77 research outputs found
Validation of water vapour transport in the tropical tropopause region in coupled Chemistry Climate Models
International audienceIn this study backward trajectories from the tropical lower stratosphere were calculated for the Northern Hemisphere (NH) winters 1995?1996, 1997?1998 (El Niño) and 1998?1999 (La Niña) and summers 1996, 1997 and 1999 using both ERA-40 reanalysis data of the European Centre for Medium-Range Weather Forecast (ECMWF) and coupled chemistry climate model (CCM) data. The calculated trajectories were analyzed to determine the distribution of points where individual air masses encounter the minimum temperature and thus minimum water vapour mixing ratio during their ascent through the tropical tropopause layer (TTL) into the stratosphere. The geographical distribution of these dehydration points and the local conditions there determine the overall water vapour entry into the stratosphere. Results of two CCMs are presented: the ECHAM4.L39(DLR)/CHEM (hereafter: E39/C) from the German Aerospace Center (DLR) and the Freie Universität Berlin Climate Middle Atmosphere Model with interactive chemistry (hereafter: FUB-CMAM-CHEM). In the FUB-CMAM-CHEM model the minimum temperatures are overestimated by about 7 K in Northern Hemisphere (NH) winter as well as in NH summer, resulting in too high water vapour entry values compared to ERA-40. However, the geographical distribution of dehydration points is fairly reproduced for NH winter 1995?1996 and 1998?1999 and in all boreal summers. The distribution of dehydration points suggests an influence of the Indian monsoon upon the water vapour transport. The E39/C model displays a temperature bias of about +3 K. Hence, the minimum water vapour mixing ratios are higher relative to ERA-40. The geographical distribution of dehydration points is satisfactory in NH winter 1995?1996 and 1997?1998 with respect to ERA-40. The distribution is not reproduced for the NH winter 1998?1999 (La Niña event) compared to ERA-40. There is excessive mass flux through warm regions e.g. Africa, leading to excessive water vapour flux in the NH winter and summer. The possible influence of the Indian monsoon on the transport is not seen in the boreal summer. Further, the residence times of air parcels in the TTL were derived from the trajectory calculations. The analysis of the residence times reveals that in both CCMs residence times in the TTL are underestimated compared to ERA-40 and the seasonal variation is hardly present
Multiplicities of charged pions and unidentified charged hadrons from deep-inelastic scattering of muons off an isoscalar target
Multiplicities of charged pions and unidentified hadrons produced in
deep-inelastic scattering were measured in bins of the Bjorken scaling variable
, the relative virtual-photon energy and the relative hadron energy .
Data were obtained by the COMPASS Collaboration using a 160 GeV muon beam and
an isoscalar target (LiD). They cover the kinematic domain in the photon
virtuality > 1(GeV/c, , and . In addition, a leading-order pQCD analysis was performed using the
pion multiplicity results to extract quark fragmentation functions
Interplay among transversity induced asymmetries in hadron leptoproduction
In the fragmentation of a transversely polarized quark several left-right
asymmetries are possible for the hadrons in the jet. When only one unpolarized
hadron is selected, it exhibits an azimuthal modulation known as Collins
effect. When a pair of oppositely charged hadrons is observed, three
asymmetries can be considered, a di-hadron asymmetry and two single hadron
asymmetries. In lepton deep inelastic scattering on transversely polarized
nucleons all these asymmetries are coupled with the transversity distribution.
From the high statistics COMPASS data on oppositely charged hadron-pair
production we have investigated for the first time the dependence of these
three asymmetries on the difference of the azimuthal angles of the two hadrons.
The similarity of transversity induced single and di-hadron asymmetries is
discussed. A new analysis of the data allows to establish quantitative
relationships among them, providing for the first time strong experimental
indication that the underlying fragmentation mechanisms are all driven by a
common physical process.Comment: 6 figure
The Spin Structure Function of the Proton and a Test of the Bjorken Sum Rule
New results for the double spin asymmetry and the proton
longitudinal spin structure function are presented. They were
obtained by the COMPASS collaboration using polarised 200 GeV muons scattered
off a longitudinally polarised NH target. The data were collected in 2011
and complement those recorded in 2007 at 160\,GeV, in particular at lower
values of . They improve the statistical precision of by
about a factor of two in the region . A next-to-leading order
QCD fit to the world data is performed. It leads to a new determination
of the quark spin contribution to the nucleon spin, ranging
from 0.26 to 0.36, and to a re-evaluation of the first moment of .
The uncertainty of is mostly due to the large uncertainty in
the present determinations of the gluon helicity distribution. A new evaluation
of the Bjorken sum rule based on the COMPASS results for the non-singlet
structure function yields as ratio of the axial and
vector coupling constants , which validates the sum rule to an accuracy of about
9\%.Comment: 19 pages, 8 figures and table
Resonance Production and S-wave in at 190 GeV/c
The COMPASS collaboration has collected the currently largest data set on
diffractively produced final states using a negative pion
beam of 190 GeV/c momentum impinging on a stationary proton target. This data
set allows for a systematic partial-wave analysis in 100 bins of three-pion
mass, GeV/c , and in 11 bins of the reduced
four-momentum transfer squared, (GeV/c) . This
two-dimensional analysis offers sensitivity to genuine one-step resonance
production, i.e. the production of a state followed by its decay, as well as to
more complex dynamical effects in nonresonant production. In this paper,
we present detailed studies on selected partial waves with , , , , and . In these waves, we observe
the well-known ground-state mesons as well as a new narrow axial-vector meson
decaying into . In addition, we present the results
of a novel method to extract the amplitude of the subsystem with
in various partial waves from the
data. Evidence is found for correlation of the and
appearing as intermediate isobars in the decay of the known
and .Comment: 96 page
Transverse-momentum-dependent Multiplicities of Charged Hadrons in Muon-Deuteron Deep Inelastic Scattering
A semi-inclusive measurement of charged hadron multiplicities in deep
inelastic muon scattering off an isoscalar target was performed using data
collected by the COMPASS Collaboration at CERN. The following kinematic domain
is covered by the data: photon virtuality (GeV/), invariant
mass of the hadronic system GeV/, Bjorken scaling variable in the
range , fraction of the virtual photon energy carried by the
hadron in the range , square of the hadron transverse momentum
with respect to the virtual photon direction in the range 0.02 (GeV/ (GeV/). The multiplicities are presented as a
function of in three-dimensional bins of , , and
compared to previous semi-inclusive measurements. We explore the
small- region, i.e. (GeV/), where
hadron transverse momenta are expected to arise from non-perturbative effects,
and also the domain of larger , where contributions from
higher-order perturbative QCD are expected to dominate. The multiplicities are
fitted using a single-exponential function at small to study
the dependence of the average transverse momentum on , and . The power-law behaviour of the
multiplicities at large is investigated using various
functional forms. The fits describe the data reasonably well over the full
measured range.Comment: 28 pages, 20 figure
Leading-order determination of the gluon polarisation from semi-inclusive deep inelastic scattering data
Using a novel analysis technique, the gluon polarisation in the nucleon is
re-evaluated using the longitudinal double-spin asymmetry measured in the cross
section of semi-inclusive single-hadron muoproduction with photon virtuality
. The data were obtained by the COMPASS experiment at
CERN using a 160 GeV/ polarised muon beam impinging on a polarised LiD
target. By analysing the full range in hadron transverse momentum ,
the different -dependences of the underlying processes are separated
using a neural-network approach. In the absence of pQCD calculations at
next-to-leading order in the selected kinematic domain, the gluon polarisation
is evaluated at leading order in pQCD at a hard scale of . It is determined in three intervals
of the nucleon momentum fraction carried by gluons, , covering the
range ~ and does not exhibit a significant
dependence on . The average over the three intervals, at
, suggests that the gluon polarisation
is positive in the measured range.Comment: 14 pages, 6 figure
Multiplicities of charged kaons from deep-inelastic muon scattering off an isoscalar target
Precise measurements of charged-kaon multiplicities in deep inelastic scattering were performed. The results are presented in three-dimensional bins of the Bjorken scaling variable x, the relative virtual-photon energy y, and the fraction z of the virtual-photon energy carried by the produced hadron. The data were obtained by the COMPASS Collaboration by scattering 160 GeV muons off an isoscalar 6LiD target. They cover the kinematic domain View the MathML source in the photon virtuality, 0.0045 GeV/c2 in the invariant mass of the hadronic system. The results from the sum of the z -integrated K+ and K 12 multiplicities at high x point to a value of the non-strange quark fragmentation function larger than obtained by the earlier DSS fit
Estimates of ozone return dates from Chemistry-Climate Model Initiative simulations
We analyse simulations performed for the
Chemistry-Climate Model Initiative (CCMI) to estimate the
return dates of the stratospheric ozone layer from depletion
caused by anthropogenic stratospheric chlorine and bromine.
We consider a total of 155 simulations from 20 models, including
a range of sensitivity studies which examine the impact
of climate change on ozone recovery. For the control
simulations (unconstrained by nudging towards analysed meteorology)
there is a large spread (±20 DU in the global average)
in the predictions of the absolute ozone column. Therefore,
the model results need to be adjusted for biases against
historical data. Also, the interannual variability in the model
results need to be smoothed in order to provide a reasonably
narrow estimate of the range of ozone return dates. Consistent
with previous studies, but here for a Representative
Concentration Pathway (RCP) of 6.0, these new CCMI simulations
project that global total column ozone will return to
1980 values in 2049 (with a 1σ uncertainty of 2043–2055).
At Southern Hemisphere mid-latitudes column ozone is projected
to return to 1980 values in 2045 (2039–2050), and at
Northern Hemisphere mid-latitudes in 2032 (2020–2044). In
the polar regions, the return dates are 2060 (2055–2066) in
the Antarctic in October and 2034 (2025–2043) in the Arctic
in March. The earlier return dates in the Northern Hemisphere
reflect the larger sensitivity to dynamical changes.
Our estimates of return dates are later than those presented
in the 2014 Ozone Assessment by approximately 5–17 years,
depending on the region, with the previous best estimates
often falling outside of our uncertainty range. In the tropics
only around half the models predict a return of ozone to
1980 values, around 2040, while the other half do not reach
the 1980 value. All models show a negative trend in tropical
total column ozone towards the end of the 21st century. The
CCMI models generally agree in their simulation of the time
evolution of stratospheric chlorine and bromine, which are
the main drivers of ozone loss and recovery. However, there
are a few outliers which show that the multi-model mean results
for ozone recovery are not as tightly constrained as possible.
Throughout the stratosphere the spread of ozone return
dates to 1980 values between models tends to correlate with
the spread of the return of inorganic chlorine to 1980 values.
In the upper stratosphere, greenhouse gas-induced cooling
speeds up the return by about 10–20 years. In the lower
stratosphere, and for the column, there is a more direct link
in the timing of the return dates of ozone and chlorine, especially
for the large Antarctic depletion. Comparisons of total
column ozone between the models is affected by different
predictions of the evolution of tropospheric ozone within
the same scenario, presumably due to differing treatment
of tropospheric chemistry. Therefore, for many scenarios,
clear conclusions can only be drawn for stratospheric ozone
columns rather than the total column. As noted by previous
studies, the timing of ozone recovery is affected by the evolution
of N2O and CH4. However, quantifying the effect in the
simulations analysed here is limited by the few realisations
available for these experiments compared to internal model
variability. The large increase in N2O given in RCP 6.0 extends
the ozone return globally by ∼ 15 years relative to N2O
fixed at 1960 abundances, mainly because it allows tropical
column ozone to be depleted. The effect in extratropical latitudes
is much smaller. The large increase in CH4 given in the
RCP 8.5 scenario compared to RCP 6.0 also lengthens ozone
return by ∼ 15 years, again mainly through its impact in the
tropics. Overall, our estimates of ozone return dates are uncertain
due to both uncertainties in future scenarios, in particular
those of greenhouse gases, and uncertainties in models.
The scenario uncertainty is small in the short term but increases
with time, and becomes large by the end of the century.
There are still some model–model differences related
to well-known processes which affect ozone recovery. Efforts
need to continue to ensure that models used for assessment
purposes accurately represent stratospheric chemistry
and the prescribed scenarios of ozone-depleting substances,
and only those models are used to calculate return dates. For
future assessments of single forcing or combined effects of
CO2, CH4, and N2O on the stratospheric column ozone return
dates, this work suggests that it is more important to
have multi-member (at least three) ensembles for each scenario
from every established participating model, rather than
a large number of individual models
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