Dispersion in the Wake of a Rectangular Building: Validation of Two Reynolds-Averaged Navier–Stokes Modelling Approaches

When modelling the turbulent dispersion of a passive tracer using Reynolds-averaged Navier–Stokes (RANS) simulations, two different approaches can be used. The first consists of solving a transport equation for a scalar, where the governing parameters are the mean velocity field and the turbulent diffusion coefficient, given by the ratio of the turbulent viscosity and the turbulent Schmidt number Sc _t . The second approach uses a Lagrangian particle tracking algorithm, where the governing parameters are the mean velocity and the fluctuating velocity field, which is determined from the turbulence kinetic energy and the Lagrangian time T _L . A comparison between the two approaches and wind-tunnel data for the dispersion in the wake of a rectangular building immersed in a neutral atmospheric boundary layer (ABL) is presented. Particular attention was paid to the influence of turbulence model parameters on the flow and concentration field. In addition, an approach to estimate Sc _t and T _L based on the calculated flow field is proposed. The results show that applying modified turbulence model constants to enable correct modelling of the ABL improves the prediction for the velocity and concentration fields when the modification is restricted to the region for which it was derived. The difference between simulated and measured concentrations is smaller than 25% or the uncertainty of the data on 76% of the points when solving the transport equation for a scalar with the proposed formulation for Sc _t , and on 69% of the points when using the Lagrangian particle tracking with the proposed formulation for T _L .     

Safe climate policy is affordable—12 reasons

There is a widespread sense that a sufficiently stringent climate mitigation policy, that is, a considerable reduction of greenhouse gas emissions to avoid extreme climate change, will come with very high economic costs for society. This is supported by many cost–benefit analyses (CBA) and policy cost assessments of climate policy. All of these, nevertheless, are based on debatable assumptions. This paper will argue instead that safe climate policy is not excessively expensive and is indeed cheaper than suggested by most current studies. To this end, climate CBA and policy cost assessments are critically evaluated, and as a replacement twelve complementary perspectives on the cost of climate policy are offered.     

Spatial variability and temporal stability of throughfall water under a dominant beech (Fagus sylvatica L.) tree in relationship to canopy cover

Although the spatial heterogeneity of throughfall water (TF) under forest canopies has been related to vegetation structure in several forest types, few reports have been made of the driving factors of small-scale TF variability in deciduous stands. Therefore, the spatial variability of the amount of TF water under one dominant beech (Fagus sylvatica L.) tree was quantified in high temporal and spatial resolution over a 2-year period to examine the temporal stability of spatial TF variability and to relate spatial TF patterns to canopy cover determined photographically above each TF collector (n = 48). The spatial variability of TF was significantly higher during the leafed periods (coefficient of variation (CV) = 18%) than during the leafless periods (CV = 8%), and a strong negative relationship was observed between the CV of event TF and the TF fraction of rainfall in the open field. Geostatistical analysis showed that the cumulative TF water during the leafed periods was spatially correlated up to a distance of 3–4 m. There was a significant temporal stability of spatial TF patterns in the growing periods and in the dormant periods, but patterns differed largely between the two periods of the year. TF water during the growing periods significantly decreased with increasing canopy cover above the sampling locations (r = −0.54, p = 0.014, n = 20), but was more closely correlated with branch cover (r = −0.77, p < 0.001). However, the spatial pattern of TF during defoliated conditions was not related to the measured variation in branch cover.     

GIA-induced secular variations in the Earth's long wavelength gravity field: Influence of 3-D viscosity variations

Predictions of present day secular variations in the Earth's long wavelength geopotential driven by glacial isostatic adjustment (GIA) have previously been analyzed to infer the radial profile of mantle viscosity and to constrain ongoing cryospheric mass balance. These predictions have been based on spherically symmetric Earth models. We explore the impact of lateral variations in mantle viscosity using a new finite-volume formulation for computing the response of 3-D Maxwell viscoelastic Earth models. The geometry of the viscosity field is constrained from seismic-to-mographic images of mantle structure, while the amplitude of the lateral viscosity variations is tuned by a free parameter in the modeling. We focus on the zonal J˙_? harmonics for degrees ? = 2,…,8 and demonstrate that large-scale lateral viscosity variations of two to three orders of magnitude have a modest, 5-10%, impact on predictions of J˙₂. In contrast, predictions of higher degree harmonics show a much greater sensitivity to lateral variation in viscosity structure. We conclude that future analyses of secular trends (for degree ? > 2) estimated from ongoing (GRACE, CHAMP) satellite missions must incorporate GIA predictions based on 3-D viscoelastic Earth models.     

Vertically integrated moisture flux convergence as a predictor of thunderstorms

Vertically Integrated Moisture Flux Convergence (VIMFC) alone and in combination with the lifted stability index of the most unstable layer (SMUL) is evaluated as a thunderstorm predictor. By using six-hourly standard pressure weather analysis data from the European Centre for Medium-range Weather Forecasts (ECMWF) during 30 days in the summers of 1992 and 1994 containing several severe weather events along with quiescent events in northwestern Europe 17,206 events are obtained. The location and time of a lightning discharge are obtained from the Arrival Time Difference (ATD) sferics lightning location system from the UK Meteorological Office. Using the Heidke Skill Score (HEIDKE) to determine the best threshold we conclude that VIMFC alone, does not perform well as a dichotomous thunderstorm predictor compared to the stability index. However, the Thundery Case Probability (TCP) tested as function of VIMFC results in a high correlation with thunderstorms. By combining SMUL and VIMFC the surplus value as a thunderstorm predictor of VIMFC was established. TCP percentages up to 95% were found in an unstable environment with high positive values of VIMFC. In a marginally unstable environment with a high positive VIMFC the thunderstorm probability is higher than in a very unstable environment with no or negative VIMFC. These results are illustrated with a study of the case of the disastrous flash flood at Vaison-La-Romaine (southeastern France) on September 22, 1992. Although latent instability was present in a large area surrounding Vaison-La-Romaine, nearly all and especially the most severe thunderstorm activity occurred within the smaller area with positive VIMFC and latent instability.     

Suspension-feeding bivalves and the fate of primary production: An estuarine model applied to Chesapeake Bay

A probabilistic mathematical model of bivalve suspension-feeding in estuaries is based on bivalve abundance, filtering capacities, and water mixing parameters. We applied the model to five regions of the upper Chesapeake Bay, ranging from shallow tidal fresh habitats to deep mesohaline habitats, for the years 1985 to 1987. Model results indicated that existing suspension-feeding bivalves could consume more than 50% of annual primary production in shallow freshwater and oligohaline reaches of the upper Chesapeake Bay and Potomac River. In deep mesohaline portions of the Chesapeake Bay and Potomac River, suspension-feeding bivalves could consume only 10% of primary production. Independent estimates of benthic carbon demand based on benthic production supported the model predictions. Hydrodynamics of large estuaries restrict the potential of benthic suspension-feeders to crop phytoplankton production because the width and depth of these estuaries limit transport of pelagic waters to the littoral flanks of the estuaries where benthic suspension-feeders can be abundant. Benthic suspension-feeders are dominant consumers in shallow segments of the Chesapeake Bay system, but are suppressed in deeper segments. The suppression is below that set by hydrodynamic limits, and may be due to periodic hypoxia or other factors. Our results suggest that the proposed use of suspension-feeding bivalves to improve water quality of large estuaries will be limited by the depth and width of the estuary, unless the bivalves are suspended in the water column by artificial means.