Abstract
The accuracy, reliability, and skill of several objective supercell identification methods are evaluated using 113 simulations from an idealized cloud model with 1-km horizontal grid ...spacing. Horizontal cross sections of vorticity and radar reflectivity at both mid- and low levels were analyzed for the presence of a supercell, every 5 min of simulation time, to develop a “truth” database. Supercells were identified using well-known characteristics such as hook echoes, inflow notches, bounded weak-echo regions (BWERs), and the presence of significant vertical vorticity.
The three objective supercell identification techniques compared were the Pearson correlation (PC) using an analysis window centered on the midlevel storm updraft; a modified Pearson correlation (MPC), which calculates the PC at every point in the horizontal using a small 3 km × 3 km analysis window; and updraft helicity (UH). Results show that the UH method integrated from 2 to 5 km AGL, and using a threshold value of 180 m
2
s
−2
, was equally as accurate as the MPC technique—averaged from 2 to 5 km AGL and using a minimum updraft threshold of 7 m s
−1
with a detection threshold of 0.3—in discriminating between supercells and nonsupercells for 1-km horizontal grid spacing simulations. At courser resolutions, the UH technique performed best, while the MPC technique produced the largest threat scores for higher-resolution simulations. In addition, requiring that the supercell detection thresholds last at least 20 min reduced the number of false alarms.
Solvent-free polymer lithography via the Kirkendall effect Thompson, Richard L.
Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms,
06/2010, Letnik:
268, Številka:
11
Journal Article
Recenzirano
An alternative route to form micron scale features in a PMMA resist that does not rely on solvent-based developers is reported. Instead of dissolving out irradiated regions of the resist, annealing ...is used to diffuse the irradiated material into the surrounding areas. Radiation-induced chain scission results in regions of reduced molecular weight adjacent to the virgin material, which have a much greater diffusion coefficient than the surrounding material. When annealed above the glass transition temperature, the fast-diffusing irradiated material swells the relatively immobile virgin PMMA. The extent of swelling is analysed to estimate the effective molecular weight of the irradiated material, and potential applications for solvent-free lithography based on proton beam writing are discussed.
Abstract
A severe thunderstorm wind gust climatology spanning 2003–09 for the contiguous United States is developed using measured Automated Surface Observing System (ASOS) and Automated Weather ...Observing System (AWOS) wind gusts. Archived severe report information from the National Climatic Data Center publication Storm Data and single-site volumetric radar data are used to identify severe wind gust observations ≥50 kt (25.7 m s−1) associated with thunderstorms and to classify the convective mode of the storms. The measured severe wind gust distribution, comprising only 2% of all severe gusts, is examined with respect to radar-based convective modes. The convective mode scheme presented herein focuses on three primary radar-based storm categories: supercell, quasi-linear convective systems (QLCSs), and disorganized. Measured severe gust frequency revealed distinct spatial patterns, where the high plains received the greatest number of gusts and occurred most often in the late spring and summer months. Severe wind gusts produced by supercells were most frequent over the plains, while those from QLCS gusts were most frequent in the plains and Midwest. Meanwhile, disorganized storms produced most of their severe gusts in the plains and Intermountain West. A reverse spatial distribution signal exists in the location between the maximum measured severe wind gust corridor located over the high plains and the maximum in all severe thunderstorm wind reports from Storm Data, located near and west of the southern Appalachians.
This paper investigates the relationships between short-term convective mode evolution, the orientations of vertical shear and mean wind vectors with respect to the initiating synoptic boundary, the ...motion of the boundary, and the role of forcing for ascent. The dominant mode of storms (linear, mixed mode, and discrete) was noted 3 h after convective initiation along cold fronts, drylines, or prefrontal troughs. Various shear and mean wind vector orientations relative to the boundary were calculated near the time of initiation. Results indicate a statistical correlation between storm mode at 3 h, the normal components of cloud-layer and deep-layer shear vectors, the boundary-relative mean cloud-layer wind vector, and the type of initiating boundary. Thunderstorms, most of which were initially discrete, tended to evolve more quickly into lines or mixed modes when the normal components of the shear vectors and boundary-relative mean cloud-layer wind vectors were small. There was a tendency for storms to remain discrete for larger normal shear and mean wind components. Smaller normal components of mean cloud-layer wind were associated with a greater likelihood that storms would remain within the zone of linear forcing along the boundary for longer time periods, thereby increasing the potential for upscale linear growth. The residence time of storms along the boundary is also dependent on the speed of the boundary. It was found that the boundary-relative normal component of the mean cloud-layer wind better discriminates between mode types than does simply the ground-relative normal component. The influence of mesoscale forcing for ascent and type of boundary on mode evolution was also investigated. As expected, it was found that the magnitude and nature of the forcing play a role in how storms evolve. For instance, strong linear low-level convergence often contributes to rapid upscale linear growth, especially if the boundary motion relative to the mean cloud-layer wind prevents storms from moving away from the boundary shortly after initiation. In summary, results from this study indicate that, for storms initiated along a synoptic boundary, convective mode evolution is modulated primarily by the residence time of storms within the zone of linear forcing, the nature and magnitude of linear forcing, and secondarily by the normal component of the cloud-layer shear.
Numerous applications require specific properties at polymer surfaces that differ from the bulk. Herein we describe the novel synthesis of a series of multi-end functionalised poly(styrene) and ...poly(isoprene) additives carrying 1 to 3 fluoroalkyl (CF) groups. The additives were prepared by endcapping the living chain ends of polymers prepared via living anionic polymerisation. The resulting polymers have been used as additives to render the surface of polymer films hydrophobic/lipophobic and we have characterised these polymer films using static contact angle measurements with water as the contact fluid. We have found that the additive molecular weight, the number of CF groups, additive concentration and annealing conditions have a significant impact upon the resulting surface properties. Increasing the additive concentration and/or number of CF groups resulted in higher contact angles whereas increasing the molecular weight of additive reduced contact angles and surface hydrophobicity. It has been discovered that these additives undergo rapid adsorption to the surface of a thin film in the time taken to produce the film by spin coating and the result is significantly enhanced surface properties. Annealing polystyrene films above the glass transition temperature revealed some interesting behaviour in so much that it demonstrated that on many occasions it is preferable to anneal films containing very small quantities of additive rather than to simply add greater quantities of additive. In addition to contact angles measurements, Rutherford backscattering (RBS) analysis has been carried out on examples of modified poly(isoprene) films to quantitatively analyse the effect of additive molecular weight and number of fluoroalkyl groups on the near surface elemental composition of the modified thin films and confirming the relationship (described above) between these additive molecular parameters and surface adsorption. Finally, we have described a model which compares the behaviour of the additives in thin films to surfactants in solution.
Multihydroxyl end functional polyethylenes have been prepared with controlled molecular weight, microstructure, and functionalization. These materials, designed as interfacially active blend ...additives for polar interfaces, are thermally stable up to ∼250 °C and to have similar crystallinity and dynamics to their unfunctionalized homopolymer analogues. The polymers segregated strongly to silicon oxide interfaces, with adsorbed layers forming spontaneously at annealed polymer interfaces, having surface excess concentrations approaching 2R g and a maximum areal density of approximately 0.6 adsorbed chains per nm2. This interfacial activity is achieved almost without detriment to the bulk physical properties of the polymer as evidenced by thermal analysis, quasi-elastic neutron scattering, and small-angle neutron scattering (SANS). SANS experiments show little evidence for aggregation of the dihydroxyl functionalized polymers in blends with PE homopolymers, which is thought to explain why these additives have particularly strong interfacial adsorption, even at relatively high concentrations. A modest level of segregation of the additives to exposed blend surfaces was also seen, particularly when the additive molecular weight was significantly lower than that of the matrix. We attribute this to a combination of the relatively low molecular weight of the additives and the marginally lower surface energy associated with deuterated polymers.
Development of novel prevention and treatment strategies for herpes simplex virus (HSV) mediated diseases is dependent upon an accurate understanding of the central molecular events underlying the ...regulation of latency and reactivation. We have recently shown that the transactivation function of the virion protein VP16 is a critical determinant in the exit from latency in vivo. HSV-1 strain SJO2 carries a single serine to alanine substitution at position 375 in VP16 which disrupts its interaction with its essential co-activator Oct-1. Here we report that SJO2 is severely impaired in its ability to exit latency in vivo. This result reinforces our prior observations with VP16 transactivation mutant, in1814, in which VP16 interaction with Oct-1 is also disrupted and solidifies the importance of the VP16–Oct-1 interaction in the early steps in HSV-1 reactivation.
We have prepared and characterized a series of multifluorocarbon end-functional polyethylene additives, which when blended with polyethylene matrices increase surface hydrophobicity and ...lipophobicity. Water contact angles of >112° were observed on spin-cast blended film surfaces containing less than 1% fluorocarbon in the bulk, compared to ∼98° in the absence of any additive. Crystallinity in these films gives rise to surface roughness that is an order of magnitude greater than is typical for amorphous spin-cast films but is too little to give rise to superhydrophobicity. X-ray photoelectron spectroscopy (XPS) confirms the enrichment of the multifluorocarbon additives at the air surface by up to 80 times the bulk concentration. Ion beam analysis was used to quantify the surface excess of the additives as a function of composition, functionality, and molecular weight of either blend component. In some cases, an excess of the additives was also found at the substrate interface, indicating phase separation into self-stratified layers. The combination of neutron reflectometry and ion beam analysis allowed the surface excess to be quantified above and below the melting point of the blended films. In these films, where the melting temperatures of the additive and matrix components are relatively similar (within 15 °C), the surface excess is almost independent of whether the blended film is semicrystalline or molten, suggesting that the additive undergoes cocrystallization with the matrix when the blended films are allowed to cool below the melting point.
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Surface modification of amorphous PET in incompatible blends is demonstrated using fluorocarbon end-functional polystyrenes. Contact angles with water and decane were consistent with ...high levels of surface fluorocarbon, even for spin-cast films with no further processing required. Hydrophobicity and lipophobicity were further increased by annealing above the glass transition temperature. High resolution depth profiling using complementary ion beam analysis and specular neutron reflectometry has enabled accurate characterisation of the composition profile of the additive including the minimum in additive concentration found just below the surface enriched layer. This analysis quantified the very low compatibility between the modifying polymer and the amorphous PET and was consistent with the highly segregated nature of the adsorbing species and its sharp interface with the subphase. For these incompatible polymer blends, surfaces enriched with the surface active polymer could coexist at equilibrium with extremely low (∼0.4%) bulk loadings of the additive. This suggests that for thicker films at even lower additive concentrations than the minimum 1% that we studied, it may be possible to achieve efficient surface modification. However, at this concentration, the efficiency of surface modification is limited by the processing conditions. Finally we note that in higher loadings of surface active additive there is clear evidence for lateral phase separation into patterned domains of differing composition. The enhancement in surface properties is due to local reorganisation rather than bulk redistribution of the components within the film, as the composition versus depth distributions of the polymer blend components was observed to be relatively unaffected by annealing.
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