Spatial Flow Patterns Research Articles

Deep learning based short-term air traffic flow prediction considering temporal–spatial correlation

In order to improve the accuracy and stability of air traffic flow prediction, an end-to-end deep learning-based model is proposed in this paper. By analyzing spatial correlations of adjacent areas and temporal correlations of historical traffic on given area, we firstly apply the gridded map method (including the flight levels) to encode the whole air traffic flow situation into a new data representation, i.e., traffic flow matrix (TFM). By the proposed data representation, inherent features of air traffic flow and their transition patterns on different cells and flight levels can be represented comprehensively. Learning from the powerful ability of convolutional neural network and recurrent neural network on modeling spatial and temporal correlations, the ConvLSTM module is proposed to build a trainable model for air traffic flow prediction. Since the output of the proposed model is also the TFM and shows the overall air traffic situation of studied regions, we call the proposed model as an end-to-end one. Experimental results on real data show the superior performance over existing approaches on prediction accuracy and stability. Furthermore, the proposed model can also predict the flow distribution on different flight levels in our application, which promotes the level of air traffic management. By analyzing the distribution of prediction errors on different cells, flight levels and predicting instants, we can draw the conclusion that spatial and temporal transition patterns of flight flow in air traffic system are fully learned by the proposed model. With the proposed model, more efficient air traffic flow measures are expected to be fulfilled to improve the operation efficiency.

Haemodynamics in Different Flow Lumen Configurations of Customised Aortic Repair for Infrarenal Aortic Aneurysms

Customised aortic repair (CAR) is a new and minimally invasive technique for the endovascular treatment of abdominal aortic aneurysms (AAAs). The aneurysm is completely sealed with a non-contained, non-cross linked polymer, while a new flow lumen is created with balloons. For CAR, the haemodynamically most favourable balloon and flow lumen configuration has not been established before; therefore, four flow parameters were assessed in an invitro model. Three invitro balloon configurations were implanted in an invitro AAA model; a configuration with crossing balloons (CC) and two parallel configurations (PC1 and PC2). These three models were consecutively placed in a flow system that mimics physiological flow conditions. Laser particle imaging velocimetry (PIV) was used to resolve spatial and temporal flow patterns during the cardiac cycle. In house built algorithms were used to analyse the PIV data for the computing of (i) flow velocity; (ii) vorticity; (iii) wall shear stress (WSS); and (iv) time averaged wall shear stress (TAWSS). Suprarenal flow patterns were similar in all models. The CC showed a higher infrarenal velocity than PC1 and PC2 (38cm/s vs. 23cm/s vs. 23cm/s), and a higher vorticity at the crossing of the lumens (CC: 337/s; PC1 127/s; PC2: 112/s). The lowest vorticity was observed in PC2, especially in the infrarenal neck (CC: 200/s; PC1 164/s; PC2: 98/s). Although WSS and TAWSS varied between configurations, values were in the within non-pathological range. The flow lumens created by three balloon configurations used in an invitro model of CAR have been studied, and resulted in different haemodynamics. The differences in velocity and lower vorticity, especially at the crossing section of the two balloons, showed that PC2 has favourable haemodynamics compared with the CC and PC1. Future research will be focused on the clinical applicability of CAR based on the PC2 design.

The Climatic Water Balance and Topography Control Spatial Patterns of Atmospheric Demand, Soil Moisture, and Shallow Subsurface Flow

AbstractCatchment hydrometeorology and the organization of shallow subsurface flow are key drivers of active contributing areas and streamflow generation. However, understanding how the climatic water balance and complex topography contribute to these processes from hillslope to catchment scales remains difficult. We compared time series of vapor pressure deficits and soil moisture to the climatic water balance and topographic variables across six zero‐order catchments in the Lubrecht Experimental Forest (Montana, USA). We then evaluated how local hydrometeorology (volumetric water content and atmospheric vapor pressure deficit) affected the spatial occurrence of shallow subsurface flow. Generalized linear mixed model analysis revealed significant, temporally stable (monthly and seasonal average) patterns of hydrometeorology that can be predicted by the topographic wetness index and the dynamic climatic water deficit (CWD = potential evapotranspiration − actual evapotranspiration). Intracatchment patterns were significantly correlated to the topographic wetness index, while intercatchment patterns were correlated to spatiotemporal variance in the CWD during each time period. Spatial patterns of shallow subsurface flow were related to the hydrometeorological conditions of the site. We observed persistent shallow subsurface flow in convergent hillslope positions, except when a catchment was positioned in locations with high CWDs (low elevations and southerly aspects). Alternatively, we observed persistent subsurface flow across all hillslope positions (even 70‐m upslope from the hollow) when catchments were positioned in locations with especially low CWDs (northerly aspects and high elevations). These results highlight the importance of considering the superposition of the catchment‐scale climatic water balance and hillslope‐scale topography when characterizing hydrometeorology and shallow subsurface flow dynamics.

Forecasting the spatiotemporal variability of soil CO2 emissions in sugarcane areas in southeastern Brazil using artificial neural networks.

Carbon dioxide (CO2) is considered one of the main greenhouse effect gases and contributes significantly to global climate change. In Brazil, the agricultural areas offer an opportunity to mitigate this effect, especially with the sugarcane crop, since, depending on the management system, sugarcane stores large amounts of carbon, thereby removing it from the atmosphere. The CO2 production in soil and its transport to the atmosphere are the results of biochemical processes such as the decomposition of organic matter and roots and the respiration of soil organisms, a phenomenon called soil CO2 emissions (FCO2). The objective of the study was to investigate the use of neural networks with backpropagation algorithm to predict the spatial patterns of soil CO2 emission during short periods in sugarcane areas. FCO2 values were collected in three commercial crop areas in the São Paulo state, southeastern Brazil, registered through the LI-8100 system during the years 2008 (Motuca), 2010 (Guariba city), and 2012 (Pradópolis), in the period after the mechanical harvesting (green cane). A neural network multilayer perceptron with a backpropagation algorithm was applied to estimate the FCO2 in 2012, using data from 2008 and 2010 as training for the neural network. The neural network initially presented a mean absolute percentage error (MAPE) of 18.3852 and a coefficient of determination (R2) of 0.9188. Data obtained from the observed and estimated values of FCO2 present moderate spatial dependence, and it is observed from the maps of the spatial pattern of the CO2 flow that the results from the neural network show considerable similarity to the observed data. The model results identify the higher and lower characteristics in sample points of CO2 emissions and produce an overestimation of the range of spatial dependence (0.45m) and an underestimation of the interpolated values in the field (R2 = 0.80; MAPE = 12.0591), when compared to the actual soil CO2 emission values. Therefore, the results indicate that the artificial neural network provides reliable estimates for the evaluation of FCO2 from data of the soil's physical and chemical attributes and describes the spatial variability of FCO2 in sugarcane fields, thereby contributing to the reduction of uncertainties associated with FCO2 accountings in these areas.

Dynamics of three-layer convection in a two-dimensional spherical domain with a growing innermost layer: Implications for whole solid-earth dynamics

I have performed numerical simulations of thermal convection with a highly viscous outermost layer (HVL), low-viscosity middle layer (LVL), and highly viscous innermost layer (IML) growing with time in a 2-D spherical domain. The results show that the average temperature of the LVL significantly decreases and the root-mean-square velocity of the LVL fluctuates significantly when the IML grows with time, compared to the model without the growing IML. This result suggests that the formation of the IML cools the LVL effectively. However, the existence of the growing IML barely affects the magnitude and spatial pattern of the heat flow at the HVL–LVL interface and the convection pattern of the HVL. In addition, the growing IML lengthens the lateral scale of the LVL convection owing to the basal heating from the LVL–IML interface. The present numerical results imply that the growing inner core, which corresponds to the IML in this model, throughout the Earth’s history may barely affect the convection pattern of the mantle (HVL) and the heat flow escaping from the outer core (i.e., LVL) to the mantle via the core–mantle boundary (i.e., the HVL–LVL interface). A previous geodynamo simulation suggested that the lateral thermal heterogeneity of the temperature just under the core–mantle boundary strengthens the geomagnetic field in the outer core. The present numerical results may suggest that the growing inner core throughout the Earth’s history facilitates the strong lateral thermal heterogeneity and strengthens the geomagnetic field.

Contrast-enhanced ultrasound to visualize hemodynamic changes after rodent spinal cord injury.

OBJECTIVE Traumatic spinal cord injury (tSCI) causes an almost complete loss of blood flow at the site of injury (primary injury) as well as significant hypoperfusion in the penumbra of the injury. Hypoperfusion in the penumbra progresses after injury to the spinal cord and is likely to be a major contributor to progressive cell death of spinal cord tissue that was initially viable (secondary injury). Neuroprotective treatment strategies seek to limit secondary injury. Clinical monitoring of the temporal and spatial patterns of blood flow within the contused spinal cord is currently not feasible. The purpose of the current study was to determine whether ultrafast contrast-enhanced ultrasound (CEUS) Doppler allows for detection of local hemodynamic changes within an injured rodent spinal cord in real time. METHODS A novel ultrafast CEUS Doppler technique was developed utilizing a research ultrasound platform combined with a 15-MHz linear array transducer. Ultrafast plane-wave acquisitions enabled the separation of higher-velocity blood flow in macrocirculation from low-velocity flow within the microcirculation (tissue perfusion). An FDA-approved contrast agent (microbubbles) was used for visualization of local blood flow in real time. CEUS Doppler acquisition protocols were developed to characterize tissue perfusion both during contrast inflow and during the steady-state plateau. A compression injury of the thoracic spinal cord of adult rats was induced using iris forceps. RESULTS High-frequency ultrasound enabled visualization of spinal cord vessels such as anterior spinal arteries as well as central arteries (mean diameter [± SEM] 145.8 ± 10.0 µm; 76.2 ± 4.5 µm, respectively). In the intact spinal cord, ultrafast CEUS Doppler confirmed higher perfusion of the gray matter compared to white matter. Immediately after compression injury of the thoracic rodent spinal cord, spinal cord vessels were disrupted in an area of 1.93 ± 1.14 mm2. Ultrafast CEUS Doppler revealed a topographical map of local tissue hypoperfusion with remarkable spatial resolution. Critical loss of perfusion, defined as less than 40% perfusion compared to the surrounding spared tissue, was seen within an area of 2.21 ± 0.6 mm2. CONCLUSIONS In our current report, we introduce ultrafast CEUS Doppler for monitoring of spinal vascular structure and function in real time. Development and clinical implementation of this type of imaging could have a significant impact on the care of patients with tSCI.

Reduced gene flow in a vulnerable species reflects two centuries of habitat loss and fragmentation

AbstractUnderstanding the effects of landscape modification on gene flow of fauna is central to informing conservation strategies that promote functional landscape connectivity and population persistence. We explored the effects of large‐scale habitat loss and fragmentation on spatial and temporal patterns of gene flow in a threatened Australian woodland bird: the Grey‐crowned Babbler Pomatostomus temporalis. Using microsatellite data, we (1) investigated historical (i.e., pre‐fragmentation) and contemporary (i.e., post‐fragmentation) levels of gene flow among subpopulations and/or regions, (2) identified first‐generation migrants and likely dispersal events, (3) tested for signatures of genetic bottlenecks, (4) estimated contemporary and historical effective population sizes, and (5) explored the relative influences of drift and migration in shaping contemporary population structure. Results indicated that the functional connectivity of landscapes used by the Grey‐crowned Babbler is severely compromised in the study area. The proportion of individuals that were recent immigrants among all subpopulations were low. Habitat fragmentation has led to a clear division between subpopulations in the east and west, and the patterns of gene flow exchange between these two regions have changed over time. The effective population size estimates for these two regions are now well below that required for long‐term population viability (Ne < 100). Demographic history models indicate that genetic drift was a greater influence on subpopulations than gene flow, and most subpopulations show signatures of bottlenecks. Translocations to promote gene flow and boost genetic diversity in the short term and targeted habitat restoration to improve landscape functional connectivity in the long term represent promising conservation management strategies that will likely have benefits for many other woodland bird species.

A Stepwise Spatio-Temporal Flow Clustering Method for Discovering Mobility Trends

Massive flows that represent the individual level of movements and communications can be easily obtained in the age of big data. Generalizing spatial and temporal flow patterns from such data is essential to demonstrate spatial connections and mobility trends. Clustering approaches provide effective methods to handle data sets that contain massive individual-level flows. However, existing flow clustering studies obscure the geometric properties of flow data, such as direction and length, which significantly indicate movement trends. In addition, temporal information is often ignored because previous approaches have mainly focused on the perspective of spatial clusters of flow data, resulting in a loss of temporal patterns. In this paper, we introduce new spatial and temporal similarity measurements between flows and propose a new clustering approach of flow data based on a stepwise strategy. This method can identify clusters from distinct flow distributions and discover significant spatio-temporal trends from large flow data. Simulated experiments with synthetic flows and a case study using Beijing taxi trip data are conducted to validate the usefulness of the proposed method.

Gas Dynamics of a Recessed Nozzle in Its Displacement in the Radial Direction

Numerical simulation of gasdynamic processes accompanying the operation of the recessed nozzle of a solid-propellant rocket motor in its linear displacement is carried out. Reynolds-averaged Navier–Stokes equations closed using the equations of a k–e turbulence model are used for calculations. The calculations are done for different rates of flow of the gas in the main channel and in the over-nozzle gap, and also for different displacements of the nozzle from an axisymmetric position. The asymmetry of geometry gives rise to a complicated spatial flow pattern characterized by the presence of singular points of spreading and by substantially inhomogeneous velocity and pressure distributions. The vortex flow pattern resulting from the linear displacement of the nozzle from an axisymmetric position is compared with the data of experimental visualization. The change in the vortex pattern of the flow and in the position of the singular points as a function of the flow coefficient and the displacement of the nozzle from the symmetry axis is discussed.

A 3-D numerical model of the influence of meanders on groundwater discharge to a gaining stream in an unconfined sandy aquifer

Groundwater discharge to streams depends on stream morphology and groundwater flow direction, but are not always well understood. Here a 3-D groundwater flow model is employed to investigate the impact of meandering stream geometries on groundwater discharge to streams in an unconfined and homogenous sandy aquifer at the reach scale (10–200m). The effect of meander geometry was examined by considering three scenarios with varying stream sinuosity. The interaction with regional groundwater flow was examined for each scenario by considering three groundwater flow directions. The sensitivity of stream morphology and flow direction to other parameters was quantified by varying the stream width, the meander amplitude, the magnitude of the hydraulic gradient, the hydraulic conductivity, and the aquifer thickness. Implications for a real stream were then investigated by simulating groundwater flow to a stream at a field site located in Grindsted, Denmark. The simulation of multiple scenarios was made possible by the employment of a computationally efficient coordinate transform numerical method. Comparison of the scenarios showed that the geometry of meanders greatly affect the spatial distribution of groundwater flow to streams. The shallow part of the aquifer discharges to the outward pointing meanders, while deeper groundwater flows beneath the stream and enters from the opposite side. The balance between these two types of flow depends on the aquifer thickness and meander geometry. Regional groundwater flow can combine with the effect of stream meanders and can either enhance or smooth the effect of a meander bend, depending on the regional flow direction. Results from the Grindsted site model showed that real meander geometries had similar effects to those observed for the simpler sinuous streams, and showed that despite large temporal variations in stream discharge, the spatial pattern of flow is almost constant in time for a gaining stream.

Regional air–sea coupled model simulation for two types of extreme heat in North China

Extreme heat (EH) over North China (NC) is affected by both large scale circulations and local topography, and could be categorized into foehn favorable and no-foehn types. In this study, the performance of a regional coupled model in simulating EH over NC was examined. The effects of regional air–sea coupling were also investigated by comparing the results with the corresponding atmosphere-alone regional model. On foehn favorable (no-foehn) EH days, a barotropic cyclonic (anticyclonic) anomaly is located to the northeast (northwest) of NC, while anomalous northwesterlies (southeasterlies) prevail over NC in the lower troposphere. In the uncoupled simulation, barotropic anticyclonic bias occurs over China on both foehn favorable and no-foehn EH days, and the northwesterlies in the lower troposphere on foehn favorable EH days are not obvious. These biases are significantly reduced in the regional coupled simulation, especially on foehn favorable EH days with wind anomalies skill scores improving from 0.38 to 0.47, 0.47 to 0.61 and 0.38 to 0.56 for horizontal winds at 250, 500 and 850 hPa, respectively. Compared with the uncoupled simulation, the reproduction of the longitudinal position of Northwest Pacific subtropical high (NPSH) and the spatial pattern of the low-level monsoon flow over East Asia are improved in the coupled simulation. Therefore, the anticyclonic bias over China is obviously reduced, and the proportion of EH days characterized by anticyclonic anomaly is more appropriate. The improvements in the regional coupled model indicate that it is a promising choice for the future projection of EH over NC.

Development and evaluation of microsatellite markers for the critically endangered birch Betula chichibuensis (Betulaceae).

Premise of the study:Microsatellite markers were developed and characterized for the critically endangered birch Betula chichibuensis (Betulaceae) to investigate the genetic structure of this species for conservation purposes.Methods and Results:Sixteen microsatellite markers with di-, tri-, and tetranucleotide repeat motifs were developed and optimized using MiSeq paired-end sequencing. Of these, 14 were polymorphic, with two to five alleles per locus, in 47 individuals from two newly discovered populations of B. chichibuensis in Japan. Observed and unbiased expected heterozygosities per locus ranged from 0.000 to 0.617 and from 0.000 to 0.629, respectively. These markers were tested for cross-species amplification in B. maximowicziana, B. platyphylla var. japonica, and B. schmidtii.Conclusions:This set of microsatellite markers, the first developed for B. chichibuensis, will help elucidate spatial patterns of gene flow and levels of inbreeding in this species to aid its conservation.

Comment on a paper by Schito et al. (2017) “Thermal evolution of Paleozoic successions of the Holy Cross Mountains (Poland)”

The paper by Schito et al. (2017) contains many misquotations of the regional literature on the Holy Cross Mountains, that may confuse the reader. Present comment suggests more appropriate references related to the stratigraphic, tectonic and palaeothermal aspects of the commented paper. The burial-thermal history as interpreted by Schito et al. is based on doubtful, poorly documented or even unsubstantiated thermal maturity and stratigraphical data, ignoring important regional evidence, such as Caledonian (sub-Devonian) unconformity and Permian thermal anomaly. The 1-D modeling study performed by Schito et al. (2017) did not consider published alternative concepts of the temporal and spatial heat flow patterns. The resulting models, assuming uniformly low heat flow between the Ordovician and the earliest Cretaceous, are inconsistent with independent regional data pointing to a hotter Variscan (Carboniferous-Permian) thermal regime.

Modelling and estimating modal share in European transport: a comparative analysis using interregional freight flow data

This paper explores the use of different statistical techniques to examine spatial flow patterns of freight transport among competing transport modes. Freight transport has certain peculiarities that are different from passenger transport. It is argued that a logit model, the most commonly used technique in empirical analysis of passenger flows, is not always appropriate for analysis of freight flows, unless the interdependence between the decisionmaking regarding the shipment of individual units of freight belonging to the same shipment is correctly modeled. The focus of this work is on analysis of aggregate freight transport flows of the type that may be generated from conventional spatial interaction models. In particular, adjusted estimation techniques alternative to the logit model are employed to analyze the transport flows of 2 products, chemical products and foodstuffs, based on a European interregional data set. Various caveats that arose during empirical analysis are also discussed.

Historical processes and contemporary ocean currents drive genetic structure in the seagrass Thalassia hemprichii in the Indo-Australian Archipelago.

Understanding spatial patterns of gene flow and genetic structure is essential for the conservation of marine ecosystems. Contemporary ocean currents and historical isolation due to Pleistocene sea level fluctuations have been predicted to influence the genetic structure in marine populations. In the Indo-Australian Archipelago (IAA), the world's hotspot of marine biodiversity, seagrasses are a vital component but population genetic information is very limited. Here, we reconstructed the phylogeography of the seagrass Thalassia hemprichii in the IAA based on single nucleotide polymorphisms (SNPs) and then characterized the genetic structure based on a panel of 16 microsatellite markers. We further examined the relative importance of historical isolation and contemporary ocean currents in driving the patterns of genetic structure. Results from SNPs revealed three population groups: eastern Indonesia, western Indonesia (Sunda Shelf) and Indian Ocean; while the microsatellites supported five population groups (eastern Indonesia, Sunda Shelf, Lesser Sunda, Western Australia and Indian Ocean). Both SNPs and microsatellites showed asymmetrical gene flow among population groups with a trend of southwestward migration from eastern Indonesia. Genetic diversity was generally higher in eastern Indonesia and decreased southwestward. The pattern of genetic structure and connectivity is attributed partly to the Pleistocene sea level fluctuations modified to a smaller level by contemporary ocean currents.

Spatial and temporal dynamics of groundwater flow across a wet meadow, Polar Bear Pass, Bathurst island, Nunavut

Interest is growing about how groundwater supplies will shift in warming northern terrains. We evaluated the seasonal and spatial pattern of groundwater flow in a wet meadow bordered by a late-lying snowbed and tundra ponds at Polar Bear Pass, Bathurst Island (75.7°N, 98.7°W). A water budget approach signalled the relative importance of groundwater inflow to tundra ponds. Groundwater flow across the wet meadow was estimated using a modified Darcy's equation, which requires information on both water and frost tables, and hydraulic conductivity. These data were obtained from 2007 to 2009 along a series of water wells extending from a late-lying snowbed across the wet meadow to a nearby study pond. Groundwater fluxes across the wet meadow were limited in magnitude and duration in a warm/dry year (2007), when the late-lying snowbed was the main external water source, a response differing from rainy/cool years (2008 and 2009). Overall, seasonal water budgets indicate that groundwater fluxes were minimal in the wet meadow and an adjacent tundra pond. Late-lying snowbeds play a limited role in sustaining wet meadows and ponds here. Summer precipitation and evaporation continue to drive wet meadow and tundra pond hydrological response in low-gradient wetlands, especially in the post-snowmelt season. Copyright © 2016 John Wiley & Sons, Ltd.

Spatial-temporal traffic flow pattern identification and anomaly detection with dictionary-based compression theory in a large-scale urban network

Traffic flow pattern identification, as well as anomaly detection, is an important component for traffic operations and control. To reveal the characteristics of regional traffic flow patterns in large road networks, this paper employs dictionary-based compression theory to identify the features of both spatial and temporal patterns by analyzing the multi-dimensional traffic-related data. An anomaly index is derived to quantify the network traffic in both spatial and temporal perspectives. Both pattern identifications are conducted in three different geographic levels: detector, intersection, and sub-region. From different geographic levels, this study finds several important features of traffic flow patterns, including the geographic distribution of traffic flow patterns, pattern shifts at different times-of-day, pattern fluctuations over different days, etc. Both spatial and temporal traffic flow patterns defined in this study can jointly characterize pattern changes and provide a good performance measure of traffic operations and management. The proposed method is further implemented in a case study for the impact of a newly constructed subway line. The before-and-after study identifies the major changes of surrounding road traffic near the subway stations. It is found that new metro stations attract more commute traffic in weekdays as well as entertaining traffic during weekends.

Profiles evolutionary design and evaluation of mixing performance of screw elements for intermeshing counter-rotating twin-screw kneader

The profile designs of screw elements are very crucial to improving the mixing performance for the intermeshing counter-rotating twin-screw kneader. In order to find the inherent law among different profiles of screw elements and to derive a mathematical model which evolves different types of end cross-section profiles of screw elements, a universal mathematical model of end cross-section profiles of screw elements is presented in this work. Different types of profiles of screw elements, including those traditionally used can be obtained by evolutionary design method after changing the tooth number of female and male elements and other design parameters. Several typical screw elements with different types of end cross-section profiles are selected to analyze the mixing performance. Spatial flow patterns in flow channel of the screw elements are presented with particle tracking analysis by using mesh superposition technology. The mixing performance of different screw elements including distributive and dispersive mixing, contrastive analysis for some parameters, such as residence time distribution density function, segregation scale, rate of stretching, rate of shear, etc. are proposed. The results in this paper provide a data basis for the profiles selection of screw element for intermeshing counter-rotating twin-screw kneader.

Divergence history of the Carpathian and smooth newts modelled in space and time.

Information about demographic history is essential for the understanding of the processes of divergence and speciation. Patterns of genetic variation within and between closely related species provide insights into the history of their interactions. Here, we investigated historical demography and genetic exchange between the Carpathian (Lissotriton montandoni, Lm) and smooth (L.vulgaris, Lv) newts. We combine an extensive geographical sampling and multilocus nuclear sequence data with the approximate Bayesian computation framework to test alternative scenarios of divergence and reconstruct the temporal and spatial pattern of gene flow between species. A model of recent (last glacial period) interspecific gene flow was favoured over alternative models. Thus, despite the relatively old divergence (4-6mya) and presumably long periods of isolation, the species have retained the ability to exchange genes. Nevertheless, the low migration rates (ca. 10(-6) per gene copy per generation) are consistent with strong reproductive isolation between the species. Models allowing demographic changes were favoured, suggesting that the effective population sizes of both species at least doubled as divergence reaching the current ca. 0.2 million in Lm and 1 million in Lv. We found asymmetry in rates of interspecific gene flow between Lm and one evolutionary lineage of Lv. We suggest that intraspecific polymorphism for hybrid incompatibilities segregating within Lv could explain this pattern and propose further tests to distinguish between alternative explanations. Our study highlights the importance of incorporating intraspecific genetic structure into the models investigating the history of divergence.

Surface circulation in Block Island Sound and adjacent coastal and shelf regions: A FVCOM-CODAR comparison

CODAR-derived surface currents in Block Island Sound over the period of June 2000 through September 2008 were compared to currents computed using the Northeast Coastal Ocean Forecast System (NECOFS). The measurement uncertainty of CODAR-derived currents, estimated using statistics of a screened nine-year time series of hourly-averaged flow field, ranged from 3 to 7cm/s in speed and 4° to 14° in direction. The CODAR-derived and model-computed kinetic energy spectrum densities were in good agreement at subtidal frequencies, but the NECOFS-derived currents were larger by about 28% at semi-diurnal and diurnal tidal frequencies. The short-term (hourly to daily) current variability was dominated by the semidiurnal tides (predominantly the M2 tide), which on average accounted for ∼87% of the total kinetic energy. The diurnal tidal and subtidal variability accounted for ∼4% and ∼9% of the total kinetic energy, respectively. The monthly-averaged difference between the CODAR-derived and model-computed velocities over the study area was 6cm/s or less in speed and 28° or less in direction over the study period. An EOF analysis for the low-frequency vertically-averaged model current field showed that the water transport in the Block Island Sound region was dominated by modes 1 and 2, which accounted for 89% and 7% of the total variance, respectively. Mode 1 represented a relatively stationary spatial and temporal flow pattern with a magnitude that varied with season. Mode 2 was characterized mainly by a secondary cross-shelf flow and a relatively strong along-shelf flow. Process-oriented model experiments indicated that the relatively stationary flow pattern found in mode 1 was a result of tidal rectification and its magnitude changed with seasonal stratification. Correlation analysis between the flow and wind stress suggested that the cross-shelf water transport and its temporal variability in mode 2 were highly correlated to the surface wind forcing. The mode 2 derived onshore and offshore water transport, and was consistent with wind-driven Ekman theory. The along-shelf water transport over the outer shelf, where a large portion of the water flowed from upstream Nantucket Shoals, was not highly correlated to the surface wind stress.