Improved water table dynamics in MODFLOW

The standard formulation of a block-centered finite-difference model, such as MODFLOW, uses the center of the cell as the location of a cell node. Simulations of a dynamic water table can be improved if the node of a cell containing the water table is located at the water table rather than at the center of the cell. The LPF package of MOD-FLOW-2000 was changed to position a cell's node at the water table in convertible cells with a water table. Improved accuracy in the upper regions of an unconfined aquifer is demonstrated for pumping from a partially penetrating well. The change introduces a nonlinearity into the solution of the flow equations that results in slightly slower convergence of the flow solution, 7% slower in the presented demonstration. Accuracy of simulations is improved where vertical flow is dominated by a moving water table, but not when a large water table gradient dominates over the water table movement.     

Medieval Warming, Little Ice Age, and European impact on the environment during the last millennium in the lower Hudson Valley, New York, USA

Establishing natural climate variability becomes particularly important in large urban areas in anticipation of droughts. We present a well-dated bi-decadal record of vegetation, climate, land use, and fire frequency from a tidal marsh in the Hudson River Estuary. The classic Medieval Warm Period is evident through striking increases in charcoal and Pinus dominance from 800–1300 A.D., paralleling paleorecords southward along the Atlantic seaboard. Higher inputs of inorganic sediment during this interval suggest increased watershed erosion during drought conditions. The presence of the Little Ice Age ensues with increases in Picea and Tsuga, coupled with increasing organic percentages due to cooler, moister conditions. European impact is manifested by a decline in arboreal pollen due to land clearance, increased weedy plant cover (i.e., Ambrosia, Plantago, and Rumex), and an increase in inorganic particles to the watershed.     

A mechanism for anomalous decline in radon precursory to an earthquake

Mechanisms for interpreting anomalous decreases in radon in ground water prior to earthquakes are examined with the help of a case study to show that radon potentially is a sensitive tracer of strain changes in the crust preceding an earthquake. The 2003 Chengkung earthquake of magnitude (M) 6.8 on December 10, 2003, was the strongest earthquake near the Chengkung area in eastern Taiwan since 1951. The Antung radon-monitoring station was located 20 km from the epicenter. Approximately 65 d prior to the 2003 Chengkung earthquake, precursory changes in radon concentration in ground water were observed. Specifically, radon decreased from a background level of 780 pCi/L to a minimum of 330 pCi/L. The Antung hot spring is situated in a fractured block of tuffaceous sandstone surrounded by ductile mudstone. Given these geological conditions, we hypothesized that the dilation of brittle rock mass occurred at a rate faster than the recharge of pore water and gas saturation developed in newly created cracks preceding the earthquake. Radon partitioning into the gas phase may explain the anomalous decrease of radon precursory to the 2003 Chengkung earthquake. To support the hypothesis, vapor-liquid, two-phase radon-partitioning experiments were conducted at formation temperature (60 degrees C) using formation brine from the Antung hot spring. Experimental data indicated that the decrease in radon required a gas saturation of 10% developed in rock cracks. The observed decline in radon can be correlated with the increase in gas saturation and then with the volumetric strain change for a given fracture porosity.     

Comparison of Two Closely Located Meteorological Measurement Sites and Consequences for Their Areal Representativity

We compare meteorological data collected at the Met Office Research Unit, Cardington, UK with similar data from a temporary meteorological station located approximately 8.5 km away. Data were examined for a period of 10 months to ascertain differences in mean quantities, and in heat and radiation budgets, at different heights between the two locations, one of which is located in a wide shallow valley, the other on a plateau at the valley edge. Results reveal that screen-level variables at the two sites show the greatest differences in mean quantities, for most conditions, but that at 50 m the differences are negligible, indicating that temperatures had become aggregated and homogeneous at that height. For flux measurements between 10 and 50 m, however, significant differences were observed at certain times of the year, which appear to be related to local vegetation and soil conditions, and these are discussed. The study also presents data for stable conditions that show that temperature differences at screen level are again the most significant difference between the two sites. On average these were not large, but on occasions discrepancies at low levels (up to approximately 10 m) as large as 5°C were observed. It is thought that these greater differences during stable conditions may be caused by cold air pooling at the valley site, despite the very shallow orography there. Data from the two sites have been compared with forecasts from two Met Office mesoscale models. Results show that, during daytime hours, model-predicted values lie outside the range of values observed at the two sites, indicating a possible model bias. However, the opposite was true for most nighttime hours, during which model values fell within the range observed at the two sites, indicating that at night the model predictions are representative of the region. In this case, however, comparison of the model prediction with either one of the available observations could lead to the false conclusion that the model temperatures were either too high or too low, depending on which observational site was used for the comparison.     

A proposal for a new scenario framework to support research and assessment in different climate research communities

In this paper, we propose a scenario framework that could provide a scenario “thread” through the different climate research communities (climate change – vulnerability, impact, and adaptation - and mitigation) in order to support assessment of mitigation and adaptation strategies and climate impacts. The scenario framework is organized around a matrix with two main axes: radiative forcing levels and socio-economic conditions. The radiative forcing levels (and the associated climate signal) are described by the new Representative Concentration Pathways. The second axis, socio-economic developments comprises elements that affect the capacity for mitigation and adaptation, as well as the exposure to climate impacts. The proposed scenarios derived from this framework are limited in number, allow for comparison across various mitigation and adaptation levels, address a range of vulnerability characteristics, provide information across climate forcing and vulnerability states and span a full century time scale. Assessments based on the proposed scenario framework would strengthen cooperation between integrated-assessment modelers, climate modelers and vulnerability, impact and adaptation researchers, and most importantly, facilitate the development of more consistent and comparable research within and across these research communities.     

57-second oscillations in Nova Centauri 1986 (V842 Cen)

High-speed photometry in 2008 shows that the light curve of V842 Cen possesses a coherent modulation at 56.825 s, with sidebands at 56.598 and 57.054 s. These have appeared since this nova remnant was observed in 2000 and 2002. We deduce that the dominant signal is the rotation period of the white dwarf primary and the sidebands are caused by reprocessing from a surface moving with an orbital period of 3.94 h. Thus, V842 Cen is an intermediate polar (IP) of the DQ Herculis subclass, is the fastest rotating white dwarf among the IPs and is the third fastest known in a cataclysmic variable. As in other IPs, we see no dwarf nova oscillations, but there are often quasi-periodic oscillations in the range 350–1500 s. There is a strong brightness modulation with a period of 3.78 h, which we attribute to negative superhumps, and there is an even stronger signal at 2.886 h which is of unknown origin but is probably a further example of that seen in GW Lib and some other systems. We used the Swift satellite to observe V842 Cen in the ultraviolet and in X-rays, although no periodic modulation was detected in the short observations. The X-ray luminosity of this object appears to be much lower than that of other IPs in which the accretion region is directly visible.     

The unusual X-ray light curve of GRB 080307: the onset of the afterglow?

Swift -detected GRB 080307 showed an unusual smooth rise in its X-ray light curve around 100 s after the burst, at the start of which the emission briefly softened. This 'hump' has a longer duration than is normal for a flare at early times and does not demonstrate a typical flare profile. Using a two-component power-law-to-exponential model, the rising emission can be modelled as the onset of the afterglow, something which is very rarely seen in Swift -X-ray light curves. We cannot, however, rule out that the hump is a particularly slow early-time flare, or that it is caused by upscattered reverse shock electrons.     

Ouachita, Appalachian, and Ancestral Rockies deformations recorded in mesoscale structures on the foreland Ozark plateaus

Mesoscale structures in Paleozoic rocks of the Ozark plateaus reveal four Pennsylvanian deformation episodes in midcontinent North America. The two earliest episodes can be assigned to progressive northwestward docking of the Ouachita terrane with North America. Early extensional structures (Event 1) indicate a northwest/southeast maximum horizontal stress (Hmax) during Early Pennsylvanian Ouachita terrane advance. Event 2 extensional and strike-slip structures indicate Hmax across the Ozark plateaus that varies systematically from north-northwest/south-southeast in the south to northeast/southwest in the north. This suggests development of a slip-line deformation field in response to minor northeastward lateral escape of lithospheric blocks away from the northwestward-moving Ouachita terrane's leading edge, which acted as an indenter in western Arkansas, southeastern Oklahoma, and Texas. Younger contractional and strike-slip structures of Event 3 indicate northeast/southwest Hmax across the entire Ozark plateaus, and deformation orientation and intensity are not readily assigned to Ouachita foreland deformation and may be related to Middle Pennsylvanian Ancestral Rockies contractional deformation. Finally, Event 4 contractional structures indicate northwest/southeast Hmax consistent with southern Appalachian late stage convergence.Deformation episodes are localized along basement fault zones, particularly at major bends, suggesting minor restraining-bend uplifts along strike-slip faults. Geometries of conjugate normal fault and hybrid shear joint arrays indicate localized areas of high differential stress consistent with basement block uplift at these bends. High-angle faults reactivated in a reverse sense and bedding-parallel veins suggest tensile minimum stresses and pore fluid pressures exceeding lithostatic stress, consistent with brine pulses driven into the midcontinent during Late Paleozoic orogeny (as proposed by other authors).