Comparing MODFLOW simulation options for predicting intra‐meander flux

During the evolution of meander bends, the intra‐meander groundwater head gradients steepen and generate zones of accelerated water and nutrient intra‐meander fluxes important for ecosystem processes. This paper compares and contrasts three MODFLOW groundwater model packages based on their simulation of intra‐meander flux for two stages of meander evolution observed in a sandbox river table and one level of river bed clogging, where the hydraulic conductivity in the river bed is lower than in the adjacent aquifer. These packages are the Time‐Variant Specified Head package [constant head (CHD)], River package (RIV), and Streamflow‐Routing package (SFR2), each controlling the groundwater or river head bounding the intra‐meander region. The RIV and SFR2 packages fix river stage and allow for variation in groundwater head below the river, which is suggested for simulating intra‐meander flux for all sinuosities with and without river bed clogging whenever river bed parameters are available. The CHD package fixes below river groundwater head and fails to simulate intra‐meander head loss and flux in meanders with high sinuosity or river bed clogging. In low sinuosity meanders and in cases without river bed clogging, there were no significant differences between MODFLOW packages for simulating river intra‐meander head loss and flux. This research demonstrates why MODFLOW users need to consider the limitations of each package when simulating intra‐meander flux in reaches with river bed clogging, high sinuosity, or similarly steep hydraulic gradients. Copyright © 2014 John Wiley & Sons, Ltd.     

The simulation of European heat waves from an ensemble of regional climate models within the EURO-CORDEX project

The ability of a large ensemble of regional climate models to accurately simulate heat waves at the regional scale of Europe was evaluated. Within the EURO-CORDEX project, several state-of-the art models, including non-hydrostatic meso-scale models, were run for an extended time period (20 years) at high resolution (12 km), over a large domain allowing for the first time the simultaneous representation of atmospheric phenomena over a large range of spatial scales. Eight models were run in this configuration, and thirteen models were run at a classical resolution of 50 km. The models were driven with the same boundary conditions, the ERA-Interim re-analysis, and except for one simulation, no observations were assimilated in the inner domain. Results, which are compared with daily temperature and precipitation observations (ECA&D and E-OBS data sets) show that, even forced by the same re-analysis, the ensemble exhibits a large spread. A preliminary analysis of the sources of spread, using in particular simulations of the same model with different parameterizations, shows that the simulation of hot temperature is primarily sensitive to the convection and the microphysics schemes, which affect incoming energy and the Bowen ratio. Further, most models exhibit an overestimation of summertime temperature extremes in Mediterranean regions and an underestimation over Scandinavia. Even after bias removal, the simulated heat wave events were found to be too persistent, but a higher resolution reduced this deficiency. The amplitude of events as well as the variability beyond the 90th percentile threshold were found to be too strong in almost all simulations and increasing resolution did not generally improve this deficiency. Resolution increase was also shown to induce large-scale 90th percentile warming or cooling for some models, with beneficial or detrimental effects on the overall biases. Even though full causality cannot be established on the basis of this evaluation work, the drivers of such regional differences were shown to be linked to changes in precipitation due to resolution changes, affecting the energy partitioning. Finally, the inter-annual sequence of hot summers over central/southern Europe was found to be fairly well simulated in most experiments despite an overestimation of the number of hot days and of the variability. The accurate simulation of inter-annual variability for a few models is independent of the model bias. This indicates that internal variability of high summer temperatures should not play a major role in controlling inter-annual variability. Despite some improvements, especially along coastlines, the analyses conducted here did not allow us to generally conclude that a higher resolution is clearly beneficial for a correct representation of heat waves by regional climate models. Even though local-scale feedbacks should be better represented at high resolution, combinations of parameterizations have to be improved or adapted accordingly.     

Human impact on the forests of Bermuda: the decline of endemic cedar and palmetto since 1609, recorded in the Holocene pollen record of Devonshire Marsh

The wreck of the Sea Venture on Bermuda reefs in 1609 initiated continuous habitation by humans on these islands. Colonization brought significant changes to the native and endemic flora of Bermuda. Original floral diversity was low, due to the effects of isolation and lack of previous anthropogenic influences. Two dominant endemic components of the flora, Bermuda cedar (Juniperus bermudiana) and Bermuda palmetto (Sabal bermudana), were extensively utilized by the colonists. Cedar was used for housing, furniture, shipbuilding and export, while the palmetto was used for roof thatch, basketry, food and drink. Exploitation of these species occurred to such extent that the General Laws of Bermuda included resolutions protecting them as early as 1622. Later, in the period between 1946 and 1951, two accidentally introduced scale insects eliminated 95% of the existing cedar population.While the flora and geography of upland habitats on Bermuda have been drastically modified by humans, the peat marsh basins have remained relatively unaffected. From the peats of one of these, Devonshire Marsh, a 9-m core was extracted for pollen analysis and to evaluate the potential for further study. The pollen record below 1.6 m indicated very little change in the native and endemic flora of Bermuda, but above that depth statistically significant changes in the relative abundances of pollen of Bermuda cedar and Bermuda palmetto are noted. Relative abundances of both species decreases significantly above this depth. This decrease is interpreted to represent the arrival of colonists and their impact on the cedar and palmetto populations. A second decrease in relative abundance of Bermuda cedar is recorded above 0.7 m. This reflects the scale infestation and decimation of the remaining cedar population. Coincident with decreases in cedar and palmetto are increases in relative abundance of Poaceae and Asteraceae, indicative of cleared land and the spread of weedy taxa. A radiocarbon date of 520±70 yr BP at a depth of 1.7 m and major changes in relative abundance of palynomorphs in proximity to the marsh surface allows their interpretation as a record of human impact on the endemic flora of Bermuda.This is the 19th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

A Lagrangian Cloud Model for the Study of Marine Fog

A large-eddy simulation model is coupled with a Lagrangian cloud model to study marine fog. In this model, aerosols and droplets are treated from a Lagrangian frame of reference, in contrast to the traditional bulk and bin microphysical models. Droplet growth via condensation is governed by Köhler theory and environmental conditions local to the droplet. Coupling to the vapour and temperature fields of the flow ensures mass, momentum, and energy conservation between the air and droplet phases. Based on the recent C-FOG field campaign, a simulation is performed which highlights the benefits and potential of this type of model. By initializing the simulation with the measured aerosol size distribution and making assumptions about the chemical composition of the multiple peaks, the simulations provide a clear explanation for the observed bimodal droplet distribution during C-FOG: high supersaturation levels cause condensational growth of nearly all coarse-mode aerosols (presumed to be composed of marine salt), as well as a large number of accumulation model aerosols (presumed to be of continental origin with a lower hygroscopicity). The largest peak in the resulting droplet distribution is created from coarse-mode aerosols with high hygroscopicity, while the secondary peak is only possible due to the limited impact of the largest peak on saturation levels inside the fog. Thus, for the simulated levels of supersaturation, it is the limited number of coarse-mode aerosols which is responsible for the emergence of a larger second peak.

     

Diversity and genetic differentiation in Artemia species and populations detected by AFLP markers

Amplified Fragment Length Polymorphism (AFLP) markers were successfully employed to analyze 15Artemia species and strains for genetic diversity. AFLP markers are extremely sensitive to even a small sequence variation. They are stable and more polymorphic than RAPD. Twelve pairs of primer combinations were used to detect AFLP bands, of which 384 were polymorphic, and DNA fingerprintings were obtained by using silver staining. The polymorphism analysis leads us to the following conclusions: 1.Artemia tibetiana seems to differentiate fromA. sinica. 2. The parthenogenetic populations from inland salt lakes could follow an evolutionary path that is different from that of the coastal parthenogenetic populations.     

Multifractal Analysis of the Arnea,Greece Seismicity with Potential Implications for Earthquake Prediction

Two-dimensional multifractal analysis is performed in a seismic area of Northern Greece responsible for recent strong earthquakes, including the Arnea sequence of May 1995, culminating in a M_w 5.3 event on 4/5/1995. It is found that multifractality gradually increases prior to the major seismic activity and that declusterization replaces clusterization not long before its initialization. The fractal dimensions D(q) (q > 0) abruptly drop for aftershocks, reflecting their very strong spatial clustering. The observed seismicity patterns seem to be compatible with a percolation process. Before the main sequence, the fractal dimension is consistently in the range 1.67–1.96 (standard deviation included). Percolation theory predicts 1.9 for 2D percolation clusters and 1.8 for the backbone of 3D percolation clusters. If the observed gradual increase in multifractality is due to multifractality reaching a maximum prior to the major slip (percolation), this may enable us to roughly estimate its time of occurrence.