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INTRODUCTION Recently much attention has been paid to different formulations of surface wave force in wave-driven ocean and coastal circulations (e.g., McWilliams et al., 2004, Mellor, 2003, Smith, 2006 and others). Basically, the analytical expressions for surface wave force and wave-current interaction can be classified into two types. One is the classical wave ‘radiation stress’ concept presented by Longuet-Higgins and Stewart (1962, 1964) and many others in depth-integrated and short wave-averaged equations. Mellor (2003) recently used the same concept to derive short wave-averaged 3-D equations with a depth-dependent wave-induced force. A direct application of this kind of depth-dependent wave-induced force was conducted by Xia et al. (2004) who related the vertical variation of current to the vertical structure of radiation stresses. Another type of wave force is the surface wave force initially derived by Garrett (1976) in the study of Langmuir circulation generation. The wave driving forces include the wave dissipation term and the wave-averaged vortex forcing term which has been identified later by Leibovich (1980) and Smith (1980) as the vertically integrated form of the ’CL vortex-force’ derived by Craik and Leibovich (1976). Dingemans et al. (1987) also presented a similar formulation of this type of wave driving force, though the current refraction, that may result in the vortex-force term, was recognized to give insignificant contributions under the conditions of slowly varying wave fields. Smith (2006) extended the formulation of Garrett (1976) to include finite-depth effects and provided some insight into physical interpretation of each forcing term in depth-integrated equations. A

Tidal currents are a promising source for future power generation given their periodicity and predictability. Therefore, numerical hydrodynamic models are frequently utilized for resource assessments. However, the relevant scales of the simulations and hence modeling techniques depend on the problem at hand. This paper shows the potential of the grid refinement technique for the assessment of tidal current energy for particular sites given its relatively low computational expense and high accuracy for the regions with the refined resolution. Example applications are described for mapping the tidal resources near two facilities (Portsmouth Naval Shipyard in Maine and Key West Naval Station in Florida) for possible future deployments of Marine Hydro-Kinetic technologies. The grid refinement capability in the coupled ocean-atmosphere-wave-sediment transport modeling system is used to improve the spatial resolution in the regions of interest, and has shown significant advantage over the original simulation results for the tidal power assessment. The numerical simulation results from both the original coarse grids and the refined grids are validated with measured tidal constituents at available locations. This study demonstrates how the enhancement of a model system for a regional tidal energy with grid refinement can assist with the performance of site specific resource assessments with modest computational expense.

This data report presents oceanographic observations made at eight locations in Long Bay, South Carolina, from October 2003 through April 2004. These locations were offshore of Myrtle Beach, South Carolina, and centered on a shore-oblique sand deposit that is approximately 10 kilometers long, 2 kilometers wide, and in excess of 3 meters thick. The observations were collected through a collaborative effort with the U.S. Geological Survey (USGS), the University of South Carolina (USC), and Georgia Institute of Technology (GT) as part of a larger study to understand the physical processes that control the transport of sediments in Long Bay. Funding for this work was provided by the South Carolina Coastal Erosion Project, a cooperative study supported by the USGS and the South Carolina Sea Grant Consortium (Sea Grant Project No: R/CP-11).

Current pulsations from a longshore bar and trough rip system located on the eastern coast of Moreton Island, Australia are presented. These pulsations occur over 10-20minute intervals through the rip system and are correlated to both water level gradients and wave energy variations. The field measurements suggest that rip current pulsations can be driven by fluctuating mass transport over the shore parallel inner bar.

This study evaluated the effectiveness of signs alerting drivers to double traffic fines in highway work zones, school zones and safety corridors. The evaluation was based on a driver survey that investigated the decision to exceed speed limits across a range of driving contexts and risk categories. The findings indicated that personal assessments of risk change from one hypothetical situation to another, suggesting that people make a more or less calculated decision to violate the speed limit, based on those risks. The findings also suggested that people perceive a higher relative risk associated with traffic fines if the situation is one in which a doubling of traffic fines may apply. Finally, the results showed that interview subjects who reported being aware of double-fine zones tended to have somewhat higher assessments of the risk associated with traffic citations, traffic fines and higher insurance rates.

Mean water level, pressure and velocity measurements of a small rip current system taken on Moreton Island, Australia in December 2000 are presented. During high tide, gradients in the mean water level over the bar and in the trough are weakly directed away from the channel thereby producing no feeder or rip currents. However as the tide falls, the gradients strengthen toward the channel, generating feeder and rip currents. Numerical modeling of the rip system reproduces similar behavior of the circulation patterns. The model indicates that the changing breaking pattern as the water level decreases with the falling tide is the primary factor for the differing hydraulic gradients and determining whether a rip current is generated.

[1] The focus of the paper is the analysis of the flow in rip current systems generated by channels in longshore bars on a beach. The horizontal variations of rip current systems are described through the use of the quasi-three-dimensional nearshore circulation model SHORECIRC. Model predictions are compared to laboratory measurements of waves and current velocities throughout the entire rip current system and show reasonable agreement. The rips in the two channels are found to behave differently because of the depth variation across the basin. It is found that higher bottom stress leads to more stable flow where the rip current meanders less and fewer eddies are generated. The wave current interaction creates forcing which reduces the distance rip currents flow offshore and can lead to a slow pulsation of the rip current. This pulsation is in addition to the instabilities of a jet which can also be present in rip currents. The three dimensionality of the rip current system is found to have a significant effect on the overall circulation patterns. INDEX TERMS: 4255 Oceanography: General: Numerical modeling; 4512 Oceanography: Physical: Currents; 4546 Oceanography: Physical: Nearshore processes; KEYWORDS: rip currents, nearshore circulation, numerical modeling, waves

[1] Most studies on shear waves to date have assumed the flow is depth uniform (two dimensional). In the present study, we utilize the quasi-three-dimensional (quasi-3D) nearshore circulation model SHORECIRC to study shear waves. Our results show that shear wave flow is more organized in the quasi-3D simulation than in the 2D simulation. In the 2D simulation, the vortices are moving farther offshore of the bar, while in the quasi-3D simulation, they are more confined to the shoreward side of the bar. Moreover, the shear waves in the quasi-3D simulation are much less energetic than in the 2D simulation, though the total momentum mixing for the two cases is not significantly different. To understand which mechanisms cause the differences in the 2D and the quasi-3D simulation, the momentum, kinetic energy, and enstrophy equations for the mean flow and the shear waves are derived. The momentum, energy, and enstrophy balances are discussed using the numerical results from the idealized SUPERDUCK topography and the wave conditions on October 16, 1986. The effects of the quasi-3D dispersion due to the depth varying currents on shear waves are illustrated. Analysis of the mean momentum balance shows that both the shear waves and the quasi-3D current pattern contribute to the momentum transfer, and the momentum transfer provided by the shear waves is sometimes larger than that by the quasi-3D dispersive terms. The kinetic energy balance of the shear waves shows that the quasi-3D dispersive terms will extract kinetic energy from the depth-averaged shear waves. Furthermore, the enstrophy equation demonstrates that the quasi-3D dispersion terms provide vortex tilting, which allows three-dimensional vortex interactions.

[1] This paper describes the three-dimensional variation of rip currents through the use of experimental measurements of rip currents in a directional wave basin. The rip currents are unstable, appearing sporadically at any given location. The vertical profile of the rip current is found to vary from depth-uniform inside the channel to depth-varying further offshore. Offshore from the channel the rip current has much stronger velocities at the surface than near the bottom. The instantaneous profiles twist rapidly over depth farther offshore and are fairly uniform in the channel. The depth variations are shown to be sensitive to the total volume flux in the rip.

The present paper describes the 3-D variation of rip currents through the use of physical and numerical models. Experimental measurements of rip currents are made in a directional wave basin. The vertical profile of the rip current is found to vary from depth uniform inside the channel to depth varying further offshore. Offshore from the channel the rip current has much stronger velocities at the surface than near the bottom. A nearshore circulation numerical modeling system is applied to the wave basin. The results from the model show qualitative and quantitative agreement with the laboratory measurements.

Fast IPSCs in the brain are predominantly caused by presynaptic release of GABA that activates GABAAreceptor (GABAAR) channels. The IPSCs are shaped by the gating and desensitization properties of postsynaptic GABAARs. Specifically, fast desensitization has been suggested to decrease IPSC amplitude and to increase IPSC duration by slowing deactivation; however, the mechanisms underlying desensitization, deactivation, and their coupling are poorly understood. Consistent with this suggestion, α1β3γ2L GABAARs desensitize with a prominent fast phase and deactivate slowly, whereas α1β3δ GABAARs desensitize without a fast phase and deactivate rapidly. Using the concentration-jump technique applied to excised patches, we studied GABAARs containing chimeras or exchange mutants between δ and γ2L subunits to gain insight into the structural bases for fast desensitization and its coupling to deactivation. We demonstrated that the N terminus and two adjacent residues (V233, Y234) in the first transmembrane domain (TM1) of the δ subunit were both required to abolish fast desensitization. Additionally, these residues in TM1 of the γ2L subunit (Y235, F236) were critical for desensitized states to prolong deactivation after removal of GABA, because mutations resulted in accelerated deactivation despite unaltered desensitization time course. Interestingly, control of desensitization and deactivation was independent of the identity (γ2L or δ subunit sequence) of TM2, indicating that structures related to the putative channel gate may play a less direct role in desensitization than previously suggested.

To understand the mechanisms underlying activation and deactivation of GABA(A) receptor (GABAR) channels, we studied the properties of an identified GABAR isoform under conditions similar to those present at central GABAergic synapses. Recombinant alpha5beta3gamma2L GABARs were expressed in L929 fibroblasts and studied using patch-clamp recording techniques. Brief application of a high GABA concentration to outside-out membrane patches elicited transient currents that resembled those reported for miniature inhibitory postsynaptic currents (mIPSCs), as well as native GABAR currents recorded under similar conditions. Characteristic of these currents was a rapid activation phase followed by a prolonged biphasic deactivation phase that far outlasted GABA application. Single-channel recordings revealed unique patterns of channel activity with two channel conductance states of 22 and 16 pS. The prolonged deactivation phase appeared to be sustained by entry into and reopening from long-lasting closures or desensitized states. Agonist affinity determined the time course of deactivation, indicating that occupied receptors drove the channel activity underlying the decay of current. The time course of deactivation was also longer at depolarized membrane potentials. The similarities between transient activation kinetics of recombinant alpha5beta3gamma2L GABARs to activation of synaptic GABARs (rapid activation and prolonged, voltage-dependent deactivation) suggest that intrinsic channel properties determine much of the response patterns of native GABARs.

Physiological properties of GABAA receptors from acutely dissociated rat dentate granule cells. Study of fast, GABAA receptor-mediated, inhibitory postsynaptic currents (IPSCs) in hippocampal dentate granule cells has suggested that properties of GABAA receptors influence the amplitude and time course of the IPSCs. This study describes the physiological properties of GABAA receptors present on hippocampal dentate granule cells acutely isolated from 18- to 35-day-old rats. Rapid application of 1 mM GABA to outside-out macropatches excised from granule cells produced GABAA receptor currents with rapid rise time and biexponential decay of current after removal of GABA. After activation, granule cell GABAA receptor currents desensitized incompletely. During a 400-ms application of 1 mM GABA, peak current only desensitized ∼40%. In symmetrical chloride solutions there was no outward rectification of whole cell current. Activation rates and peak currents elicited by rapid application of GABA to macropatches were also similar at positive and negative holding potentials. However, deactivation of GABAA receptor currents was slower at positive holding potentials. When whole cell currents were recorded without ATP in the pipette, current run-down was not apparent for 30 min in 50% of neurons, but run-down appeared to start soon after access was established in the remaining neurons. When 2 mM ATP was included in the recording pipette no run-down was apparent in 30 min of recording. The efficacy and potency of GABA were lower in cells recorded with no ATP in the pipette and during run-down compared with those recorded with 2 mM ATP and no run-down.

GABAA receptors (GABARs) mediate the majority of fast inhibitory neurotransmission in the mammalian brain. Functional GABARs are ligand-gated chloride ion channels composed of five individual subunits. These subunits derive from six identified families, many with multiple subtypes (α1-6, β1-3, γ1-3, δ, e and π). When expressed in Xenopus oocytes or mammalian cells, different GABAR subunit combinations form receptors with unique pharmacological and biophysical properties (Macdonald & Olsen, 1994). These subunits do not assemble randomly, however, for while αβ subunit combinations readily express in mammalian cells, addition of a γ subunit drives expression of αβγ isoforms (Angelotti & Macdonald, 1993). The majority of native receptors are believed to be formed by combinations of αβγ and αβδ subunits (McKernan & Whiting, 1996), although the recently characterized e and π subunits may substitute for γ or δ subunits in some instances (Hedblom & Kirkness, 1997; Davies et al. 1997). In the rat, the γ2 subtype becomes the dominant γ subunit expressed at later developmental stages, and mRNA and membrane protein for this subtype are expressed in most brain regions. In contrast, the δ subunit is restricted only to a few cell populations in the postnatal rat that include thalamic relay neurons, cerebellar granule neurons and dentate granule neurons of the hippocampus (Laurie et al. 1992a,b; Wisden et al. 1992; Sperk et al. 1997). While the δ subunit has been shown to combine preferentially with the α6 subtype in cerebellar granule neurons (Jones et al. 1997), the GABAR subtypes that it combines with in dentate granule neurons remain unknown. The potential importance of hippocampal δ subunit-containing GABARs is underscored, however, by δ subunit knockout mice that exhibit spontaneous seizures (Olsen et al. 1997). For this investigation, we chose the α1β3γ2L and α1β3δ GABAR isoforms to determine the roles of γ and δ subunits in shaping GABAR currents. α1β3γ2L and α1β3δ GABAR whole-cell currents have been characterized previously in L929 fibroblasts, where incorporation of the δ subunit resulted in higher apparent GABA affinity, slower and less complete whole-cell current desensitization, and smaller whole-cell currents compared with receptors containing the γ2 subtype (Saxena & Macdonald, 1994). α1β3 currents also desensitized more rapidly than α1β3δ currents, although both faster (Fisher & Macdonald, 1997) and slower desensitization (Dominguez-Perrot et al. 1996) relative to α1β3γ2L currents have been reported. In addition, α1β3 single channels had a smaller main channel conductance level (13 pS), while α1β3γ2L and α1β3δ channels had a similar larger main conductance level (27 pS). The γ2L subtype, however, conferred a change on the open and closed properties of the receptor, leading to a tendency for longer duration openings and longer bursts of openings (Fisher & Macdonald, 1997). While suggesting major differences in channel gating and desensitization, these analyses did not resolve the rapid phases of activation, desensitization and deactivation of GABAR currents. These rapid kinetic properties are critical to understanding the potential synaptic roles of the α1β3γ2L and α1β3δ GABAR isoforms. In previous studies of native receptors, rapid application of GABA to outside-out membrane patches containing many GABARs reproduced the rapid activation and deactivation of IPSCs (Maconochie et al. 1994; Jones & Westbrook, 1995; Tia et al. 1996; Galaretta & Hestrin, 1997; Mellor & Randall, 1997, 1998). Also, with this rapid application protocol, it was demonstrated that GABAR desensitization was an important factor in shaping the deactivation time course of macropatch responses (Jones & Westbrook, 1995, 1996). In addition, macropatch deactivation kinetics were altered by allosteric modulators of GABARs such as benzodiazepines (Lavoie & Twyman, 1996; Mellor & Randall, 1997) and the anaesthetic propofol (Zhu & Vicini, 1997), as well as by intracellular phosphatase activity (Jones & Westbrook, 1997). Moreover, different recombinant GABAR isoforms displayed unique rapid kinetic properties (Verdoorn, 1994; Tia et al. 1996; Lavoie et al. 1997) that probably contribute to the diversity in GABAergic synaptic responses. In addition to predicting the synaptic behaviour of recombinant GABAR isoforms, rapid kinetic analysis of macroscopic currents may serve as a bridge between single-channel and whole-cell analysis, allowing for the development of more comprehensive kinetic models of GABAR behaviour that incorporate desensitization (Macdonald & Twyman, 1992). For this study, we implemented a GABA application system that allowed very rapid solution exchange (10–90 % rise time < 400 μs) during electrophysiological recordings from outside-out membrane patches containing multiple receptor channels. Using this technique, we determined the rapid activation, desensitization and deactivation kinetics of α1β3γ2L and α1β3δ GABAR currents and used these kinetic data in combination with steady-state single-channel analysis to develop comprehensive models of GABAR kinetic behaviour for these isoforms. In some instances, the α1β3 isoform was examined so that contributions of the γ and δ subunits could be more thoroughly assessed.

The presented study reports on numerical simulations of flows in tidal channels near Rose Dhu Island, GA, which is used to identify hotspots of hydrokinetic energy and to assess the tidal stream energy potential at this site. The numerical simulations are complemented with field measurements of local current velocities and water surface heights, which are used to validate the simulations. Both velocity distributions and water surface heights as predicted by the numerical model are in good agreement with observed data. The simulations reveal a tidal asymmetry in the encompassing Ogeechee estuary with the ebb tide currents dominating over the flood tide ones. The model is able to successfully predict the distribution of discharge into the smaller creeks around Rose Dhu Island and thereby capturing the location of local hotspots of hydrokinetic energy. It is found that local hotspots do exist near the island, and the analysis suggests the maximum available annual power of 4.75 MW, with a peak estimated extraction surpassing 4 KW during Spring tides.

To extract mechanistic information of activated processes, we propose to decompose potential energy and free energy differences between configurations into contributions from individual atoms, functional groups, or residues. Decomposition is achieved by calculating the mechanical work associated with the displacements and forces of each atom along a path that connects two states, i.e., following the flow of work. Specifically, we focus on decomposing energy or free energy differences along representative pathways such as minimum energy paths (MEPs) and minimum free energy paths (MFEPs), and a numerical metric is developed to quantify the required accuracy of the reaction path. A statistical mechanical analysis of energy decomposition is also presented to illustrate the generality of this approach. Decomposition along MEP and MFEP is demonstrated on two test cases to illustrate the ability to derive quantitative mechanistic information for different types of activated processes. First, the MEP of alanine dipeptide isomerization in vacuum and the MFEP of isomerization in explicit water is studied. Our analysis shows that carbonyl oxygen and amide hydrogen contribute to most of the energetic cost for isomerization and that explicit water solvation modulates the free energy landscape primarily through hydrogen bonding with these atoms. The second test case concerns the formation of tetrahedral intermediate during a transesterification reaction. Decomposition analysis shows that water molecules not only have strong stabilization effects on the tetrahedral intermediate but also constitute a sizable potential energy barrier due to their significant structural rearrangement during the reaction. We expect that the proposed method can be generally applied to develop mechanistic understanding of catalytic and biocatalytic processes and provide useful insight for strategies of molecular engineering.

We present an unusual case of recurrent cough syncope in a 43-year-old woman, which was initially thought to be seizures. Syncopal episodes were triggered by paroxysms of cough and were characterized by unresponsiveness and myoclonic jerks in her extremities. She had a left-sided glomus jugulare tumor that extended into the posterior cranial fossa with evidence of worsening communicating hydrocephalus on brain imaging. We postulate that bouts of cough produced increased intracranial pressure both by raising intrathoracic and intraabdominal pressures as well as by transient obstruction to cerebrospinal fluid flow secondary to intermittent tonsillar herniation during cough. This resulted in diffuse decrease in cerebral blood flow causing syncope. The patient's syncopal episodes decreased in frequency once an external ventricular drain was placed followed by a ventriculoperitoneal shunt. Search for factors that can increase intracranial pressure seems warranted in patients with recurrent cough syncope.