Two-dimensional equilibrium morphological modelling of a tidal inlet: an entropy based approach

The management of tidal inlets requires the accurate prediction of equilibrium morphologies. In areas where the flow from rivers is highly regulated, it is important to give decision makers the ability to determine optimal flow management schemes, in order to allow tidal inlets to function as naturally as possible, and minimise the risk of inlet closure. The River Murray Mouth in South Australia is one such problem area. Drought and the retention of water for irrigation and urban water consumption have limited the amount of water entering the estuary. As a result, sediment from the coastal environment is being deposited in the mouth of the estuary, reducing the effect of further coastal interactions. Currently, situations such as this are modelled using traditional process-based methods, where wave, current, sediment transport and sediment balance modules are linked together in a time-stepping process. The modules are reapplied and assessed until a stable morphology is formed. In this paper, new options for modelling equilibrium morphologies of tidal inlets are detailed, which alleviate some of the shortfalls of traditional process-based models, such as the amplification of small errors and reliance on initial conditions. The modelling problem is approached in this paper from a different angle and involves the use of entropy based objective functions, which are optimised in order to find equilibrium morphologies. In this way, characteristics of a system at equilibrium can be recognised and a stable system predicted without having to step through time. This paper also details the use of self-organisation based modelling methods, another non-traditional model application, where local laws and feedback result in the formation of a global stable equilibrium morphology. These methods represent a different approach to traditional models, without some of the characteristics that may add to their limitations. Responsible Editor: Alejandro Souza     

Relationships among satellite chlorophylla, river inputs, and hypoxia on the Louisiana Continental shelf, Gulf of Mexico

SeaWiFS ocean color measurements were used to investigate interannual, monthly, and weekly variations in chlorophylla (chla) on the Louisiana shelf and to assess relationships with river discharge, nitrate load, and hypoxia. During the study period (2000–2003), interannual changes in shelf-wide chla concentrations averaged over January–July ranged from +57% to −33% of the 4-yr average, in accord with freshwater discharge changes of +20% to −29% and nitrate load changes of +20% to −35% from the Mississippi and Atchafalaya Rivers. Chla variations were largest on the shelf between the Mississippi and Atchafalaya Deltas. Within this region, which corresponds spatially to the area of most frequent hypoxia, lowest January–July mean chla concentrations (5.5 mg m⁻³ over 7,000 km²) occurred during 2000, the year of lowest freshwater discharge (16,136 m³ s⁻¹) and nitrate load (55,738 MT N d⁻¹) onto the shelf. Highest January–July mean chla concentrations (13 mg m⁻³ over 7,000 km²) were measured in 2002, when freshwater discharge (27,440 m³s⁻¹) and nitrate load (101,761 MT N d⁻¹) were highest and second highest, respectively. Positive correlations (R²=0.4–0.5) were found between chla and both fresh water and nitrate loads with 0 to 1 month lags, with the strongest relationships just west of the Mississippi Delta. In 2001, unusually clear skies allowed the identification of distinct spring and summer chla blooms west of the Mississippi Delta 4–5 wk after peaks in river discharge. East of the delta, the chla concentrations peaked in June and July, following the seasonal reversal in the coastal current. A clear linkage was not detected between satellite-measured chla and hypoxia during the 4-yr period, based on a time series of bottom oxygen concentrations at one station within the area of most frequent hypoxia. Clear relationships are confounded by the interaction of physical processes (wind stress effects) with the seasonal cycle of nutrient-enhanced productivity and are influenced by the prior year's nitrate load and carbon accumulation at the seabed.     

Strike-slip faulting, rotation, and along-strike elongation in the Kopeh Dagh mountains, NE Iran

The Kopeh Dagh is a linear mountain range separating the shortening in Iran from the stable, flat Turkmenistan platform. In its central part is an array of active right-lateral strike-slip faults that obliquely cut the range and produce offsets of several kilometres in the geomorphology and geological structure. They are responsible for major destructive earthquakes in the 19th and 20th centuries and represent an important seismic hazard for this now-populous region of NE Iran. These strike-slip faults all end in thrusts, revealed by the uplift and incision of Late Quaternary river terraces, and do not continue beyond the Atrak river valley, which forms the southern margin of the Kopeh Dagh. The cumulative offset on these strike-slip faults, and their associated rotation about vertical axes, can account for ∼60 km of N–S shortening. This value is similar to estimates of the Late Quaternary N–S right-lateral shear between central Iran and Afghanistan, which must be accommodated in NE Iran. The strike-slip faults also require ∼30 km of along-strike extension of the Kopeh Dagh, which is taken up by the westward component of motion between the South Caspian Basin and both Eurasia and Central Iran. It is probable that these motions occurred over the last ∼10 Ma.     

Spin-dependent limits from the DRIFT-IId directional dark matter detector

Data are presented from the DRIFT-IId detector operated in the Boulby Underground Science Facility in England. A 0.8 m³ fiducial volume, containing partial pressures of 30 Torr CS₂ and 10 Torr CF₄, was exposed for a duration of 47.4 live-time days with sufficient passive shielding to provide a neutron free environment within the detector. The nuclear recoil events seen are consistent with a remaining low-level background from the decay of radon daughters attached to the central cathode of the detector. However, charge from such events must drift across the entire width of the detector, and thus display large diffusion upon reaching the readout planes of the device. Exploiting this feature, it is shown to be possible to reject energy depositions from these Radon Progeny Recoil events while still retaining sensitivity to fiducial-volume nuclear recoil events. The response of the detector is then interpreted, using the F nuclei content of the gas, in terms of sensitivity to proton spin-dependent WIMP-nucleon interactions, displaying a minimum in sensitivity cross section at 1.8 pb for a WIMP mass of 100 GeV/c². This sensitivity was achieved without compromising the direction sensitivity of DRIFT.     

The first high-precision radial velocity search for extra-solar planets

The reflex motion of a star induced by a planetary companion is too small to detect by photographic astrometry. The apparent discovery in the 1960s of planetary systems around certain nearby stars, in particular Barnard’s star, turned out to be spurious. Conventional stellar radial velocities determined from photographic spectra at that time were also too inaccurate to detect the expected reflex velocity changes. In the late 1970s and early 1980s, the introduction of solid-state, signal-generating detectors and absorption cells to impose wavelength fiducials directly on the starlight, reduced radial velocity errors to the point where such a search became feasible. Beginning in 1980, our team from UBC introduced an absorption cell of hydrogen fluoride gas in front of the CFHT coudé spectrograph and, for 12 years, monitored the radial velocities of some 29 solar-type stars. Since it was assumed that extra-solar planets would most likely resemble Jupiter in mass and orbit, we were awarded only three or four two-night observing runs each year. Our survey highlighted three potential planet hosting stars, γ Cep (K1 IV), β Gem (K0 III), and ? Eri (K2 V). The putative planets all resembled Jovian systems with periods and masses of: 2.5 years and 1.4 M_J, 1.6 years and 2.6 M_J, and 6.9 years and 0.9 M_J, respectively. All three were subsequently confirmed from more extensive data by the Texas group led by Cochran and Hatzes who also derived the currently accepted orbital elements.None of these three systems is simple. All five giant stars and the supergiant in our survey proved to be intrinsic velocity variables. When we first drew attention to a possible planetary companion to γ Cep in 1988 it was classified as a giant, and there was the possibility that its radial velocity variations and those of β Gem (K0 III) were intrinsic to the stars. A further complication for γ Cep was the presence of an unseen secondary star in an orbit with a period initially estimated at some 30 years. The implication was that the planetary orbit might not be stable, and a Jovian planet surviving so close to a giant then seemed improbable. Later observations by others showed the stellar binary period was closer to 67 years, the primary was only a sub-giant and a weak, apparently synchronous chromospheric variation disappeared. Chromospheric activity was considered important because κ¹ Cet, one of our program stars, showed a significant correlation of its radial velocity curve with chromospheric activity.? Eri is a young, magnetically active star with spots making it a noisy target for radial velocities. While the signature of a highly elliptical orbit (e = 0.6) has persisted for more than three planetary orbits, some feel that even more extensive coverage is needed to confirm the identification despite an apparent complementary astrometric acceleration detected with the Hubble Space Telescope.We confined our initial analyses of the program stars to looking for circular orbits. In retrospect, it appears that some 10% of our sample did in fact have Jovian planetary companions in orbits with periods of years.     

Cloud-to-ground lightning signatures of long-lived tornadic supercells on 27–28 April 2011

This study integrates past research methodologies, data from the National Lightning Detection Network (NLDN), and geographic information system techniques to assess the lightning and severe weather hazard relationship for the 27–28 April 2011 United States tornado outbreak. NLDN and Doppler radar data are used to examine the cloud-to-ground (CG) lightning characteristics associated with seven supercell thunderstorms that produced long-track, significant and/or violent tornadoes. Analyses indicate that CG lightning flashes alone do not provide enough information for the detection of a lightning jump prior to tornadogenesis. All seven supercells were dominated by negative-polarity CG lightning flashes; which is expected due to the geographic location and elevated low-level moisture found in the outbreak environment. The correlation between low-level mesocyclone strength and total CG lightning flash rate was varied and inconsistent among all storms despite their formation and sustenance in similar environmental and geographic space. Additional case studies, as well as climatological approaches, are required to discover if the varying lightning–tornado relationships found in this study are consistent with other tornadic environments.     

Reynolds stress and sand transport over a foredune

Reynolds shear stress (RS = –u′ w′) and sand transport patterns over a vegetated foredune are explored using three‐dimensional velocity data from ultrasonic anemometers (at 0 · 2 and 1 · 2 m) and sand transport intensity from laser particle counters (at 0 · 014 m). A mid‐latitude cyclone on 3–4 May 2010 generated storm‐force winds (exceeding 20 m s^–1) that shifted from offshore to obliquely alongshore. Quadrant analysis was used to characterize the spatial variation of RS quadrant components (Q1 through Q4) and their relative contributions were parameterized using the flow exuberance relation, EX_FL = (Q1 + Q3)/(Q2 + Q4). The magnitudes of RS and sand transport varied somewhat independently over the dune as controlled by topographic forcing effects on flow dynamics. A ‘flow exuberance effect’ was evident such that Q2 (ejection‐like) and Q4 (sweep‐like) quadrants (that contribute positively to RS) dominated on the beach, dune toe, and lower stoss, whereas Q1 and Q3 (that contribute negatively to RS) dominated near the crest. This exuberance effect was not expressed, however, in sand transport patterns. Instead, Q1 and Q4, with above‐average streamwise velocity fluctuations (+u′), were most frequently associated with sand transport. Q4 activity corresponded with most sand transport at the beach, toe, and stoss locations (52, 60, 100%). At the crest, 25 to 86% of transport was associated with Q1 while Q4 corresponded with most of the remaining transport (13 to 59%). Thus, the relationship between sand transport and RS is not as straightforward as in traditional equations that relate flux to stress in increasing fashion. Generally, RS was poorly associated with sand transport partly because Q1 and Q4 contributions offset each other in RS calculations. Thus, large amounts of transport can occur with small RS. Turbulent kinetic energy or Reynolds normal stresses (u′², w′²) may provide stronger associations with sand transport over dunes, although challenges exist on how to normalize and compare these quantities. Copyright © 2013 John Wiley & Sons, Ltd.     

Comparison of the satellite and ground-based measurements of the hydrogen fluoride content in the atmosphere

Satellite and ground-based measurements of the hydrogen fluoride (HF) total content (TC) are analyzed and compared. The HF profiles measured with an FTS device on the ACE satellite are used to calculate the TC and compare it with the ground-based measurements near St. Petersburg in 2009–2011. A comparison indicated that the seasonal variations in HF TC based on two independent measurements are in good qualitative agreement. Rare (nine) cases of direct comparison between two measurement types coordinated with respect to time (during the day) and site (no farther than 500 km) gave the following characteristics: the average difference is 8% and satellite data predominate over ground data; the standard deviation of a difference is 7%. In two cases of close measurement pairs (closer than 200 km), a comparison gave differences of 1 and 7%. The statistical characteristics of differences between two measurement types are in good agreement with the independent comparison of the ACE-FTS HF TC measurements with the NDACC network data.