Efficiency of encounter-controlled reaction between diffusing reactants in a finite lattice: topology and boundary effects

The role of dimensionality (Euclidean versus fractal), spatial extent, boundary effects and system topology on the efficiency of diffusion-reaction processes involving two simultaneously-diffusing reactants is analyzed. We present numerically-exact values for the mean time to reaction, as gauged by the mean walklength before reactive encounter, obtained via application of the theory of finite Markov processes, and via Monte Carlo simulation. As a general rule, we conclude that for sufficiently large systems, the efficiency of diffusion-reaction processes involving two synchronously diffusing reactants (two-walker case) relative to processes in which one reactant of a pair is anchored at some point in the reaction space (one walker plus trap case) is higher, and is enhanced the lower the dimensionality of the system. This differential efficiency becomes larger with increasing system size and, for periodic systems, its asymptotic value may depend on the parity of the lattice. Imposing confining boundaries on the system enhances the differential efficiency relative to the periodic case, while decreasing the absolute efficiencies of both two-walker and one walker plus trap processes. Analytic arguments are presented to provide a rationale for the results obtained. The insights afforded by the analysis to the design of heterogeneous catalyst systems are also discussed.Comment: 15 pages, 8 figures, uses revtex4, accepted for publication in Physica

Historical and Projected Distributions of Daily Temperature and Pressure in the Arctic

Changes in extreme temperatures and pressures in the Arctic have received little attention in the context of climate change. Here we examine the distributions and extremes of surface air temperature and pressure in the Arctic for the late 20th century, using Alaskan weather station data, an atmospheric reanalysis, and general circulation models (GCMs). There is good agreement among these sources for the late 20th century, with broader distributions for both temperature and pressure in winter as compared to summer, and over land as compared to over ocean. We used the output from 21st-century greenhouse simulations by the GCMs to address the occurrence of extremes in the coming decades. The model projections of the 21st-century extremes largely agree with changes in the mean state, with record low temperatures decreasing in frequency and record high temperatures increasing in frequency. The changes in 21st-century extremes are more pronounced over the ocean, where the present-day distributions are narrower. The projected decreases of mean pressure result in more frequent occurrences of extreme low pressure, especially over the Arctic Ocean, although the extremes of pressure are less affected by changes of the means than are the extremes of temperature. Lastly, we find that the transition from sea ice to open water, and associated changes in the salinity of the surface water, can cause changes in the temperature distribution that are more complex than simple shifts in the distribution, leading to unexpected changes in the occurrence of extreme temperatures.Les changements en matière de températures et de pressions extrêmes dans l’Arctique reçoivent peu d’attention dans le contexte du changement climatique. Ici, nous examinons les répartitions et les extrêmes de la température de l’air en surface et de la pression dans l’Arctique vers la fin du XXe siècle et ce, en nous appuyant sur les données de la station météorologique de l’Alaska, sur une nouvelle analyse atmosphérique et sur des modèles de circulation générale. Il existe une bonne concordance quant aux sources visant la fin du XXe siècle, avec des répartitions plus vastes pour les températures et pressions d’hiver comparativement à celles de l’été, ainsi que pour les températures et pressions terrestres par rapport aux températures et pressions océaniques. Nous nous sommes également servi des données émanant de simulations de serres au XXIe siècle, réalisées au moyen des modèles de circulation générale afin d’être en mesure de nous pencher sur l’occurrence des extrêmes au cours des décennies à venir. Les projections de modèles relatives aux extrêmes du XXIe siècle concordent largement avec les changements caractérisant l’état de la moyenne, la fréquence des basses températures record étant à la baisse et la fréquence des hautes températures record étant à la hausse. Les changements en matière d’extrêmes au XXIe siècle sont plus prononcés au-dessus de l’océan, là où les répartitions actuelles sont plus étroites. Les diminutions projetées de la pression moyenne se traduisent par des occurrences plus fréquentes de basses pressions extrêmes, surtout au-dessus de l’océan Arctique, bien que les extrêmes en matière de pressions soient moins touchés par les changements des moyennes que les extrêmes de températures. De plus, nous estimons que la transition allant de la glace de mer à l’eau libre, de même que les changements connexes du point de vue de la salinité de l’eau de surface, peuvent entraîner des changements sur le plan de la répartition des températures qui sont plus complexes que de simples décalages de répartition, ce qui mène à des changements inattendus en matière d’occurrence de températures extrêmes

Effects of Climate Change on the Seasonality of Weather for Tourism in Alaska

This study presents a method to identify and categorize seasonal patterns of weather conducive to specific tourism activities, using a tourism climate index based on hourly weather data. We examined changes in seasonal weather patterns in recent decades (1942–2005) at two Alaska destinations, King Salmon and Anchorage. The results indicate that climate warming has had both positive and negative effects on opportunities for tourism. The overall weather conditions for sightseeing in King Salmon have improved significantly with a lengthening of the season, which now starts 10 days earlier than in the 1940s. Conversely, weather conditions for skiing in Anchorage have deteriorated, primarily because weather suitable for skiing now ends about nine days earlier than in the 1940s. Future climate change (i.e., continued warming) is very likely to extend the sightseeing season at King Salmon, but is less likely to improve the quality of the peak season. At the same time, though warming is likely to shorten the total time for skiing each year at Anchorage, it is also likely to improve the quality of the winter season and increase the frequency of years when skiing quality peaks in mid-winter. For both activities, changes of spring temperature will have the greatest impact on tourism weather. Tourism indices such as the one presented here can be tailored to the requirements of specific tourist activities, providing an opportunity for improved tourism planning and decision making.Cette étude présente une méthode d’identification et de catégorisation des variations météorologiques saisonnières propices à des activités touristiques particulières et ce, à l’aide d’un indice climatique touristique reposant sur des données météorologiques horaires. Nous avons examiné les changements ayant caractérisé les variations météorologiques saisonnières de décennies récentes (1942–2005) à deux destinations de l’Alaska, soit King Salmon et Anchorage. Les résultats indiquent que le réchauffement climatique a eu des incidences à la fois positives et négatives sur les débouchés touristiques. Dans l’ensemble, les conditions météorologiques propices aux visites touristiques à King Salmon se sont améliorées considérablement en ce sens que la saison est maintenant plus longue, celle-ci commençant dix jours plus tôt que dans les années 1940. Par contre, les conditions météorologiques pour le ski se sont détériorées à Anchorage, principalement parce que le temps se prêtant au ski se termine maintenant environ neuf jours plus tôt que dans les années 1940. Les changements climatiques à venir (c’est-à-dire le réchauffement climatique continu) auront vraisemblablement pour effet de prolonger la saison des visites touristiques à King Salmon, sans que cela n’améliore pour autant la qualité de la haute saison. Par la même occasion, bien que le réchauffement aura probablement pour effet de raccourcir le nombre total de jours de ski à Anchorage chaque année, il aura vraisemblablement pour effet d’améliorer la qualité de la saison hivernale et d’augmenter la fréquence des années où la qualité du ski sera à son meilleur au milieu de l’hiver. Dans les deux cas, les changements caractérisant les températures enregistrées au printemps auront les plus grandes incidences sur les conditions météorologiques pour le tourisme. Les indices du tourisme, tel que celui présenté ici, peuvent être adaptés aux exigences d’activités touristiques spécifiques, ce qui donne la possibilité de mieux planifier les activités touristiques et de prendre des décisions à meilleur escient

The MASSIVE Survey - VIII. Stellar Velocity Dispersion Profiles and Environmental Dependence of Early-Type Galaxies

We measure the radial profiles of the stellar velocity dispersions, $\sigma(R)$, for 90 early-type galaxies (ETGs) in the MASSIVE survey, a volume-limited integral-field spectroscopic (IFS) galaxy survey targeting all northern-sky ETGs with absolute $K$-band magnitude $M_K < -25.3$ mag, or stellar mass $M_* > 4 \times 10^{11} M_\odot$, within 108 Mpc. Our wide-field 107" $\times$ 107" IFS data cover radii as large as 40 kpc, for which we quantify separately the inner (2 kpc) and outer (20 kpc) logarithmic slopes $\gamma_{\rm inner}$ and $\gamma_{\rm outer}$ of $\sigma(R)$. While $\gamma_{\rm inner}$ is mostly negative, of the 56 galaxies with sufficient radial coverage to determine $\gamma_{\rm outer}$ we find 36% to have rising outer dispersion profiles, 30% to be flat within the uncertainties, and 34% to be falling. The fraction of galaxies with rising outer profiles increases with $M_*$ and in denser galaxy environment, with 10 of the 11 most massive galaxies in our sample having flat or rising dispersion profiles. The strongest environmental correlations are with local density and halo mass, but a weaker correlation with large-scale density also exists. The average $\gamma_{\rm outer}$ is similar for brightest group galaxies, satellites, and isolated galaxies in our sample. We find a clear positive correlation between the gradients of the outer dispersion profile and the gradients of the velocity kurtosis $h_4$. Altogether, our kinematic results suggest that the increasing fraction of rising dispersion profiles in the most massive ETGs are caused (at least in part) by variations in the total mass profiles rather than in the velocity anisotropy alone.Comment: Accepted/in press, MNRA

The Stealth Biplane: a Proposal in Response to a Low Reynolds Number Station Keeping Mission

The Stealth Biplane is conceived and constructed to serve as a remotely piloted vehicle designed to navigate a low-level figure-eight course at a target Reynolds number of 100,000. This flight vehicle will combine the latest in lightweight radio controlled hardware in conjunction with current low Reynolds number aerodynamic research to demonstrate feasible operation in a variety of applications. These potential low Reynolds number applications include high altitude atmospheric sampling, search and rescue, and even law enforcement. Design specs and fabrication technique are discussed

The MASSIVE Survey - VII. The Relationship of Angular Momentum, Stellar Mass and Environment of Early-Type Galaxies

We analyse the environmental properties of 370 local early-type galaxies (ETGs) in the MASSIVE and ATLAS3D surveys, two complementary volume-limited integral-field spectroscopic (IFS) galaxy surveys spanning absolute $K$-band magnitude $-21.5 > M_K > -26.6$, or stellar mass $8 \times 10^{9} < M_* < 2 \times 10^{12} M_\odot$. We find these galaxies to reside in a diverse range of environments measured by four methods: group membership (whether a galaxy is a brightest group/cluster galaxy, satellite, or isolated), halo mass, large-scale mass density (measured over a few Mpc), and local mass density (measured within the $N$th neighbour). The spatially resolved IFS stellar kinematics provide robust measurements of the spin parameter $\lambda_e$ and enable us to examine the relationship among $\lambda_e$, $M_*$, and galaxy environment. We find a strong correlation between $\lambda_e$ and $M_*$, where the average $\lambda_e$ decreases from $\sim 0.4$ to below 0.1 with increasing mass, and the fraction of slow rotators $f_{\rm slow}$ increases from $\sim 10$% to 90%. We show for the first time that at fixed $M_*$, there are almost no trends between galaxy spin and environment; the apparent kinematic morphology-density relation for ETGs is therefore primarily driven by $M_*$ and is accounted for by the joint correlations between $M_*$ and spin, and between $M_*$ and environment. A possible exception is that the increased $f_{\rm slow}$ at high local density is slightly more than expected based only on these joint correlations. Our results suggest that the physical processes responsible for building up the present-day stellar masses of massive galaxies are also very efficient at reducing their spin, in any environment.Comment: Accepted to MNRA

The MASSIVE Survey XIII -- Spatially Resolved Stellar Kinematics in the Central 1 kpc of 20 Massive Elliptical Galaxies with the GMOS-North Integral-Field Spectrograph

We use observations from the GEMINI-N/GMOS integral-field spectrograph (IFS) to obtain spatially resolved stellar kinematics of the central $\sim 1$ kpc of 20 early-type galaxies (ETGs) with stellar masses greater than $10^{11.7} M_\odot$ in the MASSIVE survey. Together with observations from the wide-field Mitchell IFS at McDonald Observatory in our earlier work, we obtain unprecedentedly detailed kinematic maps of local massive ETGs, covering a scale of $\sim 0.1-30$ kpc. The high ($\sim 120$) signal-to-noise of the GMOS spectra enable us to obtain two-dimensional maps of the line-of-sight velocity, velocity dispersion $\sigma$, as well as the skewness $h_3$ and kurtosis $h_4$ of the stellar velocity distributions. All but one galaxy in the sample have $\sigma(R)$ profiles that increase towards the center, whereas the slope of $\sigma(R)$ at one effective radius ($R_e$) can be of either sign. The $h_4$ is generally positive, with 14 of the 20 galaxies having positive $h_4$ within the GMOS aperture and 18 having positive $h_4$ within $1 R_e$. The positive $h_4$ and rising $\sigma(R)$ towards small radii are indicative of a central black hole and velocity anisotropy. We demonstrate the constraining power of the data on the mass distributions in ETGs by applying Jeans anisotropic modeling (JAM) to NGC~1453, the most regular fast rotator in the sample. Despite the limitations of JAM, we obtain a clear $\chi^2$ minimum in black hole mass, stellar mass-to-light ratio, velocity anisotropy parameters, and the circular velocity of the dark matter halo.Comment: Accepted to Ap

A theoretical study of the sea breeze

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Meteorology, February 1974."December, 1973." Vita.Includes bibliographical references (leaves 125-128).The linearized Boussinesq equations with rotation, viscosity, conduction, and a mean stratification are used to model the sea breeze in two dimensions. the motion is forced by a prescribed surface temperature function. The linear model produces a sea breeze with realistic velocities and spatial dimensions. Hydrostatic solutions are found to differ very little from the nonhydrostatic solutions. The phase of the solution depends on the Coriolis parameter f; the only distinguishable feature of the solution at the inertial latitude is a slight amplitude maximum far from the coastline. Both the phase and the amplitude depend on the stability parameter N². An inversion, simulated by a discontinuity in N², reduces the intensity of the circulation. The land-sea temperature difference required by the model to create a net onshore flow in opposition to a basic current agrees well with the empirical criterion defined by Lyons (1972). The computed vertical heat fluxes, when summed along the coastlines of the principal land masses, indicate that the sea breeze effect can account for several per cent of the globally averaged vertical flux of sensible heat at a height of several hundred meters. The nonlinear advection process is studied with a finite difference model based on a series of overlapping grids. The principal effect of the nonlinear terms is a landward advection of the sea breeze circulation.by John E. Walsh.Ph.D