1,248 research outputs found
A Lagrangian trajectory view on transport and mixing processes between the eye, eyewall, and environment using a high resolution simulation of Hurricane Bonnie (1998)
The transport and mixing characteristics of a large sample of air parcels within a mature and vertically sheared hurricane vortex is examined. Data from a high-resolution (2 km grid spacing) numerical simulation of "real-case" Hurricane Bonnie (1998) is used to calculate Lagrangian trajectories of air parcels in various subdomains of the hurricane (namely, the eye, eyewall, and near-environment) to study the degree of interaction (transport and mixing) between these subdomains. It is found that 1) there is transport and mixing from the low-level eye to the eyewall that carries high- Be air which can enhance the efficiency of the hurricane heat engine; 2) a portion of the low-level inflow of the hurricane bypasses the eyewall to enter the eye, that both replaces the mass of the low-level eye and lingers for a sufficient time (order 1 hour) to acquire enhanced entropy characteristics through interaction with the ocean beneath the eye; 3) air in the mid- to upper-level eye is exchanged with the eyewall such that more than half the air of the eye is exchanged in five hours in this case of a sheared hurricane; and 4) that one-fifth of the mass in the eyewall at a height of 5 km has an origin in the mid- to upper-level environment where thet(sub e) is much less than in the eyewall, which ventilates the ensemble average eyewall theta(sub e) by about 1 K. Implications of these findings to the problem of hurricane intensity forecasting are discussed
Kinematics and Metallicity of M31 Red Giants: The Giant Southern Stream and Discovery of a Second Cold Component at R = 20 kpc
We present spectroscopic observations of red giant branch (RGB) stars in the
Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck
II 10-m telescope. The three fields targeted in this study are in the M31
spheroid, outer disk, and giant southern stream. In this paper, we focus on the
kinematics and chemical composition of RGB stars in the stream field located at
a projected distance of R = 20 kpc from M31's center. A mix of stellar
populations is found in this field. M31 RGB stars are isolated from Milky Way
dwarf star contaminants using a variety of spectral and photometric
diagnostics. The radial velocity distribution of RGB stars displays a clear
bimodality -- a primary peak centered at v = -513 km/s and a secondary one at v
= -417 km/s -- along with an underlying broad component that is presumably
representative of the smooth spheroid of M31. Both peaks are found to be
dynamically cold with intrinsic velocity dispersions of sigma(v) = 16 km/s. The
mean metallicity and metallicity dispersion of stars in the two peaks is also
found to be similar: [Fe/H] = -0.45 and sigma([Fe/H]) = 0.2. The observed
velocity of the primary peak is consistent with that predicted by dynamical
models for the stream, but there is no obvious explanation for the secondary
peak. The nature of the secondary cold population is unclear: it may represent:
(1) tidal debris from a satellite merger event that is superimposed on, but
unrelated to, the giant southern stream; (2) a wrapped around component of the
giant southern stream; (3) a warp or overdensity in M31's disk at R > 50 kpc
(this component is well above the outward extrapolation of the smooth
exponential disk brightness profile).Comment: 32 pages, 13 figures, 1 table. Accepted for publication in Ap
Observations of the Extended Distribution of Ionized Hydrogen in the Plane of M31
We have used the Wisconsin H-Alpha Mapper (WHAM) to observe the spatially
extended distribution of ionized hydrogen in M31 beyond the stellar disk. We
obtained five sets of observations, centered near the photometric major axis of
M31, that extend from the center of the galaxy to just off the edge of the
southwestern HI disk. Beyond the bright stellar disk, but within the HI disk,
weak H-alpha is detected with an intensity I(H-alpha) = 0.05 (+0.01 / -0.02)
Rayleighs. Since M31 is inclined 77 degrees with respect to the line of sight,
this implies that the ambient intergalactic ionizing flux onto each side of M31
is Phi_0 <= 1.6 x 10^4 photons cm^-2 s^-1. Just beyond the outer boundary of
the HI disk we find no significant detection of H-alpha and place an upper
limit I(H-alpha) <= 0.019 Rayleighs.Comment: To appear in ApJ Letters; 12 pages, 4 figure
Deep Photometry in a Remote M31 Major Axis Field Near G1
We present photometry from Hubble Space Telescope (HST)/Wide Field Planetary
Camera 2 parallel imagery of a remote M31 field at a projected distance of
about 34 kpc from the nucleus near the SW major axis. This field is near the
globular cluster G1, and near one of the candidate tidal plumes identified by
Ferguson et al. (2002). The F606W (V) and F814W (I) images were obtained in
parallel with Space Telescope Imaging Spectrograph spectroscopy of G1 (GO-9099)
and total 7.11 hours of integration time -- the deepest HST field in the outer
disk of M31 to date, reaching to V ~ 28. The color-magnitude diagram of the
field shows a clearly-defined red clump at V = 25.25 and a red giant branch
consistent with [Fe/H] ~ -0.7. The lack of a blue horizontal branch contrasts
with other M31 halo fields, the Andromeda dwarf spheroidals, and with the
nearby globular cluster G1. Comparing the observed luminosity function to the
Padova models, we find that at least some of the stellar population must be
younger than 6 - 8 Gyr. The outermost detected neutral hydrogen gas disk of M31
lies only 2 kpc in projection from our field. The finding that some giants in
the field have radial velocities close to that of the neutral hydrogen gas
(Reitzel, Guhathakurta, & Rich 2003) leads us to conclude that our field
samples the old, low-surface-brightness disk rather than the true Population II
spheroid.Comment: 15 pages, 3 figures. accepted for publication in the A
Microjansky radio sources in DC0107-46 (Abell 2877)
The cluster DC0107-46 (Abell 2877) lies within the Phoenix Deep Survey, made
at 1.4 GHz with the Australia Telescope Compact Array. Of 89 known optical
cluster members, 70 lie within the radio survey area. Of these 70 galaxies, 15
(21%) are detected, with luminosities as faint as 10^20 W/Hz. Spectroscopic
observations are available for 14/15 of the radio-detected cluster galaxies.
Six galaxies show only absorption features and are typical low-luminosity AGN
radio sources. One galaxy hosts a Seyfert 2 nucleus, two are star-forming
galaxies, and the remaining five may be star-forming galaxies, AGNs, or both.Comment: 14 pages, 3 figures, Accepted by ApJS (v128n2p JUN 2000 issue
High Velocity Cloud Complex H: A Satellite of the Milky Way in a Retrograde Orbit?
Observations with the Green Bank Telescope of 21cm HI emission from the
high-velocity cloud Complex H suggest that it is interacting with the Milky
Way. A model in which the cloud is a satellite of the Galaxy in an inclined,
retrograde circular orbit reproduces both the cloud's average velocity and its
velocity gradient with latitude. The model places Complex H at approximately 33
kpc from the Galactic Center on a retrograde orbit inclined about 45 degrees to
the Galactic plane. At this location it has an HI mass > 6 10^6 Msun and
dimensions of at least 10 by 5 kpc. Some of the diffuse HI associated with the
cloud has apparently been decelerated by interaction with Galactic gas. Complex
H has similarities to the dwarf irregular galaxy Leo A and to some compact
high-velocity clouds, and has an internal structure nearly identical to parts
of the Magellanic Stream, with a pressure P/k about 100 cm^{-3} K.Comment: 12 pages includes 4 figures. To be published in Astrophysical Journal
Letters, 1 July 200
Ambiguity, multiple streams, and EU policy
The multiple streams framework draws insight from interactions between agency and institutions to explore the impact of context, time, and meaning on policy change and to assess the institutional and issue complexities permeating the European Union (EU) policy process. The authors specify the assumptions and structure of the framework and review studies that have adapted it to reflect more fully EU decision-making processes. The nature of policy entrepreneurship and policy windows are assessed to identify areas of improvement. Finally, the authors sketch out a research agenda that refines the logic of political manipulation which permeates the lens and the institutional complexity which frames the EU policy process
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Small Systems, Big Targets: Power Sector Reforms and Renewable Energy Development in Small Electricity Systems
The dominant focus of much policy attention of late has been on the suitability of electricity market reform carried out under the 'standard' or prescriptive approach - the end point of which is market liberalization - for the integration of intermittent renewables. There is now a growing consensus around the argument that traditional energy-only electricity markets where prices are based on system marginal cost cannot function efficiently with both fossil fuels and renewables, potentially resulting in market disruptions and price volatility. Consequently, most policy discussion has focused on finding ways to successfully integrate the two through adopting advanced competitive solutions (such as the use of capacity markets in addition to energy-only markets) in larger electricity systems. This paper however argues that the effectiveness of competition is limited by the size of an electricity system - in other words, there is a minimum threshold size (and other associated characteristics such as tropical locations, lack of access, and the prevalence of remote communities of consumers) under which competition will not produce expected outcomes, and for which distinctive policy solutions are required. This paper contributes to the policy discourse by discussing the reform of small electricity systems to integrate renewable energy via the means of three case studies: Nicaragua, El Salvador, and their application to Australia's Northern Territory. The paper draws some policy lessons that can be considered for other small electricity systems in island economies and territories across Africa, the Caribbean, and the Asia-Pacific, that are pursuing a triad of objectives including electricity sector reform, large-scale renewables development and improving energy access
A possible radio supernova in the outer part of NGC 3310
As part of an on-going radio supernova monitoring program, we have discovered
a variable, compact steep spectrum radio source ~65 arcsec (~4 kpc) from the
centre of the starburst galaxy NGC 3310. If the source is at the distance of
NGC 3310, then its 5 GHz luminosity is ~3 x 10^{19} WHz^-1. The source
luminosity, together with its variability characteristics, compact structure
(<17 mas) and its association with a group of HII regions, leads us to propose
that it is a previously uncatalogued type II radio supernova. A search of
archival data also shows an associated X-ray source with a luminosity similar
to known radio supernova.Comment: 5 pages, 3 figures. Accepted by MNRA
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