through predominantly allogenic processes (eustatic sea-level fall, the effect of which was locally modified by differential subsidence). In two sections, five well-developed elementary sequences constitute the small-scale sequence. In the other sections, the identification of elementary sequences often is difficult because sedimentation was dominated by autogenic processes that overruled the influence of sea-level fluctuations. In low-energy, tidal-flat and internal-lagoonal settings, orbitally induced sea-level changes were recorded more faithfully, while high-energy shoals were mainly submitted to autogenic processes and the allogenic signal is masked. Consequently, the studied Jura platform experienced a combination of auto- and allogenic processes, which created a complex facies mosaic and a complex stacking of depositional sequences. Nevertheless, the 100-kyr orbital signal was strong enough to create correlatable sequence boundaries. Within a 100-kyr sequence, however, the unambiguous definition of sequences related to the 20-kyr orbital cycle is often difficult and the prediction of their lateral or vertical facies evolution impossible.     

Environmental geochemistry of an active Hg mine in Xunyang,Shaanxi Province,China

The Xunyang Hg mine (XMM) situated in Shaanxi Province is an active Hg mine in China. Gaseous elemental Hg (GEM) concentrations in ambient air were determined to evaluate its distribution pattern as a consequence of the active mining and retorting in the region. Total Hg (HgT) and methylmercury (MeHg) concentrations in riparian soil, sediment and rice grain samples (polished) as well as Hg speciation in surface water samples were measured to show local dispersion of Hg contamination. As expected, elevated concentrations of GEM were found, ranging from 7.4 to 410 ng m⁻³. High concentrations of HgT and MeHg were also obtained in riparian soils, ranged from 5.4 to 120 mg kg⁻¹ and 1.2 to 11 μg kg⁻¹, respectively. Concentrations of HgT and MeHg in sediment samples varied widely from 0.048 to 1600 mg kg⁻¹ and 1.0 to 39 μg kg⁻¹, respectively. Surface water samples showed elevated HgT concentrations, ranging from 6.2 to 23,500 ng L⁻¹, but low MeHg concentrations, ranging from 0.022 to 3.7 ng L⁻¹. Rice samples exhibited high concentrations of 50–200 μg kg⁻¹ in HgT and of 8.2–80 μg kg⁻¹ in MeHg. The spatial distribution patterns of Hg speciation in the local environmental compartments suggest that the XMM is the source of Hg contaminations in the study area.     

Effect of fluid pressure distribution on the structural evolution of accretionary wedges

Numerical experiments on evolving accretionary wedges usually implement predefined weak basal décollements and constant strength parameters for overlying compressed sequences, although fluid pressure ratio, and therefore brittle strength, can vary strongly in sedimentary basins. A two‐dimensional finite difference model with a visco‐elasto‐plastic rheology is used to investigate the influence of different simplified fluid pressure ratio distributions on the structural evolution of accretionary wedge systems. Results show that a linear increase in fluid pressure ratio towards the base leads to toeward‐verging thrust sheets and underplating of strata, while simulations with a predefined décollement form conjugate shear zones supporting box‐fold‐type frontal accretion. Surface tapers are in agreement with the critical wedge theory, which here is modified for cases of varying fluid pressure ratio. Furthermore, the numerical results resemble findings from natural examples of accretionary wedges.