Large Number Of Stations Research Articles

EVALUATING THE QUALITY OF CARGO DELIVERY USING THE COEFFICIENT OF DEVIATION IN THE ARRIVAL TIME OF TRAINS

Purpose. The research is aimed at considering the features of calculating the coefficient of deviation in the arrival time of trains to the station from the planned one relative to the expected deviation. Currently, for the management of cargo transportation, one uses the approaches in which such an indicator as the coefficient of deviation is not calculated and not taken into account. In practical implementation, this leads to the fact that the railway transporting cargo does not receive information on the ratio of nominal time deviations to real ones. This situation requires solving the problems of improving the current technologies of the transportation process related to the formation, organization and shipment of cargoes. Therefore, the requirements for the calculation of such factors are relevant. Methodology. To achieve the purpose, it is necessary to rank the collected statistics, group them and analyze them. A mathematical model is proposed for calculating the deviation of the train arrival time from the planned one and the dependence of this deviation from the delivery stage (station) and the number of trains (as a percentage of their total number). Findings. The concept of «deviation coefficient» has been introduced, which characterizes the deviation of the time of train arrival to a station from the planned relative to the expected deviation. Based on the results of the analysis of the deviation of the train arrival time at the station depending on the run, it can be noted that there is a direct relationship between the lateness of trains in hours and the station number (that is, distance). The result remains valid for an arbitrary number of trains (0, 25, 50, 75 and 100% were considered). Originality. The author first introduced the concept of “deviation coefficient”, which characterizes the deviation of the time of train arrival to a station from the planned relative to the expected deviation. Practical value. Based on the presented mathematical model, it is possible to foresee an approximate value of the lateness of trains for a railway line with a large number of stations, if the main characteristics of cargo transportation remain without significant changes, for example, a significant improvement or deterioration of the technical characteristics of the railway infrastructure.

Time-of-day/time-of-year response functions of planetary geomagnetic indices

Aims: To elucidate differences between commonly-used mid-latitude geomagnetic indices and study quantitatively the differences in their responses to solar forcing as a function of Universal Time (UT), time-of-year (F), and solar-terrestrial activity level. To identify the strengths, weaknesses and applicability of each index and investigate ways to correct for any weaknesses without damaging their strengths. Methods: We model how the location of a geomagnetic observatory influences its sensitivity to solar forcing. This modelling for a single station can then be applied to indices that employ analytic algorithms to combine data from different stations and thereby we derive the patterns of response of the indices as a function of UT, F and activity level. The model allows for effects of solar zenith angle on ionospheric conductivity and of the station’s proximity to the midnight-sector auroral oval: it employs coefficients that are derived iteratively by comparing data from the current aa index stations (Hartland and Canberra) to simultaneous values of the am index, constructed from chains of stations in both hemispheres. This is done separately for eight overlapping bands of activity level, as quantified by the am index. Initial estimates were obtained by assuming the am response is independent of both F and UT and the coefficients so derived were then used to compute a corrected F-UT response pattern for am. This cycle was repeated until it resulted in changes in predicted values that were below the adopted uncertainty level (0.001%). The ideal response pattern of an index would be uniform and linear (i.e., independent of both UT and F and the same at all activity levels). We quantify the response uniformity using the percentage variation at any activity level, V = 100 (σS/〈S〉), where S is the index’s sensitivity at that activity level and σS is the standard deviation of S: both S and σS were computed using the eight UT ranges of the 3-hourly indices and 20 equal-width ranges of F. As an overall metric of index performance, we take an occurrence-weighted mean of V, Vav, over the eight activity-level bins. This metric would ideally be zero and a large value shows that the index compilation is introducing large spurious UT and/or F variations into the data. We also study index performance by comparisons with the SME and SML indices, compiled from a very large number of stations, and with an optimum solar wind “coupling function”, derived from simultaneous interplanetary observations. Results: It is shown that a station’s response patterns depend strongly on the level of geomagnetic activity because at low activity levels the effect of solar zenith angle on ionospheric conductivity dominates over the effect of station proximity to the midnight-sector auroral oval, whereas the converse applies at high activity levels. The metric Vav for the two-station aa index is modelled to be 8.95%, whereas for the multi-station am index it is 0.65%. The ap (and hence Kp) index cannot be analyzed directly this way because its construction employs tabular conversions, but the very low Vav for am allows us to use 〈ap〉/〈am〉 to evaluate the UT-F response patterns for ap. This yields Vav = 11.20% for ap. The same empirical test applied to the classical aa index and the new “homogenous” aa index, aaH (derived from aa using the station sensitivity model), yields Vav of, respectively, 10.62% (i.e., slightly higher than the modelled value) and 5.54%. The ap index value of Vav is shown to be high because it exaggerates the average semi-annual variation and has an annual variation giving a lower average response in northern hemisphere winter. It also contains a strong artefact UT variation. We derive an algorithm for correcting for this uneven response which gives a corrected ap value, apC, for which Vav is reduced to 1.78%. The unevenness of the ap response arises from the dominance of European stations in the network used and the fact that all data are referred to a European station (Niemegk). However, in other contexts, this is a strength of ap, because averaging similar data gives increased sensitivity and more accurate values on annual timescales, for which the UT-F response pattern is averaged out.

Performance analysis of IEEE 802.11ah wireless local area network under the restricted access window‐based mechanism

SummaryIEEE 802.11ah is an approved amendment to IEEE 802.11 wireless local area network (WLAN) standard to support growing demand for machine‐to‐machine (M2M) applications. To enable an efficient scheme for accessing the channel by a large number of stations (STAs) within the coverage of an access point, 802.11ah has developed a novel mechanism known as Restrictred Access Window (RAW). Here, a group of STAs attempts channel access during their designated RAW slot by following the carrier sense multiple access‐collission avoidance (CSMA‐CA)‐based enhanced distributed channel access (EDCA) mechanism. In this paper, we develop an analytical model for finding the nonsaturation/saturation throughput of 802.11ah WLAN under the RAW‐based channel access mechanism. We describe an analytical model for finding the average frame delay as well. The impact of the number of STAs and number of groups on network throughput and average delay are presented. We establish that the RAW mechanism can improve the throughput while the average frame delay gets reduced. The analytical results are validated by extensive simulation studies.

SAMU: design and implementation of frequency selectivity-aware multi-user MIMO for WLANs

The traffic demand of wireless networks is expected to increase 1000-fold over the next decade. In anticipation of such increasing data demand for dense networks with a large number of stations, IEEE 802.11ax has introduced key technologies for capacity improvement including Orthogonal Frequency-Division Multiple Access (OFDMA), multi-user multi-input multi-output (MU-MIMO), and greater bandwidth. However, IEEE 802.11ax has yet to fully define a specific scheduling framework, on which the throughput improvement of networks significantly depends. Even within a 20 MHz of bandwidth, users experience heterogeneous channel orthogonality characteristics across sub-carriers, which prevents access points (APs) from achieving the ideal multi-user gain. Moreover, frequency selectivity increases as bandwidth scales and correspondingly severely deteriorates multi-user MIMO performance. In this work, we develop a novel channel adaptation scheme, named selectivity-aware multi-user MIMO (SAMU), to combat the issue of frequency selectivity and support coexistence among users in the network by jointly assigning subsets of sub-carriers to selected users and implementing downlink MU-MIMO. To do so, we first investigate the channel characteristics of an indoor environment. We then consider the frequency selectivity of current and emerging WiFi channel bandwidths to optimize multi-user MIMO by dividing the occupied sub-carrier resources into equally sized sub-channels according to the level of frequency selectivity. In our design, each sub-channel is allocated according to the largest bandwidth that can be considered frequency-flat, and an optimal subset of users is chosen to serve in each sub-channel according to spatial orthogonality. As a result, we support more simultaneous users than current 802.11 designs and achieve a significant performance improvement for all users in the network. Additionally, we propose a selectivity-aware high efficiency (SA-HE) mode, which is based on and fully backward compatible with the existing IEEE 802.11ax standard. Finally, over emulated and real indoor channels, we show that SAMU can achieve as much as 84.8% throughput improvement compared to existing multi-user MIMO schemes in IEEE 802.11ax.

Evaluation of SMAP Soil Moisture Relative to Five Other Satellite Products Using the Climate Reference Network Measurements Over USA

Satellite platforms provide a unique opportunity to retrieve global soil moisture. The Level 3 soil moisture (L3SMP), enhanced Level 3 soil moisture (L3SMP-E) and Level 4 surface soil moisture (L4SM) products of the latest soil moisture satellite mission, Soil Moisture Active Passive (SMAP), are evaluated relative to five other satellite products using the Climate Reference Network (CRN) with more than 110 stations (each with three in situ probes) over USA from 2009 to present. This large number of stations allows for the categorization of SMAP performance based on land cover types, complementing prior efforts based on the few core validation areas with many in situ observations within a single satellite pixel. The SMAP as well as Aquarius products clearly outperform the other products. Over all land cover types, L3SMP, L3SMP-E, and L4SM are better in the summer than in the winter and they perform best over short vegetation. L4SM has higher correlations compared with CRN than L3SMP over tall and short vegetation whereas L3SMP has higher correlations over crops. On average, L3SMP-E performs as well as L3SMP. There is a mismatch between the point in situ measurements and satellite pixel retrievals driven by subpixel precipitation variability, and its impact on these results is also assessed.

CHANGES IN FREQUENCY OF OCCURRENCE OF YEARLY PRECIPITATION ANOMALIES IN THE LUBLIN REGION IN THE SECOND HALF OF THE 20TH CENTURY

The aim of this paper is to characterise changes in frequency of occurrence of anomalous yearly sums of atmospheric precipitation in the Lublin region in the period 1951–2000. In the second half of the 20th century negative precipitation anomaly occurred in a larger number of stations than the positive anomaly. Particularly strong anomalies were observed in two decades (1951–1960 and 1981–1990). However, during two following decades (1961–1970 and 1971–1980) the positive anomaly occurred in more stations.

Energy Dispersive X-ray technique applied to fatigue fracture surfaces oxidation to provide information on growth rate

Gas turbine components are strongly affected by cyclic loading during operation, which makes fatigue development the most significant cause of failure or cracking within them. The nature and growth rate of that fatigue is dependent on the stress experienced at a particular area of a part, the temperature, the material properties and the applied loading frequency. Various techniques, such as striation and/or band counting can, in some cases, provide information on fatigue life. However, often such techniques are rendered useless by lack of formation or obliteration by damage and/or oxidation of such features. Once a crack has initiated and is exposed to the engine atmosphere, the level of oxidation and other contaminant elements observed on the crack surface will evolve with time, and, correspondingly, crack depth. Element distribution analysis by an Energy Dispersive x-ray Spectroscopy (EDS) technique applied along a fatigue fracture surface could help to understand the evolution of the crack with time. Using EDS, an experimental procedure was carried out to ascertain a perceived measured percentage of oxygen plus other additional chemical species at a large number of stations along the length of a fatigue crack. The analysis was performed from the initiation site to the end of the propagation area, and also within the final rupture zone which had less time exposed to air and gasses compared to the fatigue system. An exercise of comparison between that progression in oxidation across cracked in-service components versus perceived measured oxidation level analyses on trial fracture surfaces exposed in atmospheric oven conditions at high temperatures for a number of different durations was performed. The obtained results provided information on specific questions concerning the fatigue propagation life of the in-service components; thereby amplifying the use of the EDS technique in this aspect of the materials forensic field.

Early winners and losers in dialysis center pay-for-performance

BackgroundWe examined the association of dialysis facility characteristics with payment reductions and change in clinical performance measures during the first year of the United States Centers for Medicare & Medicaid Services (CMS) End Stage Renal Disease Quality Incentive Plan (ESRD QIP) to determine its potential impact on quality and disparities in dialysis care.MethodsWe linked the 2012 ESRD QIP Facility Performance File to the 2007–2011 American Community Survey by zip code and dichotomized the QIP total performance scores—derived from percent of patients with urea reduction rate > 65, hemoglobin < 10 g/dL, and hemoglobin > 12 g/dL—as ‘any’ versus ‘no’ payment reduction. We characterized associations between payment reduction and dialysis facility characteristics and neighborhood demographics, and examined changes in facility outcomes between 2007 and 2010.ResultsIn multivariable analysis, facilities with any payment reduction were more likely to have longer operation (OR 1.03 per year), a medium or large number of stations (OR 1.31 and OR 1.42, respectively), and a larger proportion of African Americans (OR 1.25, highest versus lowest quartile), all p < 0.05. Most improvement in clinical performance was due to reduced overtreatment of anemia, a decline in the percentage of patients with hemoglobin ≥ 12 g/dL; for-profits and facilities in African American neighborhoods had the greatest reduction.ConclusionsIn the first year of CMS pay-for-performance, most clinical improvement was due to reduced overtreatment of anemia. Facilities in African American neighborhoods were more likely to receive a payment reduction, despite their large decline in anemia overtreatment.

A multi-station block acknowledgment scheme in dense IoT networks

Recently, the IEEE started a new task group, i.e., the IEEE 802.11ax, to investigate and deliver next generation WLAN technologies for the scenarios of dense IoT (Internet of Things) networks with a large number of stations and access points. Indeed, accompanying with the fast growing density of nodes, new technologies have to be proposed to handle the emerging issues in dense IoT environment which are usually not existent in traditional networks under sparse or medium density. In this paper, to reduce the significant communication overheads and latency especially in dense IoT networks, a block acknowledgement scheme for uplink single-user transmissions is presented. By shortening the inter-frame interval, decreasing the access attempts and compressing the frame length, our proposed model could greatly improve the MAC efficiency gain compared with traditional schemes. In addition, by combining the traditional BA (Block Acknowledgment) and Multi-STA BA (Multi-station Block Acknowledgement) strategies together in our model, the overall experienced latency by multiple stations’ access as well as the end-to-end delay for each station could be greatly reduced. Simulation results show that our scheme could outperform the traditional acknowledgment protocols in IEEE 802.11 in terms of MAC efficiency gain and experienced delay.

Complex networks for rainfall modeling: Spatial connections, temporal scale, and network size

We apply the concepts of complex networks to investigate the properties of rainfall. Specifically, we examine the rainfall properties in terms of spatial connections, temporal scale, and network size. We employ the clustering coefficient method to rainfall data at six different temporal scales (daily, 2-day, 4-day, 8-day, 16-day, and monthly) from a large number of stations in the Murray-Darling basin in Australia. We consider different correlation thresholds to identify the existence of links between stations. To account for the influence of network size (i.e. number of stations) and length of data, we consider three different networks: (1) 430 stations with 30years of daily data; (2) 383 stations with 30years of daily data; and (3) 383 stations with 64years of daily data. The results indicate that the nature of spatial connections changes with correlation threshold, with changes occurring at different temporal scales for different thresholds. Identification of an appropriate threshold is key to understand the rainfall connectivity properties.

Parameterizing surface wind speed over complex topography

AbstractSubgrid parameterizations are used in coarse‐scale meteorological and land surface models to account for the impact of unresolved topography on wind speed. While various parameterizations have been suggested, these were generally validated on a limited number of measurements in specific geographical areas. We used high‐resolution wind fields to investigate which terrain parameters most affect near‐surface wind speed over complex topography under neutral conditions. Wind fields were simulated using the Advanced Regional Prediction System (ARPS) on Gaussian random fields as model topographies to cover a wide range of terrain characteristics. We computed coarse‐scale wind speed, i.e., a spatial average over the large grid cell accounting for influence of unresolved topography, using a previously suggested subgrid parameterization for the sky view factor. We only require correlation length of subgrid topographic features and mean square slope in the coarse grid cell. Computed coarse‐scale wind speed compared well with domain‐averaged ARPS wind speed. To further statistically downscale coarse‐scale wind speed, we use local, fine‐scale topographic parameters, namely, the Laplacian of terrain elevations and mean square slope. Both parameters showed large correlations with fine‐scale ARPS wind speed. Comparing downscaled numerical weather prediction wind speed with measurements from a large number of stations throughout Switzerland resulted in overall improved correlations and distribution statistics. Since we used a large number of model topographies to derive the subgrid parameterization and the downscaling framework, both are not scale dependent nor bound to a specific geographic region. Both can readily be implemented since they are based on easy to derive terrain parameters.

Evaluating and Improving the Impact of the Atmospheric Stability and Orography on Surface Winds in the WRF Model

AbstractThis study assesses the impact of the atmospheric stability on the turbulent orographic form drag (TOFD) generated by unresolved small-scale orography (SSO) focusing on surface winds. With this aim, several experiments are conducted with the Weather Research and Forecasting (WRF) Model and they are evaluated over a large number of stations (318 at 2-m height) in the Iberian Peninsula with a year of data. In WRF, Jiménez and Dudhia resolved the SSO by including a factor in the momentum equation, which is a function of the orographic variability inside a grid cell. It is found that this scheme can improve the simulated surface winds, especially at night, but it can underestimate the winds during daytime. This suggests that TOFD can be dependent on the PBL’s stability. To inspect and overcome this limitation, the stability conditions are included in the SSO parameterization to maintain the intensity of the drag during stable conditions while attenuating it during unstable conditions. The numerical experiments demonstrate that the inclusion of stability effects on the SSO drag parameterization improves the simulated surface winds at diurnal, monthly, and annual scales by reducing the systematic daytime underestimation of the original scheme. The correction is especially beneficial when both the convective velocity and the boundary layer height are used to characterize the unstable conditions.

Identification of Streamflow Changes across the Continental United States Using Variable Record Lengths

The study focused on investigating the presence of change patterns in 600 unimpaired streamflow stations across the continental U.S. at different time intervals to understand the change patterns that can provide significant insight regarding climate variability and change. Each station had continuous streamflow data of at least 30 years (the entire dataset covered a range of 109 years). Presence of trends and shifts were detected in water year and the four seasons (fall, winter, spring, and summer) analyzing the water year and seasonal mean flows. Two non-parametric tests, namely, the Mann-Kendall test and the Pettitt’s test were used to identify the trends and the shifts, respectively. The results showed an increasing trend in the northeast and upper-mid regions, whereas southeast and northwest regions underwent a decrease. Shifts followed similar patterns as trends with higher number of stations with significant change. Fall and spring showed the highest number of stations with increasing and decreasing change, respectively, in the seasonal analyses. Results of this study may assist water managers to understand the streamflow change patterns across the continental U.S., especially at the regional scale since this study covers a long range of years with a large number of stations in each region.

Throughput Matching Algorithm and Stochastic Models for Analysis of Open and Closed Manufacturing Systems

Order release policies such as card-controlled CONWIP policy aim at improving system responsiveness and minimizing system-wide inventory levels. It is not clear if order release policies (without card control) can be equally effective under certain settings of the production system design parameters. In this research, we study the performance of alternate order release (material control policies) under a variety of design parameter settings such as number of stations, station service time characteristics, and the location of bottleneck station using queuing network models. To compare CONWIP policy (closed system) with open control policies (such as deterministic start times), we develop a throughput matching algorithm. The throughput times in CONWIP system is about 2%-50% less in comparison to open control systems; however, we show that the number of stations and the location of the bottleneck station affects the choice of the order release policy, and the benefit of card-controlled policies diminishes for a system with large number of stations (the throughput time percentage benefit using CONWIP in comparison to open control is 1/K, where K is the number of stations). We also analyze the system for a variety of demand inter-arrival times and check its effect on the expected number of backlogs and system wide expected waiting time.

Performance report of the RHUM-RUM ocean bottom seismometer network around La Réunion, western Indian Ocean

Abstract. RHUM-RUM is a German-French seismological experiment based on the sea floor surrounding the island of La Réunion, western Indian Ocean (Barruol and Sigloch, 2013). Its primary objective is to clarify the presence or absence of a mantle plume beneath the Reunion volcanic hotspot. RHUM-RUM's central component is a 13-month deployment (October 2012 to November 2013) of 57 broadband ocean bottom seismometers (OBS) and hydrophones over an area of 2000 × 2000 km2 surrounding the hotspot. The array contained 48 wideband OBS from the German DEPAS pool and 9 broadband OBS from the French INSU pool. It is the largest deployment of DEPAS and INSU OBS so far, and the first joint experiment. This article reviews network performance and data quality: of the 57 stations, 46 and 53 yielded good seismometer and hydrophone recordings, respectively. The 19 751 total deployment days yielded 18 735 days of hydrophone recordings and 15 941 days of seismometer recordings, which are 94 and 80 % of the theoretically possible yields. The INSU seismic sensors stand away from their OBS frames, whereas the DEPAS sensors are integrated into their frames. At long periods (&gt; 10 s), the DEPAS seismometers are affected by significantly stronger noise than the INSU seismometers. On the horizontal components, this can be explained by tilting of the frame and buoy assemblage, e.g. through the action of ocean-bottom currents, but in addition the DEPAS intruments are affected by significant self-noise at long periods, including on the vertical channels. By comparison, the INSU instruments are much quieter at periods &gt; 30 s and hence better suited for long-period signals studies. The trade-off of the instrument design is that the integrated DEPAS setup is easier to deploy and recover, especially when large numbers of stations are involved. Additionally, the wideband sensor has only half the power consumption of the broadband INSU seismometers. For the first time, this article publishes response information of the DEPAS instruments, which is necessary for any project where true ground displacement is of interest. The data will become publicly available at the end of 2017.

Design rainfall in Qatar: sensitivity to climate change scenarios

Design rainfall is needed in the design of numerous engineering infrastructures such as urban drainage systems, bridges, railways, metro systems, highways and flood levees. Design rainfall is derived using regional frequency analysis approach based on observed rainfall data from a large number of stations within a homogeneous region. This paper provides an assessment of the possible impacts of climate change on design rainfalls in Qatar. The future climate conditions are established based on AR4 and A2 categories of emission scenarios (SRES) specified by the Intergovernmental Panel on Climate Change. Predicted 24-h annual maximum rainfall series for both the wet (NCAR-CCSM) and dry scenarios (CSIRO-MK3.5) for the Qatari grid points are extracted for three different periods, which are current (2000–2029), medium-term (2040–2069) and end-of-century climates (2080–2099). Using an L-moments-based index frequency approach, homogeneous regions are established and the best-fit distribution is then used to derive rainfall quantiles for average recurrence intervals (ARIs) of 2, 5, 10, 25, 50 and 100 years. The results show that there is no significant change in the design rainfalls in Qatar in the short term covering 2040–2069; however, a significant change is predicted at the end of century covering 2080–2099. Updated design rainfalls are estimated considering climate change scenarios for the period of 2080–2099 by averaging results from the wet and dry climate scenarios. The increase in 24-h annual maximum rainfall for the period 2080–2099 (compared with the current period 2000–2029) is found to be in the range of 68 and 76 % for 100-year ARI. For the typical design ARIs of 10–20 years, the increase in design rainfall is found to be in the range of 43 and 54 %. The method presented in this study can be applied to other arid regions, in particular to the Middle Eastern countries.

Direct simulation Monte Carlo method with a focal mechanism algorithm

To simulate the observation of the radiation pattern of an earthquake, the direct simulation Monte Carlo (DSMC) method is modified by implanting a focal mechanism algorithm. We compare the results of the modified DSMC method (DSMC-2) with those of the original DSMC method (DSMC-1). DSMC-2 shows more or similarly reliable results compared to those of DSMC-1, for events with 12 or more recorded stations, by weighting twice for hypocentral distance of less than 80 km. Not only the number of stations, but also other factors such as rough topography, magnitude of event, and the analysis method influence the reliability of DSMC-2. The most reliable result by DSMC-2 is obtained by the best azimuthal coverage by the largest number of stations. The DSMC-2 method requires shorter time steps and a larger number of particles than those of DSMC-1 to capture a sufficient number of arrived particles in the small-sized receiver.

A100 Key factors influencing air quality in rail subway systems

Abstract Background Air quality plans incentivise the use of public transport to abate atmospheric emissions from private road vehicles in cities. In this context rail subway systems are especially desirable as they are based on electric trains, are energetically/environmentally efficient, and help relieve surface traffic congestion. However, a number of studies have revealed poor air quality underground, especially concerning levels of Particulate Matter. Interestingly, some subway systems appear to be worse than others in terms of particle loading, and it is clearly necessary to identify the key factors involved. Methods Ambient subway aerosol particles present in trains and on platforms in the Barcelona metro system have been (and are being) intensively monitored, sampled and physico-chemically characterised, and different types of stations compared. Air monitoring equipment includes high volume samplers for chemical analysis of particles below 2.5 μm, a light-scattering laser photometer for particle concentrations in 3 sizes, and an indoor air quality meter for carbon oxides. Results Outdoor concentrations of traffic particles and gases in Barcelona city do not significantly influence subway air quality, with most particles being ferruginous and sourced underground. Air quality is better inside trains than on platforms, although passenger density clearly affects carbon dioxide levels in both. Stations with cleanest particle concentrations are those fitted with new platform screen door systems, whereas the poorest air quality is associated with older-style narrow single track-tunnel-platform designs. Particle concentration varies along the platform, being influenced by the number and position of passenger exit tunnels, the train frequency and travel direction. Particle composition and morphologies are similar in all stations, indicating a common mechanism of formation, and they are physically and chemically distinctive from particles breathed in outdoor urban air. The iron-rich particles are mostly nanometric in size and derive from mechanical processes of sliding and wear at the brake-wheel and wheel-rail interface, with minor contributions from high temperature processes such as sparking. Conclusions Air quality in the subway environment is an issue of concern due to the high particle concentrations that can be registered, especially on platforms. The variables influencing these concentrations include type and intensity of ventilation, station design, number and location of accesses to platforms and number of trains and passengers. The detailed study of these parameters in a large number of stations within the ongoing research projects METRO and IMPROVE LIFE is enabling us to develop protocols aimed at producing discernible improvements to rail subway air quality.

Trends in indices of daily temperature and precipitations extremes in Morocco

The purpose of this paper is to provide a summary of Morocco’s climate extreme trends during the last four decades. Indices were computed based on a daily temperature and precipitation using a consistent approach recommended by the ETCCDI. Trends in these indices were calculated at 20 stations from 1970 to 2012. Twelve indices were considered to detect trends in temperature. A large number of stations have significant trends and confirm an increase in temperature, showing increased warming during spring and summer seasons. The results also show a decrease in the number of cold days and nights and an increase in the number of warm days and nights. Increasing trends have also been found in the absolute warmest and coldest temperatures of the year. A clear increase is detected for warm nights and diurnal temperature range. Eight indices for precipitation were also analyzed, but the trends for these precipitation indices are much less significant than for temperature indices and show more mixed spatial patterns of change. Heavy precipitation events do not exhibit significant trends except at a few locations, in the north and central parts of Morocco, with a general tendency towards drier conditions. The correlation between these climate indices and the large-scale atmospheric circulations indices such as the NAO, MO, and WEMO were also analyzed. Results show a stronger relationship with these climatic indices for the precipitation indices compared to the temperature indices. The correlations are more significant in the Atlantic regions, but they remain moderate at the whole country scale.

A Simple but Effective Contention Aware and Adaptive Back-off Mechanism for Improving the Performance of IEEE 802.11 DCF

In IEEE 802.11 standard, Distributed Coordination Function (DCF) is used as a primary medium access mechanism to share the common wireless medium. It is now well established that performance of the DCF degrades, especially when there is a large number of stations in the network contending for the wireless medium. This occurs due to the traditional parameter setting of its Binary Exponential Back-off (BEB) mechanism, which is used for collision avoidance. Therefore, in this paper, we propose an effective Contention Window (CW) and Contention Slot Selection (CSS) mechanism, named Contention Aware and Adaptive Back-off (CAAB) mechanism, to enhance performance of the IEEE 802.11 DCF. This mechanism differs from the standard back-off mechanism in two ways. First, contention window is not reset after successful transmission. Instead, stations will be allowed to go back to the possible preceding back-off stage after successful packet transmission in order to maintain the CW selection as a continuous process. Second, back-off timer is also not uniformly chosen in the respective CW interval. Instead to it, a non-uniform CSS distribution is used to select back-off timer in order to make the selection process adaptive according to the contention level. A Markov chain model is developed to derive the throughput and delay performance of the DCF based on CAAB mechanism. Finally, performance of our mechanism is evaluated with respect to the BEB and Double Increment Double Decrement mechanisms. Simulation results show that the proposed mechanism outperforms the referenced mechanisms in almost each count.