1,225 research outputs found
The hydrological significance of mountains: from regional to global scale
International audienceMountain regions supply a large share of the world's population with fresh water. Quantification of the hydrological significance of mountains, however, is subject to great uncertainty. Instead of focusing on global averages in advance, the present analysis follows a catchment-based approach using discharge data provided by the Global Runoff Data Centre (GRDC). The River Rhine originating in the European Alps is chosen as a first study area, revealing the hydrological relationship between mountainous and lowland regions in a well-documented area. Following the findings from this analysis, different aspects of runoff characteristics for a total of 22 case-study river basins world-wide have been investigated and compared, for a global view. The view has been extended through aspects of climate and human use of mountain runoff. The particular hydrological characteristics of mountain areas are characterised by disproportionately large discharges. In humid areas, mountains supply up to 20?50% of total discharge while in arid areas, mountains contribute from 50?90% of total discharge, with extremes of over 95%. The overall assessment of the hydrological significance of mountain areas reveals that the world's major "water towers" are found in arid or semi-arid zones where they provide essential fresh water for a significant proportion of a quickly growing global population. Keywords: mountain hydrology, global comparative assessment, runoff, water resources, sustainability, Rhine River, European Alp
On the formation of low-mass black holes in massive binary stars
Recently (Brown \& Bethe 1994) it was suggested that most stars with main
sequence mass in the range of about explode, returning
matter to the Galaxy, and then go into low-mass () black
holes. Even more massive main-sequence stars would, presumably, chiefly g o
into high-mass () black holes. The Brown-Bethe estimates
gave approximately low-mass black holes in the Galaxy. A
pressing question, which we attempt to answer here, is why, with the possible
exception of the compact objects in SN1987A and 4U\,1700--37, none of these
have been seen.
We address this question in three parts. Firstly, black holes are generally
``seen'' only in binaries, by the accretion of matter from a companion star.
High mass black holes are capable of accreting more matter than low-mass black
holes, so there is a selection effect favoring them. This, in itself, would not
be sufficient to show why low-mass black holes have not been seen, since
neutron stars (of nearly the same mass) are seen in abundance.
Secondly, and this is our main point, the primary star in a binary ---the
first star to evolve--- loses its hydrogen envelope by transfer of matter to
the secondary and loss into space, and the resulting ``naked'' helium star
evolves differently than a helium core, which is at least initially covered by
the hydrogen envelope in a massive main-sequence star. We show that primary
stars in binaries can end up as neutron stars even if their initial mass
substantially exceeds the mass limit for neutron star formation from single
stars (). An example is 4U\,1223--62, in which we suggest
that the initial primary mass exceeded , yet X-ray pulsationsComment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Surface water floods in Switzerland: what insurance claim records tell us about the damage in space and time
Surface water floods (SWFs) have received increasing attention in the recent
years. Nevertheless, we still know relatively little about where, when and
why such floods occur and cause damage, largely due to a lack of data but
to some degree also because of terminological ambiguities. Therefore, in a
preparatory step, we summarize related terms and identify the need for
unequivocal terminology across disciplines and international boundaries in
order to bring the science together. Thereafter, we introduce a large
(n = 63 117), long (10–33 years) and representative
(48 % of all Swiss buildings covered) data set of spatially explicit
Swiss insurance flood claims. Based on registered flood damage to buildings,
the main aims of this study are twofold: First, we introduce a method to
differentiate damage caused by SWFs and fluvial floods based on the
geographical location of each damaged object in relation to flood hazard maps
and the hydrological network. Second, we analyze the data with respect to
their spatial and temporal distributions aimed at quantitatively answering
the fundamental questions of how relevant SWF damage really is, as well as
where and when it occurs in space and time.
This study reveals that SWFs are responsible for at least 45 % of the
flood damage to buildings and 23 % of the associated direct tangible
losses, whereas lower losses per claim are responsible for the lower loss
share. The Swiss lowlands are affected more heavily by SWFs than the alpine
regions. At the same time, the results show that the damage claims and
associated losses are not evenly distributed within each region either.
Damage caused by SWFs occurs by far most frequently in summer in almost all
regions. The normalized SWF damage of all regions shows no significant upward
trend between 1993 and 2013. We conclude that SWFs are in fact a highly
relevant process in Switzerland that should receive similar attention like
fluvial flood hazards. Moreover, as SWF damage almost always coincides with
fluvial flood damage, we suggest considering SWFs, like fluvial floods, as integrated processes of
our catchments
Retrospective analysis of a nonforecasted rain-on-snow flood in the Alps – a matter of model limitations or unpredictable nature?
A rain-on-snow flood occurred in the Bernese Alps, Switzerland, on 10 October
2011, and caused significant damage. As the flood peak was unpredicted by the
flood forecast system, questions were raised concerning the causes and the
predictability of the event. Here, we aimed to reconstruct the anatomy of
this rain-on-snow flood in the Lötschen Valley (160 km<sup>2</sup>) by
analyzing meteorological data from the synoptic to the local scale and by
reproducing the flood peak with the hydrological model WaSiM-ETH (Water Flow
and Balance Simulation Model). This in order to gain process understanding
and to evaluate the predictability.
<br><br>
The atmospheric drivers of this rain-on-snow flood were (i) sustained
snowfall followed by (ii) the passage of an atmospheric river bringing warm
and moist air towards the Alps. As a result, intensive rainfall (average of
100 mm day<sup>-1</sup>) was accompanied by a temperature increase that
shifted the 0° line from 1500 to 3200 m a.s.l. (meters above
sea level) in 24 h with a maximum increase of 9 K in
9 h. The south-facing slope of the valley received significantly more
precipitation than the north-facing slope, leading to flooding only in
tributaries along the south-facing slope. We hypothesized that the reason for
this very local rainfall distribution was a cavity circulation combined with
a seeder-feeder-cloud system enhancing local rainfall and snowmelt along the
south-facing slope.
<br><br>
By applying and considerably recalibrating the standard hydrological model
setup, we proved that both latent and sensible heat fluxes were needed to
reconstruct the snow cover dynamic, and that locally high-precipitation sums
(160 mm in 12 h) were required to produce the estimated flood
peak. However, to reproduce the rapid runoff responses during the event, we
conceptually represent likely lateral flow dynamics within the snow cover
causing the model to react "oversensitively" to meltwater.
<br><br>
Driving the optimized model with COSMO (Consortium for Small-scale
Modeling)-2 forecast data, we still failed to simulate the flood because
COSMO-2 forecast data underestimated both the local precipitation peak and
the temperature increase. Thus we conclude that this rain-on-snow flood was,
in general, predictable, but requires a special hydrological model setup and
extensive and locally precise meteorological input data. Although, this data
quality may not be achieved with forecast data, an additional model with a
specific rain-on-snow configuration can provide useful information when
rain-on-snow events are likely to occur
The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions
We model the production of OH+, H2O+, and H3O+ in interstellar clouds, using
a steady state photodissociation region code that treats the freeze-out of gas
species, grain surface chemistry, and desorption of ices from grains. The code
includes PAHs, which have important effects on the chemistry. All three ions
generally have two peaks in abundance as a function of depth into the cloud,
one at A_V<~1 and one at A_V~3-8, the exact values depending on the ratio of
incident ultraviolet flux to gas density. For relatively low values of the
incident far ultraviolet flux on the cloud ({\chi}<~ 1000; {\chi}= 1= local
interstellar value), the columns of OH+ and H2O+ scale roughly as the cosmic
ray primary ionization rate {\zeta}(crp) divided by the hydrogen nucleus
density n. The H3O+ column is dominated by the second peak, and we show that if
PAHs are present, N(H3O+) ~ 4x10^{13} cm^{-2} independent of {\zeta}(crp) or n.
If there are no PAHs or very small grains at the second peak, N(H3O+) can
attain such columns only if low ionization potential metals are heavily
depleted. We also model diffuse and translucent clouds in the interstellar
medium, and show how observations of N(OH+)/N(H) and N(OH+)/N(H2O+) can be used
to estimate {\zeta}(crp)/n, {\chi}/n and A_V in them. We compare our models to
Herschel observations of these two ions, and estimate {\zeta}(crp) ~ 4-6 x
10^-16 (n/100 cm^-3) s^-1 and \chi/n = 0.03 cm^3 for diffuse foreground clouds
towards W49N
Chemical and physical influences on aerosol activation in liquid clouds: a study based on observations from the Jungfraujoch, Switzerland
A simple statistical model to predict the number of aerosols which activate to form cloud droplets in warm clouds has been established, based on regression analysis of data from four summertime Cloud and Aerosol Characterisation Experiments (CLACE) at the high-altitude site Jungfraujoch (JFJ). It is shown that 79 % of the observed variance in droplet numbers can be represented by a model accounting only for the number of potential cloud condensation nuclei (defined as number of particles larger than 80 nm in diameter), while the mean errors in the model representation may be reduced by the addition of further explanatory variables, such as the mixing ratios of O3, CO, and the height of the measurements above cloud base. The statistical model has a similar ability to represent the observed droplet numbers in each of the individual years, as well as for the two predominant local wind directions at the JFJ (northwest and southeast). Given the central European location of the JFJ, with air masses in summer being representative of the free troposphere with regular boundary layer in-mixing via convection, we expect that this statistical model is generally applicable to warm clouds under conditions where droplet formation is aerosol limited (i.e. at relatively high updraught velocities and/or relatively low aerosol number concentrations). A comparison between the statistical model and an established microphysical parametrization shows good agreement between the two and supports the conclusion that cloud droplet formation at the JFJ is predominantly controlled by the number concentration of aerosol particles
Perspectives on Interstellar Dust Inside and Outside of the Heliosphere
Measurements by dust detectors on interplanetary spacecraft appear to
indicate a substantial flux of interstellar particles with masses exceeding
10^{-12}gram. The reported abundance of these massive grains cannot be typical
of interstellar gas: it is incompatible with both interstellar elemental
abundances and the observed extinction properties of the interstellar dust
population. We discuss the likelihood that the Solar System is by chance
located near an unusual concentration of massive grains and conclude that this
is unlikely, unless dynamical processes in the ISM are responsible for such
concentrations. Radiation pressure might conceivably drive large grains into
"magnetic valleys". If the influx direction of interstellar gas and dust is
varying on a ~10 yr timescale, as suggested by some observations, this would
have dramatic implications for the small-scale structure of the interstellar
medium.Comment: 13 pages. To appear in Space Science Review
Mid-infrared PAH and H2 emission as a probe of physical conditions in extreme PDRs
Mid-infrared (IR) observations of polycyclic aromatic hydrocarbons (PAHs) and
molecular hydrogen emission are a potentially powerful tool to derive physical
properties of dense environments irradiated by intense UV fields. We present
new, spatially resolved, \emph{Spitzer} mid-IR spectroscopy of the high
UV-field and dense photodissocation region (PDR) around Monoceros R2, the
closest ultracompact \hII region, revealing the spatial structure of ionized
gas, PAHs and H emissions. Using a PDR model and PAH emission feature
fitting algorithm, we build a comprehensive picture of the physical conditions
prevailing in the region. We show that the combination of the measurement of
PAH ionization fraction and of the ratio between the H 0-0 S(3) and S(2)
line intensities, respectively at 9.7 and 12.3 m, allows to derive the
fundamental parameters driving the PDR: temperature, density and UV radiation
field when they fall in the ranges K, cm,
respectively. These mid-IR spectral tracers thus provide a tool
to probe the similar but unresolved UV-illuminated surface of protoplanetary
disks or the nuclei of starburst galaxies.Comment: Accepted for publication in ApJ Letter
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