ALBERT

Description: [1]  This paper presents a comparative study of shortwave downward radiation (SDR) measurements and simulations, obtained with the radiative transfer model (RTM) LibRadtran, at the Baseline Surface Radiation Network (BSRN) site of Izaña Atmospheric Observatory (IZA, Spain). The analysis is based on cloud-free days between March 2009 and August 2012 (386 days), including aerosol-free and Saharan mostly pure mineral dust conditions and comparing the day-to-day, annual and inter-annual variability.The observed agreement between simulations and measurements is excellent: the variance of daily measurements overall agrees within 99% with the variance of daily simulations and the mean bias (simulations - measurements) is -0.30 ± 0.24 MJm -2 (-1.1 ± 0.9%) for global, -0.16 ± 0.34 MJm -2 (-0.4 ± 0.9%) for direct and +0.02 ± 0.25 MJm -2 (+0.9 ± 9.2%) for diffuse SDR. Furthermore, the diurnally averaged aerosol radiative forcing ( Δ DF) and radiative forcing efficiency ( ΔDF eff ) due to Saharan mostly pure mineral dust events has been computed at Izaña Observatory. The mean Δ DF values are -7 ± 1, -96 ± 5 and 44 ± 2 Wm -2 for global, direct and diffuse BSRN SDR,respectively (mean aerosol optical depth, AOD, at 500 nm of 0.18 ± 0.01), while the mean ΔDF eff values are -59 ± 6, -495 ± 11 and 230 ± 8 Wm -2 per unit of AOD at 500 nm for global, direct and diffuse BSRN SDR, respectively. These values highlight the importance of scattering processes for mineral dust aerosols: the ratio between Δ DF and the corresponding SDR without aerosols is ~ 2.5% for diffuse SDR versus 0.2% for direct SDR. This illustrates the significant potential of mineral dust particles to cool the Earth-atmosphere system.

Description: The extent of the area accommodating convergence between the African and Iberian plates, how this convergence is partitioned between crust and mantle, and the role of the plate boundary in accommodating deformation are not well-understood subjects. We calculate the structure of the lithosphere derived from its density distribution along a profile running from the Tagus Abyssal Plain to the Sahara Platform and crossing the Gorringe Bank, the NW Moroccan margin, and the Atlas Mountains. The model is based on the integration of gravity, geoid, elevation, and heat flow data and on the crustal structure across the NW Moroccan margin derived from reflection and wide-angle seismic data. The resulting mantle density anomalies suggest important variations of the lithosphere-asthenosphere boundary (LAB) topography, indicating prominent lithospheric mantle thickening beneath the margin (LAB 〉 200 km depth) followed by thinning beneath the Atlas Mountains (LAB ∼90 km depth). At crustal levels the Iberia-Africa convergence is sparsely accommodated in a ∼950 km wide area and localized in the Atlas and Gorringe regions, with an inferred shortening of ∼50 km. In contrast, mantle thickening accommodates a 400 km wide region, thus advocating for a decoupled crustal-mantle mechanical response. A combination of mantle underthrusting due to oblique convergence, together with a viscous dripping fed by lateral mantle dragging, can explain the imaged lithospheric structure. The model is consistent with crustal shortening estimates and with the accommodation of part of the Iberia-Africa convergence farther NW of the Gorringe Bank and/or off the strike of the profile.

Description: Aquifer remediation is a challenging problem with environmental, social and economic implications. As a general rule, pumping proceeds until the concentration of the target substance within the pumped water lies below a pre-specified value. In this paper we estimate the a priori potential failure of the endpoint of remediation due to a rebound of concentrations driven by back diffusion. In many cases, it has been observed that once pumping ceases, a rebound in the concentration at the well takes place. For this reason, administrative approaches are rather conservative and pumping is forced to last much longer than initially expected. While a number of physical and chemical processes might account for the presence of rebounding, we focus here on diffusion from low water mobility into high mobility zones. In this work we look specifically at the concentration rebound when pumping is discontinued while accounting for multiple mass transfer processes occurring at different time scales and parametric uncertainty. We aim to a risk-based optimal operation methodology that is capable of estimating the endpoint of remediation based on aquifer parameters characterizing the heterogeneous medium as well as pumping rate and initial size of the polluted area.

Description: Over the last three decades, ozone depletion over Antarctica has affected temperature and winds in the lower stratosphere, and even in the troposphere and at the surface. The second Chemistry Climate Model Validation activity (CCMVal2) concluded that chemistry-climate models simulate stratospheric cooling that is too large compared to observations, even though the modeled and observed ozone trends are similar. However, these comparisons were based only on radiosonde data available for 1969–1998. Here, we investigate trends in the Southern Hemisphere polar cap in the latest version of the Community Earth System Model (CESM1) with its high-top atmospheric component, WACCM4, fully coupled to an ocean model. We compare model trends with observations for different periods and with other modeling studies to show much better agreement with more recent data, and conclude that the discrepancy between observed trends and those calculated by high-top models may not be as large as previously reported.

Description: Anomalous transport in advection-dominated convergent flow tracer tests can occur due to small-scale heterogeneities in aquifer hydraulic properties. These result in fluctuations of the groundwater velocity field and complex connectivity patterns between injection and extraction wells. While detailed characterization of heterogeneity is often not possible in practice, a proper understanding of what fundamental physical mechanisms can give rise to macroscopic behaviors that are measurable is essential for proper upscaling of solute transport processes. We analyze here how heavy-tailed breakthrough curves can arise in radially convergent flow to a well. The permeability fields are three dimensional multigaussian fields with varying statistical geometry and degrees of heterogeneity. We consider transport of conservative tracers from multiple injection locations by varying distance and angle from the extraction well. Anomalous power law tailing in breakthrough curves is attributed to a variety of features including the initial vertical stratification of the solute that arises due to a flux-weighted injection, the injection distance to the well relative to the depth of the aquifer and the statistics of the heterogeneity field as defined by the correlation length and variance of the permeability. When certain conditions co-occur for a given injection, such as strong connectivity contrasts between aquifer layers, injection distances comparable to the horizontal heterogeneity integral scales and large global variances, breakthrough curves tend to scale as a power-law with unit slope at late time. These findings offer new insights to understand what physical processes must be understood to develop and choose appropriate upscaling approaches that might reproduce such anomalous transport in heterogeneous advection-dominated systems.

Description: [1]  Elevated stratopause (ES) events occurring during Northern Hemisphere winter are identified in four climate simulations of the period 1953-2005 made with the Whole Atmosphere Community Climate Model (WACCM). We find 68 ES events in 212 winters. These events are found in winters when the middle atmosphere is disturbed and there are zonal wind reversals in the stratosphere at high latitudes. These disturbances can be associated with both major and minor stratospheric sudden warming events (SSWs). The ES events occur under conditions where the stratospheric jet, the gravity wave forcing, and the residual circulation remain reversed longer than in those winters where a SSW occurs without an ES. We compare ES events with the type of SSW (vortex splitting and vortex displacement) and find that 68% of ES events form after vortex splitting events. We also present a climatology of ES events based on NASA's Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis data from 1979 to 2012 and compare it to the model results. WACCM composites of major SSW and ES also show enhanced Eliassen-Palm flux divergences in the upper mesosphere after the stratospheric warming, immediately before the formation of an ES. However, the formation of an ES in WACCM is, due primarily to adiabatic heating from gravity wave-driven downwelling, which follows the re-establishment of the eastward jet in the upper stratosphere. We find nine winters where an ES forms in the absence of any significant planetary wave activity in the upper mesosphere and illustrate one such event.

Description: The Eocene Hecho Group turbidite system of the Aínsa-Jaca foreland Basin (southcentral Pyrenees) provides an excellent opportunity to constrain compositional variations within the context of spatial and temporal distribution of source rocks during tectonostratigraphic evolution of foreland basins. The complex tectonic setting necessitated the use of petrographic, geochemical and multivariate statistical techniques to achieve this goal. The turbidite deposits comprise four unconformity-bounded tectonostratigraphic units (TSU), consisting of quartz-rich and feldspar-poor sandstones, calclithites rich in extrabasinal carbonates and hybrid arenites dominated by intrabasinal carbonates. The sandstones occur exclusively in TSU-2, whereas calclithites and hybrid arenites occur in the overlying TSU-3, TSU-4 and TSU-5. The calclithites were deposited at the base of each TSU and hybrid arenites in the uppermost parts. Extrabasinal carbonate sources were derived from the fold-and-thrust belt (mainly Cretaceous and Palaeocene limestones). Conversely, intrabasinal carbonate grains were sourced from foramol shelf carbonate factories. This compositional trend is attributed to alternating episodes of uplift and thrust propagation (siliciclastic and extrabasinal carbonates supplies) and subsequent episodes of development of carbonate platforms supplying intrabasinal detrital grains. The quartz-rich and feldspar-poor composition of the sandstones suggests derivation from intensely weathered cratonic basement rocks during the initial fill of the foreland basin. Successive sediments (calclithites and hybrid arenites) were derived from older uplifted basement rocks (feldspar-rich and, to some extent, rock fragments-rich sandstones), thrust-and-fold belt deposits and from coeval carbonate platforms developed at the basin margins. This study demonstrates that the integration of tectono-stratigraphy, petrology and geochemistry of arenites provides a powerful tool to constrain the spatial and temporal variation in provenance during the tectonic evolution of foreland basins. © 2009 The Authors. Journal Compilation © Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists.