Spatio-Temporal Surface Shear-Stress Variability in Live Plant Canopies and Cube Arrays

This study presents spatiotemporally-resolved measurements of surface shear-stress τ _s in live plant canopies and rigid wooden cube arrays to identify the sheltering capability against sediment erosion of these different roughness elements. Live plants have highly irregular structures that can be extremely flexible and porous resulting in considerable changes to the drag and flow regimes relative to rigid imitations mainly used in other wind-tunnel studies. Mean velocity and kinematic Reynolds stress profiles show that well-developed natural boundary layers were generated above the 8 m long wind-tunnel test section covered with the roughness elements at four different roughness densities (λ = 0, 0.017, 0.08, 0.18). Speed-up around the cubes caused higher peak surface shear stress than in experiments with plants at all roughness densities, demonstrating the more effective sheltering ability of the plants. The sheltered areas in the lee of the plants are significantly narrower with higher surface shear stress than those found in the lee of the cubes, and are dependent on the wind speed due to the plants ability to streamline with the flow. This streamlining behaviour results in a decreasing sheltering effect at increasing wind speeds and in lower net turbulence production than in experiments with cubes. Turbulence intensity distributions suggest a suppression of horseshoe vortices in the plant case. Comparison of the surface shear-stress measurements with sediment erosion patterns shows that the fraction of time a threshold skin friction velocity is exceeded can be used to assess erosion of, and deposition on, that surface.     

Long-lived Coronal Loop Profiles from TRACE

An initial study of long-lived loops observed with TRACE (Lenz et al., 1999) shows that they have no significant temperature stratification and that they are denser than the classic loop model predicts. Models that agree better with the observations include a loop consisting of a bundle of filaments at different temperatures and a loop with momentum input by MHD waves. Some implications for coronal heating models and mechanisms are discussed.     

Atmospheric drag coefficients over sea ice — validation of a parameterisation concept

The concept of drag partitioning to parameterise the surface roughness of sea ice is validated using topography data of regions with high sea ice concentrations. The parameterised drag is compared to measurements obtained by aircraft and ship. The form drag can well be expressed as a function of mean ridge heights and spacings averaged over flight legs of 12 km, if an improved approximation for the coefficient of resistance of a single ridge is used. We find a good agreement between the parameterised and observed drag coefficients. The highest sea ice roughness was encountered close to coastal regions and the lowest in the central Arctic.     

Plate boundary deformation and continuing deflation of the Askja volcano,North Iceland,determined with GPS, 1987–1993

GPS geodetic measurements were conducted around the Askja central volcano located at the divergent plate boundary in north Iceland in 1987, 1990, 1992 and 1993. The accuracy of the 1987 and 1990 measurements is in the range of 10 mm for horizontal components; the accuracy of the 1992 and 1993 measurements is about 4 mm in the horizontal plane. Regional deformation in the Askja region is dominated by extension. Points located outside a 30–45 km wide plate boundary deformation zone indicate a displacement of 2.4±0.5 cm/a in the direction N 99°E±12° of the Eurasian plate relative to the North American plate in the period 1987–1990. Within the plate boundary deformation zone extensional strain accumulates at a rate of 0.8 strain/a. Displacement of control points next to Askja (>7 km from the caldera center) in the periods 1990–1993 and 1992–1993 show deflation and contraction towards the caldera. These results are in accordance with the results obtained by other geodetic methods in the area, which indicate that the deflation at Askja occurs in response to a pressure decrease at about 2.8 km depth, located close to the center of the main Askja caldera. A Mogi point source was fixed at this location and the GPS data used to solve for the source strength. A central subsidence of 11±2.5 cm in the period 1990–1993 is indicated, and 5.5±1.5 cm in the period 1992–1993. The maximum tensional strain rate, according to the point source model, occurs at a horizontal distance of 2.5–6 km from the source, at the same location as the main caldera boundary. Discrepancies between the observed displacements and predicted displacements from the Mogi model near the Askja caldera can be attributed to the regional eastwest extension that occurs at Askja.     

OpenGeoSys-ChemApp: a coupled simulator for reactive transport in multiphase systems and application to CO2 storage formation in Northern Germany

Sequestration of CO₂ into a deep geological reservoir causes a complex interaction of different processes such as multiphase flow, phase transition, multicomponent transport, and geochemical reactions between dissolved CO₂ and the mineral matrix of the porous medium. A prognosis of the reservoir behaviour and the feedback from large-scale geochemical alterations require efficient process-based numerical models. For this purpose, the multiphase flow and multicomponent transport code OpenGeoSys-Eclipse have been coupled to the geochemical model ChemApp. The newly developed coupled simulator was successfully verified for correctness and accuracy of the implemented reaction module by benchmarking tests. The code was then applied to assess the impact of geochemical reactions during CO₂ sequestration at a hypothetical but typical Bunter sandstone formation in the Northern German Basin. Injection and spreading of 1.48 × 10⁷ t of CO₂ in an anticline structure of the reservoir were simulated over a period of 20 years of injection plus 80 years of post-injection time. Equilibrium geochemical calculations performed by ChemApp show only a low reactivity to the geochemical system. The increased acidity of the aqueous solution results in dissolution of small amounts of calcite, anhydrite, and quartz. Geochemical alterations of the mineral phase composition result in slight increases in porosity and permeability, which locally may reach up to +0.02 and 0.1 %, respectively.     

Determination of first-order degradation rate constants from monitoring networks

In this article, different strategies for estimating first-order degradation rate constants from measured field data are compared by application to multiple, synthetic, contaminant plumes. The plumes were generated by numerical simulation of contaminant transport and degradation in virtual heterogeneous aquifers. These sites were then individually and independently investigated on the computer by installation of extensive networks of observation wells. From the data measured at the wells, that is, contaminant concentrations, hydraulic conductivities, and heads, first-order degradation rates were estimated by three 1D centerline methods, which use only measurements located on the plume axis, and a two-dimensional method, which uses all concentration measurements available downgradient from the contaminant source. Results for both strategies show that the true rate constant used for the numerical simulation of the plumes in general tends to be overestimated. Overestimation is stronger for narrow plumes from small source zones, with an average overestimation factor of about 5 and single values ranging from 0.5 to 20, decreasing for wider plumes, with an average overestimation factor of about 2 and similar spread. Reasons for this overestimation are identified in the velocity calculation, the dispersivity parameterization, and off-centerline measurements. For narrow plumes, the one- and the two-dimensional strategies show approximately the same amount of overestimation. For wider plumes, however, incorporation of all measurements in the two-dimensional approach reduces the estimation error. No significant relation between the number of observation wells in the monitoring network and the quality of the estimated rate constant is found for the two-dimensional approach.     

Determination of the pressure coefficients for neutron monitor and multiplicity analyzer measurements

To determine the air pressure dependence of the total rates of the neutron monitor NM-64 and of the observed multiplicities 1, 2, 4 and 6 of the neutron multiplicity analyzer the one-station method with iterative least squares described by Martinelle (1968) is used. This method is extended with a criterion for optimizing the length of the filter taken from Jenkins and Watts (1969). To show the effect of this method also classical regression estimates are given. Pressure coefficients are obtained for overlapping periods of three months for the greater part of the years 1970–1974.     

Localized climate change scenarios of mean temperature and precipitation over Switzerland

There is a growing need of the climate change impact modeling and adaptation community to have more localized climate change scenario information available over complex topography such as in Switzerland. A gridded dataset of expected future climate change signals for seasonal averages of daily mean temperature and precipitation in Switzerland is presented. The basic scenarios are taken from the CH2011 initiative. In CH2011, a Bayesian framework was applied to obtain probabilistic scenarios for three regions within Switzerland. Here, the results for two additional Alpine sub-regions are presented. The regional estimates have then been downscaled onto a regular latitude-longitude grid with a resolution of 0.02° or roughly 2 km. The downscaling procedure is based on the spatial structure of the climate change signals as simulated by the underlying regional climate models and relies on a Kriging with external drift using height as auxiliary predictor. The considered emission scenarios are A1B, A2 and the mitigation scenario RCP3PD. The new dataset shows an expected warming of about 1 to 6 °C until the end of the 21st century, strongly depending on the scenario and the lead time. Owing to a large vertical gradient, the warming is about 1 °C stronger in the Alps than in the Swiss lowlands. In case of precipitation, the projection uncertainty is large and in most seasons precipitation can increase or decrease. In summer a distinct decrease of precipitation can be found, again strongly depending on the emission scenario.     

Key climate indices in Switzerland; expected changes in a future climate

Climate indices facilitate the interpretation of expected climate change impacts for many sectors in society, economy, and ecology. The new localized data set of climatic change signals for temperature and precipitation presented by Zubler et al. (Clim Change, 2013) is applied for an analysis of frequently used climate indices in Switzerland. The indices considered are: number of summer days and tropical nights, growing season length, number of frost days and ice days, heating and cooling degree days, and the number of days with fresh snow. For the future periods 2020-49, 2045-74 and 2070–2099 the indices are computed using a delta-change approach based on the reference period 1980–2009 for the emission scenarios A1B, A2, and RCP3PD. The scenario data suggest the following relevant findings: (1) a doubling of the number of summer days by the end of the century under the scenarios A1B and A2, (2) an appearance of tropical nights even above 1500 m asl, (3) a possible reduction of the number of frost days by more than 3 months at altitudes higher than 2500 m asl, (4) a decline of heating degree days by about 30 % until the end of the century, and (5) the near disappearance of days with fresh snow at low altitudes. It is also shown that the end-of-the-century projections of all indices strongly depend on the chosen emission scenario.