ABSTRACT
A case study of persistent stratocumulus over the Azores is simulated using two independent large-eddy simulation (LES) models with bin microphysics, and forward-simulated cloud radar Doppler moments and spectra are compared with observations. Neither model is able to reproduce the monotonic increase of downward mean Doppler velocity with increasing reflectivity that is observed under a variety of conditions, but for differing reasons. To a varying degree, both models also exhibit a tendency to produce too many of the largest droplets, leading to excessive skewness in Doppler velocity distributions, especially below cloud base. Excessive skewness appears to be associated with an insufficiently sharp reduction in droplet number concentration at diameters larger than ~200 µm, where a pronounced shoulder is found for in situ observations and a sharp reduction in reflectivity size distribution is associated with relatively narrow observed Doppler spectra. Effectively using LES with bin microphysics to study drizzle formation and evolution in cloud Doppler radar data evidently requires reducing numerical diffusivity in the treatment of the stochastic collection equation; if that is accomplished sufficiently to reproduce typical spectra, progress toward understanding drizzle processes is likely.
ABSTRACT
A quantum cascade distributed feedback laser operating at 5.2 microm is used to obtain sub-Doppler resolution limited saturation features in a Lamb-dip experiment on the R(13.5)1/2 and R(13.5)3/2 transitions of NO. The dips appear as transmission spikes with full widths of ~ 4.3 MHz. At this resolution the 73 MHz _-doubling of the R(13.5)3/2 line, which is normally obscured by the 130 MHz Doppler broadening, is easily resolved.
ABSTRACT
To make a gas sensor suitable for use at high temperatures, we have used a sol-gel-processing technique to bond a copper-exchanged zeolite fluorescence indicator onto the end of an all-silica optical fiber. Experimental results from single-fiber prototype sensors show they can be used to measure either the oxygen concentration or the equivalence ratio for gas mixtures containing weak or strong reductants, respectively.
ABSTRACT
In situ measurements of the fluorescence spectra, intensity, and response time were made at elevated temperatures (~500 degrees C) for samples of Cu-ZSM-5 exposed to dilute mixtures of O(2) in N(2) and to various O(2)-reductant combinations in N(2). The results of these experiments are interpreted with a physical model of the oxidation/reduction kinetics of the copper ions. The fluorescence signal depends strongly on the gas composition, providing a mechanism for a gas composition sensor. A prototype sensor configuration is described that uses a visible (488-nm) excitation source and a fiber-optic geometry to generate and detect the fluorescence. The results indicate that for a weakly reducing gas the fluorescence signal correlates well with the oxygen concentration, whereas for strongly reducing gases the signal correlates more closely with the reductant-to-oxidant ratio.
ABSTRACT
The total internal reflection of light occurring at the interface between glass and a low-index liquid containing suspended microparticles can be electrically controlled. The particles are charged and the glass is coated with a thin, transparent conductor. When the conductor is biased to attract the particles, they scatter and absorb light from the evanescent optical field near the interface, thus reducing the reflectivity. When the conductor is biased to repel the particles, total internal reflection is achieved. Experimental results are given for the time, voltage, and angle-of-incidence dependence of the reflectivity at the interface between an In-Sn-oxide-coated glass surface and a suspension of 0.47-µm-diameter silica particles in acetonitrile. The switching is found to be fast (~ 100 ms) and reproducible. In certain conditions the on/off ratio for a single reflection can be as large as 2:1. A simple theoretical model is developed to interpret these experiments. The model gives a reasonable fit to the data and allows one to extract information such as the particle mobility and the particle density in the evanescent-wave region.
ABSTRACT
A geometrical optics approach is used to develop a theoretical model for analyzing loss mechanisms in optical light pipes. Five mechanisms are identified: intrinsic absorption, bulk scattering, losses that are due to roughness at the core-cladding interface, losses that are due to large-scale defects at the core-cladding interface, and losses that are due to absorption in the cladding material; and the effects of each of these on light-pipe transmission are considered. An approximate model appropriate for slightly rough surfaces is used to estimate the loss that is due to interface roughness. Optical experiments on commercially available light pipes are done to quantify the various loss processes. These experiments indicate that the interface effects play an important role in limiting the transmission in high-quality light pipes. From the optical measurements a rms interface roughness height in the 30-70-A range is deduced, and these values are confirmed by direct surface profilometry with an atomic force microscope.
ABSTRACT
We describe frequency-domain four-wave mixing spectroscopy measurements near the fundamental band edge in room-temperature GaAs multiple quantum wells. The line-shape information gives a measure of the dynamical behavior of the material and what is to our knowledge the first observation of an interference line shape due to a dominant slow contribution to the nonlinear response.
ABSTRACT
We describe frequency-domain spectroscopy studies of glasses doped with CdS(1-x)Se(x) using low-power cw tunable dye lasers. The results show a narrow resonance (4.4 kHz at room temperature) in the backward nearly degenerate four-wave mixing spectrum, which we believe is determined by the phonon-mediated inverse lifetime of a deep level trap involved in the nonlinear response.
ABSTRACT
The data presented here show the kinetic turbidimetric LAL assay to be a highly quantitative and effective method for determining endotoxin concentrations in products. The assay allows for the accurate assessment of inhibiting or enhancing effects in products when related to a LRW standard curve. However, designating some products as inhibitors or enhancers can be both misleading and erroneous unless qualified as to the dilution and/or endotoxin concentration. Our results demonstrate that some products can yield both inhibiting and enhancing results when related to water. Due to the enhanced resolution of the kinetic turbidimetric assay, these complicating inhibition/enhancing effects can usually be avoided by diluting to the WED. Alternatively, products could be related to a PSC in which the endotoxin response is defined and quantified within the product itself. The practicality of a PSC, however, depends upon the pass/fail limit established, the "cleanliness" of the product used as a standard and the degree of product "lot to lot" variability. Current FDA Guidelines consider a PSC valid providing the value of the "unspiked" product extrapolated from the regression line of the PSC is less than 10% of lambda, the lowest endotoxin concentration used to construct the standard. All products in which LAL kinetics have not been previously analyzed will require a characterization similar to that used with the four products described. From these data, the optimal methodology for kinetically testing the product (dilution to a WED or generation of a PSC) can be determined. Although analysis of endotoxin in a product will always require a characterization of the kinetics of the LAL-endotoxin-product reaction, subsequent testing should be rapid and straightforward. More importantly, the kinetic turbidimetric assay allows the user to quantitatively assess product endotoxin levels with a degree of precision greater than that of any other methodology currently in use.