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Separation Distance (separation + distance)
Selected AbstractsTemperature-dependent pseudopotential between two pointlike electrical chargesCONTRIBUTIONS TO PLASMA PHYSICS, Issue 5-6 2003M.-M. Gombert Abstract The pair distribution functions for particles electrically charged, at a temperature T, expressed in terms of density matrices and the corresponding pseudopotentials are studied. For an electron pair, the symmetry of the wave functions is taken into account. Exact expansions with respect to the separation distance and to a quantum parameter (, T,˝) are carried out. The known results are recovered. For high temperature, accurate approximations are derived. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electrophoresis of a charge-regulated toroid normal to a large diskELECTROPHORESIS, Issue 2 2008Jyh-Ping Hsu Abstract The electrophoresis of a charge-regulated toroid (doughnut-shaped entity) normal to a large disk is investigated under the conditions of low surface potential and weak applied electric field. The system considered is capable of modeling the electrophoretic behavior of various types of biocolloids such as bacterial DNA, plasmid DNA, and anabaenopsis near a perfectly conducting planar wall. The influences of the size of the toroid, the separation distance between the toroid and the disk, the charged conditions on the surfaces of the toroid and the disk, and the thickness of electric double layer on the electrophoretic mobility of the toroid are discussed. The results of numerical simulation reveal that under typical conditions the electrophoretic behavior of the toroid can be different from that of an integrated entity. For instance, if the surface of the toroid carries both acidic and basic functional groups, its mobility may have a local maximum as the thickness of double layer varies. We show that the electrophoretic behavior of the toroid is different, both qualitatively and quantitatively, from that of the corresponding integrated particle (particle without hole). [source] A microfabricated hybrid device for DNA sequencingELECTROPHORESIS, Issue 21 2003Shaorong LiuArticle first published online: 6 NOV 200 Abstract We have created a hybrid device of a microfabricated round-channel twin-T injector incorporated with a separation capillary in order to extend the straight separation distance for high speed and long readlength DNA sequencing. Semicircular grooves on glass wafers are obtained using a photomask with a narrow line-width and a standard isotropic photolithographic etching process. Round channels are made when two etched wafers are face-to-face aligned and bonded. A two-mask fabrication process has been developed to make channels of two different diameters. The twin-T injector is formed by the smaller channels whose diameter matches the bore of the separation capillary, and the "usual" separation channel, now called the connection channel, is formed by the larger ones whose diameter matches the outer diameter of the separation capillary. The separation capillary is inserted through the connection channel all the way to the twin-T injector to allow the capillary bore flush with the twin-T injector channels. The total dead-volume of the connection is estimated to be , 5 pL. To demonstrate the efficiency of this hybrid device, we have performed four-color DNA sequencing on it. Using a 200 ,m twin-T injector coupled with a separation capillary of 20 cm effective separation distance, we have obtained readlengths of 800 plus bases at an accuracy of 98.5% in 56 min, compared to about 650 bases in 100 min on a conventional 40 cm long capillary sequencing machine under similar conditions. At an increased separation field strength and using a diluted sieving matrix, the separation time has been reduced to 20 min with a readlength of 700 bases at 98.5% base-calling accuracy. [source] Influence of Electric Field on Microstructures of Pentacene Thin-Films in Field-Effect Transistors,ADVANCED FUNCTIONAL MATERIALS, Issue 2 2008L. Cheng Abstract We report on electric-field-induced irreversible structural modifications in pentacene thin films after long-term operation of organic field-effect transistor (OFET) devices. Micro-Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source-to-drain field that parallels the a,b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long-term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long-term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long-term operations, including enhanced field-effect mobility, improved on/off current ratio, sharp sub-threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source-to-drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed. [source] Comparison of implicit solvent models for the simulation of protein,surface interactionsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2006Yu Sun Abstract Empirical force field-based molecular simulations can provide valuable atomistic-level insights into protein,surface interactions in aqueous solution. While the implicit treatment of solvation effects is desired as a means of improving simulation efficiency, existing implicit solvent models were primarily developed for the simulation of peptide or protein behavior in solution alone, and thus may not be appropriate for protein interactions with synthetic material surfaces. The objective of this research was to calculate the change in free energy as a function of surface,separation distance for peptide,surface interactions using different empirical force field-based implicit solvation models (ACE, ASP, EEF1, and RDIE with the CHARMM 19 force field), and to compare these results with the same calculations conducted using density functional theory (DFT) combined with the self-consistent reaction field (SCRF) implicit solvation model. These comparisons show that distinctly different types of behavior are predicted with each implicit solvation method, with ACE providing the best overall agreement with DFT/SCRF calculations. These results also identify areas where ACE is in need of improvement for this application and provide a basis for subsequent parameter refinement. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2006 [source] Offspring-driven local dispersal in female sand lizards (Lacerta agilis)JOURNAL OF EVOLUTIONARY BIOLOGY, Issue 6 2004K. Ryberg Abstract We report on a field study in which determinants of female breeding dispersal (i.e. the shift in the mean home range coordinates between successive breeding events) was investigated. Offspring were released in full sib groups (or half sib ones if there was within-clutch multiple paternity) at a separation distance from the females that varied between ,families'. This allowed for analysis of ,offspring nearness' effects on maternal dispersal. When a female's offspring were released more closely to her, she responded with greater dispersal. Furthermore, when the data set was truncated at 100 m maternal,offspring separation distance at offspring release (because perception at longer distances is likely to be unrealistic), maternal dispersal resulted in greater separation distance between female and offspring in the following year. A corresponding analysis for juveniles revealed no effect of maternal nearness on offspring dispersal but identified a significant effect of clutch size, to our surprise with dispersal declining with increasing clutch size. We discuss this result in a context of the ,public information hypothesis' (reinterpreted for juveniles in a nonsocial foraging species), suggesting that conspecific abundance perhaps acts as an indicator of local habitat quality. Thus, our analysis suggests a microgeographic structuring of the adult female population driven by genetic factors, either through inbreeding avoidance, or from simply avoiding individuals with a similar genotype regardless of their pedigree relatedness, while a nongenetic factor seems more important in their offspring. [source] Potential Autonomous Selfing in Gesneria citrina (Gesneriaceae), a Specialized Hummingbird Pollinated Species with Variable Expression of HerkogamyJOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 10 2009Xin-Sheng Chen Abstract Species with mixed mating systems often demonstrate variable expression of breeding system characteristics and thus represent the opportunity to understand the factors and mechanisms that promote both outcrossed and selfed seed production. Here, we investigate variation in levels of herkogamy (variation in stigma-anther separation distance) in a Puerto Rican population of hummingbird pollinated Gesneria citrina Urban. There is significant variation in herkogamy levels among individuals of this species and stigma-anther separation is negatively associated with the ability to set fruits and seeds in the absence of pollinators. The variation in levels of herkogamy may represent a mechanism to ensure the production of some self-fertilized progeny in the absence of hummingbird pollinators. We also describe a novel breeding system in G. citrina, where stamens elongate over time to reach stigma height, but stamen elongation is accelerated by pollination. These results suggest that once the flowers are pollinated, stamen elongation may favor increased pollen removal and siring success, while the reduction in stigma-anther distance no longer imposes the risk of interference between male and female functions. We discuss our findings of breeding system variation in the context of pollination system evolution in an island setting (Antillean islands). [source] Yodel: A Yield Stress Model for SuspensionsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2006Robert J. Flatt A model for the yield stress of particulate suspensions is presented that incorporates microstructural parameters taking into account volume fraction of solids, particle size, particle size distribution, maximum packing, percolation threshold, and interparticle forces. The model relates the interparticle forces between particles of dissimilar size and the statistical distribution of particle pairs expected for measured or log-normal size distributions. The model is tested on published data of sub-micron ceramic suspensions and represents the measured data very well, over a wide range of volume fractions of solids. The model shows the variation of the yield stress of particulate suspensions to be inversely proportional to the particle diameter. Not all the parameters in the model could be directly evaluated; thus, two were used as adjustable variables: the maximum packing fraction and the minimum interparticle separation distance. The values for these two adjustable variables provided by the model are in good agreement with separate determinations of these parameters. This indicates that the model and the approximations used in its derivation capture the main parameters that influence the yield stress of particulate suspensions and should help us to better predict changes in the rheological properties of complex suspensions. The model predicts the variation of the yield stress of particulate suspensions to be inversely proportional to the particle diameter, but the experimental results do not show a clear dependence on diameter. This result is consistent with previous evaluations, which have shown significant variations in this dependence, and the reasons behind the yield stress dependence on particle size are discussed in the context of the radius of curvature of particles at contact. [source] Solution Behavior of Temperature-Responsive Molecular Brushes Prepared by ATRPMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 1 2007Joanna Pietrasik Abstract Molecular brushes, with side chains consisting of two copolymers: 2-(dimethylamino)ethyl methacrylate with methyl methacrylate, and N,N -dimethylacrylamide with butyl acrylate were prepared by grafting-from via atom transfer radical polymerization (ATRP). Poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) and poly(2-(2-bromopropionyloxy)ethyl methacrylate) were used as macroinitiators. Dynamic light scattering (DLS) studies were performed for aqueous solutions of molecular brushes below and above the lower critical solution temperature (LCST), and an unusual concentration-dependent LCST was observed. Due to the compact structure of molecular brushes, intramolecular collapse can occur when the average distance between molecules is much larger than the hydrodynamic dimensions of the individual macromolecules. However, if the concentration of the solution of molecular brushes is increased to the level in which the separation distance is comparable with the brush hydrodynamic dimensions, intermolecular aggregation occurs, as typically observed for solutions of linear polymers. [source] Compact high-gain printed loop-antenna array integrated into a 5-GHz WLAN access pointMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 10 2010Tzi-Chieh Hong Abstract A printed, planar loop-antenna array having a compact structure and high-gain, directional radiation properties for WLAN operation in the 5 GHz band is presented. The antenna-array design is composed of four-folded loops arranged to be of 2 × 2 configuration, printed on both sides of a 1.6-mm thick FR4 substrate, and one system printed circuit board (PCB) serving as an efficient reflector for the loops. The antenna array is set within the boundary of the PCB (size of 50 mm ×65 mm) and stacked there above by a separation distance of 5.4 mm only. That's, the design integrates the system PCB of an access point (AP) into an internal antenna solution for compact outdoor AP applications. The results show that good input matching with voltage standing wave ratio (VSWR) below 1.5 over the 5150,5825 MHz band can be achieved. Directional radiation patterns with peak gain at about 10 dBi were also obtained. Details of the antenna design are described and discussed. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2261,2267, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25487 [source] Impedance matrix generation by using the fast matrix generation techniqueMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 1 2009Soon Jae Kwon Abstract The computation of impedance matrix for layered structures is time-consuming because each element requires the evaluation of quadruple integrals. To increase the efficiency, we propose a technique referred to as the fast matrix generation. In this method, conventional and rigorous numerical methods are still used for generating the impedance matrix elements that are associated with the near-field interactions, whereas a more efficient scheme is employed where the separation distance between basis and testing functions exceeds a threshold value. A significant saving in computation time, sometimes over 90%, can be achieved via the application of this approach, as is demonstrated by numerical results for a number of typical microwave circuits. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 204,213, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24015 [source] The optimal density of atmospheric sounder observations in the Met Office NWP systemTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 629 2007M. L. Dando Abstract Large numbers of satellite observations are discarded from the numerical weather prediction (NWP) process because high-density observations may have a negative impact on the analysis. In current assimilation schemes, the observation error covariance matrix R is usually represented as a diagonal matrix, which assumes there are no correlations in the observation errors and that each observation is an independent piece of information. This is not the case when there are strong error correlations and this can lead to a degraded analysis. The experiments conducted in this study were designed to identify the optimal density and to determine if there were circumstances when exceeding this density might be beneficial to forecast skill. The global optimal separation distance of Advanced TIROS Operational Vertical Sounder (ATOVS) observations was identified by comparing global forecast errors produced using different densities of ATOVS. The global average of the absolute forecast error produced by each different density was found for a 3-week period from December 2004 to January 2005. The results showed that, when using the Met Office NWP system with a horizontal model resolution of ,60 km, the lowest global forecast errors were produced when using separation distances of 115,154 km. However, localized regions of the atmosphere containing large gradients such as frontal regions may benefit from thinning distances as small as 40 km and therefore the global optimal separation distance is not necessarily applicable in these circumstances. Copyright © 2007 Royal Meteorological Society [source] Single-Step Electron Transfer on the Nanometer Scale: Ultra-Fast Charge Shift in Strongly Coupled Zinc Porphyrin,Gold Porphyrin DyadsCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2008Jérôme Fortage Dr. Abstract The synthesis, electrochemical properties, and photoinduced electron transfer processes of a series of three novel zinc(II),gold(III) bisporphyrin dyads (ZnPSAuP+) are described. The systems studied consist of two trisaryl porphyrins connected directly in the meso position via an alkyne unit to tert -(phenylenethynylene) or penta(phenylenethynylene) spacers. In these dyads, the estimated center to center interporphyrin separation distance varies from 32 to 45,Ĺ. The absorption, emission, and electrochemical data indicate that there are strong electronic interactions between the linked elements, thanks to the direct attachment of the spacer on the porphyrin ring through the alkyne unit. At room temperature in toluene, light excitation of the zinc porphyrin results in almost quantitative formation of the charge shifted state .+ZnPSAuP., whose lifetime is in the order of hundreds of picoseconds. In this solvent, the charge-separated state decays to the ground state through the intermediate population of the zinc porphyrin triplet excited state. Excitation of the gold porphyrin leads instead to rapid energy transfer to the triplet ZnP. In dichloromethane the charge shift reactions are even faster, with time constants down to 2,ps, and may be induced also by excitation of the gold porphyrin. In this latter solvent, the longest charge-shifted lifetime (,=2.3,ns) was obtained with the penta-(phenylenethynylene) spacer. The charge shift reactions are discussed in terms of bridge-mediated super-exchange mechanisms as electron or hole transfer. These new bis-porphyrin arrays, with strong electronic coupling, represent interesting molecular systems in which extremely fast and efficient long-range photoinduced charge shift occurs over a long distance. The rate constants are two to three orders of magnitude larger than for corresponding ZnPAuP+ dyads linked via meso -phenyl groups to oligo-phenyleneethynylene spacers. This study demonstrates the critical impact of the attachment position of the spacer on the porphyrin on the electron transfer rate, and this strategy can represent a useful approach to develop molecular photonic devices for long-range charge separations. [source] A microfluidic study of mechanisms in the electrophoresis of supercoiled DNAELECTROPHORESIS, Issue 12 2008Dammika P. Manage Abstract In this work, microfluidic chips were used to study the electrophoresis of supercoiled DNA (SC DNA) in agarose. This system allowed us to study the electrophoretic and trapping behaviours of SC DNA of various lengths, at various fields and separation distances. Near a critical electric field the DNA is trapped such that the concentration falls exponentially with distance. The trapping of such circular DNA has been explained in terms of the ,lobster trap' or ,impalement' model where shorter fibres become trapping sites at higher fields, leading to an ongoing (and gradual) increase in trapping with increasing field. By contrast, the present study suggests that under some circumstances the traps have a barrier such that only when the DNA has sufficient energy (at high enough fields) can it become trapped, leading to a sudden transition in behaviours at the critical field. We propose an ,activated impalement' mechanism of trapping in which only at sufficiently high fields is the SC DNA impaled and trapped for long times. The critical electric field appears to be inversely proportional to the length of the DNA molecule, suggesting that the force required to impale the SC DNA is constant. [source] Distance separated simultaneous sweeping, for fast, clean, vibroseis acquisitionGEOPHYSICAL PROSPECTING, Issue 1 2010Jack Bouska ABSTRACT Distance separated simultaneous sweeping DS3 is a new vibroseis technique that produces independent records, uncontaminated by simultaneous source interference, for a range of offsets and depths that span all target zones of interest. Use of DS3 on a recent seismic survey in Oman, resulted in a peak acquisition rate of 1024 records per hour. This survey employed 15 vibrators, with a distance separation of 12 km between simultaneous active sources, recorded by 8000 active channels across 22 live lines in an 18.5 km × 11 km receiver patch. Broad distribution of simultaneous sources, across an adequately sized recording patch, effectively partitions the sensors so that each trace records only one of the simultaneous sources. With proper source separation, on a scale similar to twice the maximum usable source receiver offset, wavefield overlap occurs below the zone of interest. This yields records that are indistinguishable from non-simultaneous source data, within temporal and spatial limits. This DS3 technique may be implemented using a wide variety of acquisition geometries, optimally with spatially large recording patches that enable appropriate source separation distances. DS3 improves acquisition efficiency without data quality degradation, eliminating the requirement for special data processing or noise attenuation. [source] Short-scale diversity in a dynamic rain fade environmentINTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 2 2005Cristina Enjamio Abstract This article is concerned with using time-varying uniformly spaced rainfall data to investigate the concept and values of instantaneous diversity gain. This has been obtained on a minute-by-minute basis in order to illustrate the behaviour of a VSAT communication system which switches to the most reliable link every minute. The simulations have been carried out using a whole year of rain data obtained from the recordings of a dense rain gauge network located in Barcelona. After illustrating the concept of instantaneous diversity gain, this paper shows the results obtained for various site separations and 20 GHz. These results have been plotted together with the values given by some long-term-based models. The comparison shows that such models are not appropriate for dynamic site diversity. This was found to increase the reliability of a VSAT system, especially under severe rain conditions in which the rainfall rates are spatially arranged in rain cells and the attenuation experienced by the two sites is strongly decorrelated. The correlation coefficient between the attenuation series calculated for several separation distances has also been obtained to illustrate this concept. Finally, the paper gives the attenuation distributions calculated for a VSAT system with and without site diversity and for several site separations. Copyright © 2005 John Wiley & Sons, Ltd. [source] A fractal forecasting model for financial time seriesJOURNAL OF FORECASTING, Issue 8 2004Gordon R. Richards Abstract Financial market time series exhibit high degrees of non-linear variability, and frequently have fractal properties. When the fractal dimension of a time series is non-integer, this is associated with two features: (1) inhomogeneity,extreme fluctuations at irregular intervals, and (2) scaling symmetries,proportionality relationships between fluctuations over different separation distances. In multivariate systems such as financial markets, fractality is stochastic rather than deterministic, and generally originates as a result of multiplicative interactions. Volatility diffusion models with multiple stochastic factors can generate fractal structures. In some cases, such as exchange rates, the underlying structural equation also gives rise to fractality. Fractal principles can be used to develop forecasting algorithms. The forecasting method that yields the best results here is the state transition-fitted residual scale ratio (ST-FRSR) model. A state transition model is used to predict the conditional probability of extreme events. Ratios of rates of change at proximate separation distances are used to parameterize the scaling symmetries. Forecasting experiments are run using intraday exchange rate futures contracts measured at 15-minute intervals. The overall forecast error is reduced on average by up to 7% and in one instance by nearly a quarter. However, the forecast error during the outlying events is reduced by 39% to 57%. The ST-FRSR reduces the predictive error primarily by capturing extreme fluctuations more accurately. Copyright © 2004 John Wiley & Sons, Ltd. [source] Rewetting effects and droplet motion on partially wetted powder surfacesAICHE JOURNAL, Issue 6 2009Karen P. Hapgood Abstract In high shear mixer granulation, the powder is agitated in a vessel while liquid is sprayed onto the powder. Formation of "nuclei" can be predicted using a nucleation regime map. However, this approach assumes that only dry powder enters the spray zone. Industrial granulation processes commonly add 20,50 wt % fluid, and the partially wetted powder recirculates many times through the spray zone. The effect of partially wetted powder re-entering the spray zone is not currently known. To investigate, droplets were added to a powder bed at controlled separation distances and time intervals. A strong correlation between drop penetration time and droplet motion on the powder bed surface was observed. For fast penetrating systems, nucleation was only slightly affected by the presence of the previous droplet. However, systems with long penetration times showed lateral droplet motion due to Laplace pressure differences. Implications for the nucleation regime map are discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Synchronization of enteric neuronal firing during the murine colonic MMCTHE JOURNAL OF PHYSIOLOGY, Issue 3 2005Nick J. Spencer DiI (1,1,didodecyl-3,3,3,,3,-tetramethylindocarbecyanine perchlorate) retrograde labelling and intracellular electrophysiological techniques were used to investigate the mechanisms underlying the generation of spontaneously occurring colonic migrating myoelectric complexes (colonic MMCs) in mice. In isolated, intact, whole colonic preparations, simultaneous intracellular electrical recordings were made from pairs of circular muscle (CM) cells during colonic MMC activity in the presence of nifedipine (1,2 ,m). During the intervals between colonic MMCs, spontaneous inhibitory junction potentials (IJPs) were always present. The amplitudes of spontaneous IJPs were highly variable (range 1,20 mV) and occurred asynchronously in the two CM cells, when separated by 1 mm in the longitudinal axis. Colonic MMCs occurred every 151 ± 7 s in the CM and consisted of a repetitive discharge of cholinergic rapid oscillations in membrane potential (range: 1,20 mV) that were superimposed on a slow membrane depolarization (mean amplitude: 9.6 ± 0.5 mV; half-duration: 25.9 ± 0.7 s). During the rising (depolarizing) phase of each colonic MMC, cholinergic rapid oscillations occurred simultaneously in both CM cells, even when the two electrodes were separated by up to 15 mm along the longitudinal axis of the colon. Smaller amplitude oscillations (< 5 mV) showed poor temporal correlation between two CM cells, even at short electrode separation distances (i.e. < 1 mm in the longitudinal axis). When the two electrodes were separated by 20 mm, all cholinergic rapid oscillations and IJPs in the CM (regardless of amplitude) were rarely, if ever, coordinated in time during the colonic MMC. Cholinergic rapid oscillations were blocked by atropine (1 ,m) or tetrodotoxin (1 ,m). Slow waves were never recorded from any CM cells. DiI labelling showed that the maximum projection length of CM motor neurones and interneurones along the bowel was 2.8 mm and 13 mm, respectively. When recordings were made adjacent to either oral or anal cut ends of the colon, the inhibitory or excitatory phases of the colonic MMC were absent, respectively. In summary, during the colonic MMC, cholinergic rapid oscillations of similar amplitudes occur simultaneously in two CM cells separated by large distances (up to 15 mm). As this distance was found to be far greater than the projection length of any single CM motor neurone, we suggest that the generation of each discrete cholinergic rapid oscillation represents a discreet cholinergic excitatory junction potential (EJP) that involves the synaptic activation of many cholinergic motor neurones simultaneously, by synchronous firing in many myenteric interneurones. Our data also suggest that ascending excitatory and descending inhibitory nerve pathways interact and reinforce each other. [source] Estimates of spatial and interchannel observation-error characteristics for current sounder radiances for numerical weather prediction.THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 649 2010I: Methods, application to ATOVS data Abstract This is the first part of a two-part article that uses three methods to estimate observation errors and their correlations for clear-sky sounder radiances used in the European Centre for Medium-Range Weather Forecasts (ECMWF) assimilation system. The analysis is based on covariances derived from pairs of first-guess and analysis departures. The methods used are the so-called Hollingsworth/Lönnberg method, a method based on subtracting a scaled version of mapped assumed background errors from first-guess departure covariances and the Desroziers diagnostic. The present article reports the results for the three Advanced TIROS Operational Vertical Sounder (ATOVS) instruments: the Advanced Microwave Sounding Unit (AMSU)-A, High-Resolution Infrared Radiation Sounder (HIRS) and Microwave Humidity Sounder (MHS). The findings suggest that all AMSU-A sounding channels show little or no interchannel or spatial observation-error correlations, except for surface-sensitive channels over land. Estimates for the observation error are mostly close to the instrument noise. In contrast, HIRS temperature-sounding channels exhibit some interchannel error correlations, and these are stronger for surface-sensitive channels. There are also indications for stronger spatial-error correlations for the HIRS short-wave channels. There is good agreement between the estimates from the three methods for temperature-sounding channels. Estimating observation errors for humidity-sounding channels of MHS and HIRS appears more difficult. A considerable proportion of the observation error for humidity-sounding channels appears correlated spatially for short separation distances, as well as between channels. Observation error estimates for humidity channels are generally considerably larger than the instrument noise. Observation error estimates from this study are consistently lower than those assumed in the ECMWF assimilation system. As error correlations are small for AMSU-A, the study suggests that the current use of AMSU-A data in the ECMWF system in terms of observation-error or thinning-scale choices is fairly conservative. Copyright © 2010 Royal Meteorological Society [source] The optimal density of atmospheric sounder observations in the Met Office NWP systemTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 629 2007M. L. Dando Abstract Large numbers of satellite observations are discarded from the numerical weather prediction (NWP) process because high-density observations may have a negative impact on the analysis. In current assimilation schemes, the observation error covariance matrix R is usually represented as a diagonal matrix, which assumes there are no correlations in the observation errors and that each observation is an independent piece of information. This is not the case when there are strong error correlations and this can lead to a degraded analysis. The experiments conducted in this study were designed to identify the optimal density and to determine if there were circumstances when exceeding this density might be beneficial to forecast skill. The global optimal separation distance of Advanced TIROS Operational Vertical Sounder (ATOVS) observations was identified by comparing global forecast errors produced using different densities of ATOVS. The global average of the absolute forecast error produced by each different density was found for a 3-week period from December 2004 to January 2005. The results showed that, when using the Met Office NWP system with a horizontal model resolution of ,60 km, the lowest global forecast errors were produced when using separation distances of 115,154 km. However, localized regions of the atmosphere containing large gradients such as frontal regions may benefit from thinning distances as small as 40 km and therefore the global optimal separation distance is not necessarily applicable in these circumstances. Copyright © 2007 Royal Meteorological Society [source] | ||||||||||||||||||||||