Observations and Modeling of Tropical Planetary Atmospheres

This thesis is a comprised of three different projects within the topic of tropical atmospheric dynamics. First, I analyze observations of thermal radiation from Saturn’s atmosphere and from them, determine the latitudinal distribution of ammonia vapor near the 1.5-bar pressure level. The most prominent feature of the observations is the high brightness temperature of Saturn’s subtropical latitudes on either side of the equator. After comparing the observations to a microwave radiative transfer model, I find that these subtropical bands require very low ammonia relative humidity below the ammonia cloud layer in order to achieve the high brightness temperatures observed. We suggest that these bright subtropical bands represent dry zones created by a meridionally overturning circulation.

Second, I use a dry atmospheric general circulation model to study equatorial superrotation in terrestrial atmospheres. A wide range of atmospheres are simulated by varying three parameters: the pole-equator radiative equilibrium temperature contrast, the convective lapse rate, and the planetary rotation rate. A scaling theory is developed that establishes conditions under which superrotation occurs in terrestrial atmospheres. The scaling arguments show that superrotation is favored when the off-equatorial baroclinicity and planetary rotation rates are low. Similarly, superrotation is favored when the convective heating strengthens, which may account for the superrotation seen in extreme global-warming simulations.

Third, I use a moist slab-ocean general circulation model to study the impact of a zonally-symmetric continent on the distribution of monsoonal precipitation. I show that adding a hemispheric asymmetry in surface heat capacity is sufficient to cause symmetry breaking in both the spatial and temporal distribution of precipitation. This spatial symmetry breaking can be understood from a large-scale energetic perspective, while the temporal symmetry breaking requires consideration of the dynamical response to the heat capacity asymmetry and the seasonal cycle of insolation. Interestingly, the idealized monsoonal precipitation bears resemblance to precipitation in the Indian monsoon sector, suggesting that this work may provide insight into the causes of the temporally asymmetric distribution of precipitation over southeast Asia.

Laser micromachining of chromium-rich die steels under controlled atmospheres

Diode-pumped, solid-state (DPSS) lasers with multiwavelength capability have become an industrial reality in recent years. Multiwavelength capability allows DPSS lasers to perform operations such as micromachining in a variety of engineering materials such as ceramics, metals and polymers. A series of experiments was performed to investigate how shielding gas environments and gas pressure affect the ability to cut and machine chromium-rich die steels. Results from this study reveal that traditional plasma-controlling gases have a detrimental e�ffect on the surface morphology of micromachined components.

The application and a substitution defect model for Eu3+ -> Eu2+ reduction in non-reducing atmospheres in borates containing BO4 anion groups

The variations of emission intensities of SrB4O7:Eu2+ and Sr2B5O9Cl:Eu2+ prepared in different atmospheres are discussed in view of the structure of host compounds. A model of substitution defects is proposed to explain the abnormal reduction of Eu3+ --> Eu2+ in non-reducing atmospheres of N-2, air and O-2. Experiment results show that SrB4O7:Eu2+ phosphor sample prepared in N-2 atmosphere has an emission intensity of 94% as high as that of the sample prepared in H-2 gas. This implies that the reduction of Eu2+ --> Eu2+ in non-reducing atmospheres could be potentially used in preparing phosphors, such as SrB4O7:Eu2+. (C) 1999 Elsevier Science Ltd. All rights reserved.

Restructuring and redispersion of silver on SiO2 under oxidizing/reducing atmospheres and its activity toward CO oxidation

The effects of oxygen-hydrogen pretreatments of nanosilver catalysts in cycle mode on the structure and particle size of silver particles, and subsequently the activity of the catalyst toward CO oxidation (or CO selective oxidation in the presence of H-2) are reported in this paper. Ag/SiO2 catalyst with silver particle sizes of ca. 6 similar to 8 nm shows relatively high activity in the present reaction system. The adopting of a cycle of oxidation/reduction pretreatment has a marked influence on the activity of the catalyst. Oxygen pretreatment at 500 degrees C results in the formation of subsurface oxygen and activates the catalyst. As evidenced by in-situ XRD and TEM, the following H-2 treatment at low temperatures (100 similar to 300 degrees C) causes surface faceting and redispersing of the silver particles without destroying the subsurface oxygen species. The subsequent in-situ FTIR and catalytic reaction results show that CO oxidation occurs at -75 degrees C and complete CO conversion can be obtained at 40 degrees C over such a nanosilver catalyst pretreated with oxygen at 500 degrees C followed by H-2 at 100 degrees C. However, prolonged hydrogen treatment at high temperatures (> 300 degrees C) after oxygen pretreatment at 500 degrees C induces the aggregation of silver particles and also depletes so much subsurface oxygen species that the pathway of CO oxidation by the subsurface oxygen species is inhibited. Meanwhile, the ability of the catalyst to adsorb reactants is greatly depressed, resulting in a 20 similar to 30% decrease in the activity toward CO oxidation. However, the activity of the catalyst pretreated with oxygen at 500 degrees C followed by hydrogen treatment at high temperatures (> 300 degrees C) is still higher than that directly pretreated with H,. This kind of catalytic behavior of silver catalyst is associated with physical changes in the silver crystallites because of surface restructuring and crystallite redispersion during the course of oxygen-hydrogen pretreatment steps.

Opacity in the upper atmospheres of active stars - II. AD Leonis

We present FUV and UV spectroscopic observations of AD Leonis, with the aim of investigating opacity effects in the transition regions of late-type stars. The C III lines in FUSE spectra show significant opacity during both the quiescent and flaring states of AD Leonis, with up to 30% of the expected flux being lost during the latter. Other FUSE emission lines tested for opacity include those of O VI, while C IV, Si IV and N V transitions observed with STIS are also investigated. These lines only reveal modest amounts of opacity with losses during flaring of up to 20%. Optical depths have been calculated for homogeneous and inhomogeneous geometries, giving path lengths of approximate to 20 - 60 km and approximate to 10 - 30 km, respectively, under quiescent conditions. However path lengths derived during flaring are approximate to 2 - 3 times larger. These values are in excellent agreement with both estimates of the small-scale structure observed in the solar transition region, and path lengths derived previously for several other active late-type stars.

A new look at the pulsating DB white dwarf GD 358: Line-of-sight velocity measurements and constraints on model atmospheres

We report on our findings of the bright, pulsating, helium atmosphere white dwarf GD 358, based on time-resolved optical spectrophotometry. We identify 5 real pulsation modes and at least 6 combination modes at frequencies consistent with those found in previous observations. The measured Doppler shifts from our spectra show variations with amplitudes of up to 5.5 km s-1 at the frequencies inferred from the flux variations. We conclude that these are variations in the line-of-sight velocities associated with the pulsational motion. We use the observed flux and velocity amplitudes and phases to test theoretical predictions within the convective driving framework, and compare these with similar observations of the hydrogen atmosphere white dwarf pulsators (DAVs). The wavelength dependence of the fractional pulsation amplitudes (chromatic amplitudes) allows us to conclude that all five real modes share the same spherical degree, most likely, l=1. This is consistent with previous identifications based solely on photometry. We find that a high signal-to-noise mean spectrum on its own is not enough to determine the atmospheric parameters and that there are small but significant discrepancies between the observations and model atmospheres. The source of these remains to be identified. While we infer Teff =24 kK and log g ~ 8.0 from the mean spectrum, the chromatic amplitudes, which are a measure of the derivative of the flux with respect to the temperature, unambiguously favour a higher effective temperature, 27 kK, which is more in line with independent determinations from ultra-violet spectra.