504 research outputs found
What controls deuterium excess in global precipitation?
The deuterium excess (<i>d</i>) of precipitation is widely used in the
reconstruction of past climatic changes from ice cores. However, its
most common interpretation as moisture source temperature cannot
directly be inferred from present-day water isotope
observations. Here, we use a new empirical relation between <i>d</i> and
near-surface relative humidity (RH) together with reanalysis data to
globally predict <i>d</i> of surface evaporation from the ocean. The very
good quantitative agreement of the predicted hemispherically averaged
seasonal cycle with observed <i>d</i> in precipitation indicates that
moisture source relative humidity, and not sea surface temperature, is
the main driver of <i>d</i> variability on seasonal timescales. Furthermore,
we review arguments for an interpretation of long-term palaeoclimatic <i>d</i>
changes in terms of moisture source temperature, and we conclude that
there remains no sufficient evidence that would justify to neglect the
influence of RH on such palaeoclimatic <i>d</i> variations. Hence, we suggest
that either the interpretation of <i>d</i> variations in palaeorecords should
be adapted to reflect climatic influences on RH during evaporation, in
particular atmospheric circulation changes, or new arguments for an
interpretation in terms of moisture source temperature will have to
be provided based on future research
The importance of fronts for extreme precipitation
This is the final version. Available from AGU via the DOI in this recordExtratropical cyclones and their associated frontal systems are well known to be related to heavy precipitation events. Here an objective method is used to directly link extreme precipitation events with atmospheric fronts, identified using European Centre for MediumâRange Weather Forecasts Interim Reanalysis data, to quantify the importance of fronts for precipitation extremes globally. In some parts of the major midlatitude storm track regions, over 90% of precipitation extremes are associated with fronts, with slightly more events associated with warm fronts than cold fronts. On average, 51% of global precipitation extremes are associated with fronts, with 75% in the midlatitudes and 31% in the tropics. A large proportion of extreme precipitation events occur in the presence of both a cyclone and a front, but remote fronts are responsible for many of the âfrontâonlyâ events. The fronts producing extreme precipitation events are found to have up to 35% stronger frontal gradients than other fronts, potentially providing some improved forecasting capabilities for extreme precipitation events.This study was supported by the Australian Research Council through the Linkage Project grant LP0883961, and the Discovery Project grant DP0877417
The isotopic composition of precipitation from a winter storm â a case study with the limited-area model COSMO<sub>iso</sub>
Stable water isotopes are valuable tracers of the atmospheric water cycle, and potentially provide useful information also on weather-related processes. In order to further explore this potential, the water isotopes H<sub>2</sub><sup>18</sup>O and HDO are incorporated into the limited-area model COSMO. In a first case study, the new COSMO<sub>iso</sub> model is used for simulating a winter storm event in January 1986 over the eastern United States associated with intense frontal precipitation. The modelled isotope ratios in precipitation and water vapour are compared to spatially distributed δ</sub><sup>18</sup>O observations. COSMO<sub>iso</sub> very accurately reproduces the statistical distribution of δ</sub><sup>18</sup>O in precipitation, and also the synoptic-scale spatial pattern and temporal evolution agree well with the measurements. Perpendicular to the front that triggers most of the rainfall during the event, the model simulates a gradient in the isotopic composition of the precipitation, with high δ</sub><sup>18</sup>O values in the warm air and lower values in the cold sector behind the front. This spatial pattern is created through an interplay of large scale air mass advection, removal of heavy isotopes by precipitation at the front and microphysical interactions between rain drops and water vapour beneath the cloud base. This investigation illustrates the usefulness of high resolution, event-based model simulations for understanding the complex processes that cause synoptic-scale variability of the isotopic composition of atmospheric waters. In future research, this will be particularly beneficial in combination with laser spectrometric isotope observations with high temporal resolution
Formation of Short-Period Binary Pulsars in Globular Clusters
We present a new dynamical scenario for the formation of short-period binary
millisecond pulsars in globular clusters. Our work is motivated by the recent
observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc,
most neutron stars acquire binary companions through exchange interactions with
primordial binaries. The resulting systems have semimajor axes in the range
\~0.1-1 AU and neutron star companion masses ~1-3 Msun. For many of these
systems we find that, when the companion evolves off the main sequence and
fills its Roche lobe, the subsequent mass transfer is dynamically unstable.
This leads to a common envelope phase and the formation of short-period neutron
star - white dwarf binaries. For a significant fraction of these binaries, the
decay of the orbit due to gravitational radiation will be followed by a period
of stable mass transfer driven by a combination of gravitational radiation and
tidal heating of the companion. The properties of the resulting short-period
binaries match well those of observed binary pulsars in 47 Tuc.Comment: To appear in ApJ Letters, slightly abbreviated version with only
minor change
Extratropical Cyclones in Idealized Simulations of Changed Climates
Cyclones are a key element of extratropical weather and frequently lead to extreme events like wind storms and heavy precipitation. Understanding potential changes of cyclone frequency and intensity is thus essential for a proper assessment of climate change impacts. Here the behavior of extratropical cyclones under strongly varying climate conditions is investigated using idealized climate model simulations in an aquaplanet setup. A cyclone tracking algorithm is applied to assess various statistics of cyclone properties such as intensity, size, lifetime, displacement velocity, and deepening rates. In addition, a composite analysis of intense cyclones is performed. In general, the structure of extratropical cyclones in the idealized simulations is very robust, and changes in major cyclone characteristics are relatively small. Median cyclone intensity, measured in terms of minimum sea level pressure and lower-tropospheric relative vorticity, has a maximum in simulations with global mean temperature slightly warmer than present-day Earth, broadly consistent with the behavior of the eddy kinetic energy analyzed in previous studies. Maximum deepening rates along cyclone tracks behave similarly and are in agreement with linear quasigeostrophic growth rates if the effect of latent heat release on the stratification is taken into account. In contrast to moderate cyclones, the relative vorticity of intense cyclones continues to increase with warming to substantially higher temperatures, and this is associated with enhanced lower-tropospheric potential vorticity anomalies likely caused by increased diabatic heating. Moist processes may, therefore, lead to the further strengthening of intense cyclones in warmer climates even if cyclones weaken on average.National Science Foundation (U.S.) (GS-1148594
What are Hybrid Development Methods Made Of?
Regardless of company size or industry sector, a majority of project teams and companies use customized processes that combine different development methods-so-called hybrid development methods. Even though such hybrid development methods are highly individualized, a common understanding of how to systematically construct synergetic practices is missing. Based on 1,467 data points from a large-scale online survey among practitioners, we study the current state of practice in process use to answer the question: What are hybrid development methods made of? Our findings reveal that only eight methods and few practices build the core of modern software development. This small set allows for statistically constructing hybrid development methods
Cost studies for commercial fuselage crown designs
Studies were conducted to evaluate the cost and weight potential of advanced composite design concepts in the crown region of a commercial transport. Two designs from each of three design families were developed using an integrated design-build team. A range of design concepts and manufacturing processes were included to allow isolation and comparison of cost centers. Detailed manufacturing/assembly plans were developed as the basis for cost estimates. Each of the six designs was found to have advantages over the 1995 aluminum benchmark in cost and weight trade studies. Large quadrant panels and cobonded frames were found to save significant assembly labor costs. Comparisons of high- and intermediate-performance fiber systems were made for skin and stringer applications. Advanced tow placement was found to be an efficient process for skin lay up. Further analysis revealed attractive processes for stringers and frames. Optimized designs were informally developed for each design family, combining the most attractive concepts and processes within that family. A single optimized design was selected as the most promising, and the potential for further optimization was estimated. Technical issues and barriers were identified
A numerical process study on the rapid transport of stratospheric air down to the surface over western North America and the Tibetan Plateau
Upper-level fronts are often associated with the rapid transport of
stratospheric air along tilted isentropes to the middle or lower troposphere,
where this air leads to significantly enhanced ozone concentrations. These
plumes of originally stratospheric air can only occasionally be observed at
the surface because (i)Â stable boundary layers prevent an efficient vertical
transport down to the surface, and (ii)Â even if boundary layer turbulence
were strong enough to enable this transport, the originally stratospheric air
mass can be diluted by mixing, such that only a weak stratospheric signal can
be recorded at the surface. Most documented examples of stratospheric air
reaching the surface occurred in mountainous regions. This study investigates
two such events, using a passive stratospheric air mass tracer in a mesoscale
model to explore the processes that enable the transport down to the surface.
The events occurred in early May 2006 in the Rocky Mountains and in mid-June
2006 on the Tibetan Plateau. In both cases, a tropopause fold associated with
an upper-level front enabled stratospheric air to enter the troposphere. In
our model simulation of the North American case, the strong frontal zone
reaches down to 700 hPa and leads to a fairly direct vertical transport of
the stratospheric tracer along the tilted isentropes to the surface. In the
Tibetan Plateau case, however, no near-surface front exists and a reservoir
of high stratospheric tracer concentrations initially forms at 300â400 hPa,
without further isentropic descent. However, entrainment at the top of the
very deep boundary layer (reaching to 300 hPa over the Tibetan Plateau) and
turbulence within the boundary layer allows for downward transport of
stratospheric air to the surface. Despite the strongly differing dynamical
processes, stratospheric tracer concentrations at the surface reach peak
values of 10 %â20 % of the imposed stratospheric value in both
cases, corroborating the potential of deep stratosphere-to-troposphere
transport events to significantly influence surface ozone concentrations in
these regions.</p
What are Hybrid Development Methods Made Of? An Evidence-Based Characterization
Among the multitude of software development processes available, hardly any is used by the book. Regardless of company size or industry sector, a majority of project teams and companies use customized processes that combine different development methodsâ so-called hybrid development methods. Even though such hybrid development methods are highly individualized, a common understanding of how to systematically construct synergetic practices is missing. In this paper, we make a first step towards devising such guidelines. Grounded in 1,467 data points from a large-scale online survey among practitioners, we study the current state of practice in process use to answer the question: What are hybrid development methods made of? Our findings reveal that only eight methods and few practices build the core of modern software development. This small set allows for statistically constructing hybrid development methods. Using an 85% agreement level in the participantsâ selections, we provide two examples illustrating how hybrid development methods are characterized by the practices they are made of. Our evidence-based analysis approach lays the foundation for devising hybrid development methods
- âŚ