2,629 research outputs found
Aerosol absorption over the clear-sky oceans deduced from POLDER-1 and AERONET observations
We estimate aerosol absorption over the clear-sky oceans using aerosol geophysical products from POLDER-1 space measurements and absorption properties from ground-based AERONET measurements. Our best estimate is 2.5 Wm-2 averaged over the 8-month lifetime of POLDER-1. Low and high absorption estimates are 2.2 and 3.1 Wm-2 based on the variability in aerosol single scattering albedo observed by AERONET. Main sources of uncertainties are the discrimation of the aerosol type from satellite measurements, and potential clear-sky bias induced
by the cloud-screening procedure
Thermodynamic Properties of the Spin-1/2 Antiferromagnetic ladder Cu2(C2H12N2)2Cl4 under Magnetic Field
Specific heat () measurements in the spin-1/2
Cu(CHN)Cl system under a magnetic field up to
are reported and compared to the results of numerical calculations
based on the 2-leg antiferromagnetic Heisenberg ladder. While the temperature
dependences of both the susceptibility and the low field specific heat are
accurately reproduced by this model, deviations are observed below the critical
field at which the spin gap closes. In this Quantum High Field phase,
the contribution of the low-energy quantum fluctuations are stronger than in
the Heisenberg ladder model. We argue that this enhancement can be attributed
to dynamical lattice fluctuations. Finally, we show that such a Heisenberg
ladder, for , is unstable, when coupled to the 3D lattice, against a
lattice distortion. These results provide an alternative explanation for the
observed low temperature ( -- ) phase (previously
interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped
state.Comment: Minor changes, list of authors complete
The Influence of Aerosol Hygroscopicity on Precipitation Intensity During a Mesoscale Convective Event
We examine how aerosol composition affects precipitation intensity using the Weather and Research Forecasting Model with Chemistry (version 3.6). By changing the prescribed default hygroscopicity values to updated values from laboratory studies, we test model assumptions about individual component hygroscopicity values of ammonium, sulfate, nitrate, and organic species. We compare a baseline simulation (BASE, using default hygroscopicity values) with four sensitivity simulations (SULF, increasing the sulfate hygroscopicity; ORG, decreasing organic hygroscopicity; SWITCH, using a concentrationâdependent hygroscopicity value for ammonium; and ALL, including all three changes) to understand the role of aerosol composition on precipitation during a mesoscale convective system (MCS). Overall, the hygroscopicity changes influence the spatial patterns of precipitation and the intensity. Focusing on the maximum precipitation in the model domain downwind of an urban area, we find that changing the individual component hygroscopicities leads to bulk hygroscopicity changes, especially in the ORG simulation. Reducing bulk hygroscopicity (e.g., ORG simulation) initially causes fewer activated drops, weakened updrafts in the midtroposphere, and increased precipitation from larger hydrometeors. Increasing bulk hygroscopicity (e.g., SULF simulation) simulates more numerous and smaller cloud drops and increases precipitation. In the ALL simulation, a stronger cold pool and downdrafts lead to precipitation suppression later in the MCS evolution. In this downwind region, the combined changes in hygroscopicity (ALL) reduces the overprediction of intense events (>70 mm dâ1) and better captures the range of moderate intensity (30â60 mm dâ1) events. The results of this single MCS analysis suggest that aerosol composition can play an important role in simulating highâintensity precipitation events.Key PointsAerosol composition can affect spatial patterns of precipitationHygroscopicity and hydrometeor vertical distributions are sensitive to aerosol composition and impact precipitation processesAltering speciated aerosol hygroscopicity can influence the simulation of precipitation intensityPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141976/1/jgrd54341.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141976/2/jgrd54341_am.pd
Charge Ordering and Spin Dynamics in NaV2O5
We report high-resolution neutron inelastic scattering experiments on the
spin excitations of NaV2O5. Below Tc, two branches associated with distinct
energy gaps are identified. From the dispersion and intensity of the spin
excitation modes, we deduce the precise zig-zag charge distribution on the
ladder rungs and the corresponding charge order (about 0.6). We argue that the
spin gaps observed in the low-T phase of this compound are primarily due to the
charge transfer.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Geoengineering by stratospheric SO<sub>2</sub> injection: results from the Met Office HadGEM2 climate model and comparison with the Goddard Institute for Space Studies ModelE
We examine the response of the Met Office Hadley Centre's HadGEM2-AO climate model to simulated geoengineering by continuous injection of SO<sub>2</sub> into the lower stratosphere, and compare the results with those from the Goddard Institute for Space Studies ModelE. Despite the differences between the models, we find a broadly similar geographic distribution of the response to geoengineering in both models in terms of near-surface air temperature and mean JuneâAugust precipitation. The simulations also suggest that significant changes in regional climate would be experienced even if geoengineering was successful in maintaining global-mean temperature near current values, and both models indicate rapid warming if geoengineering is not sustained
The scavenging processes controlling the seasonal cycle in Arctic sulphate and black carbon aerosol
This is the final version of the article. Available from European Geosciences Union via the DOI in this record.The seasonal cycle in Arctic aerosol is typified by high concentrations of large aged anthropogenic particles transported from lower latitudes in the late Arctic winter and early spring followed by a sharp transition to low concentrations of locally sourced smaller particles in the summer. However, multi-model assessments show that many models fail to simulate a realistic cycle. Here, we use a global aerosol microphysics model (GLOMAP) and surface-level aerosol observations to understand how wet scavenging processes control the seasonal variation in Arctic black carbon (BC) and sulphate aerosol. We show that the transition from high wintertime concentrations to low concentrations in the summer is controlled by the transition from ice-phase cloud scavenging to the much more efficient warm cloud scavenging in the late spring troposphere. This seasonal cycle is amplified further by the appearance of warm drizzling cloud in the late spring and summer boundary layer. Implementing these processes in GLOMAP greatly improves the agreement between the model and observations at the three Arctic ground-stations Alert, Barrow and Zeppelin Mountain on Svalbard. The SO4 model-observation correlation coefficient (R) increases from:-0.33 to 0.71 at Alert (82.5 N), from-0.16 to 0.70 at Point Barrow (71.0 N) and from-0.42 to 0.40 at Zeppelin Mountain (78 N). The BC model-observation correlation coefficient increases from-0.68 to 0.72 at Alert and from-0.42 to 0.44 at Barrow. Observations at three marginal Arctic sites (Janiskoski, Oulanka and Karasjok) indicate a far weaker aerosol seasonal cycle, which we show is consistent with the much smaller seasonal change in the frequency of ice clouds compared to higher latitude sites. Our results suggest that the seasonal cycle in Arctic aerosol is driven by temperature-dependent scavenging processes that may be susceptible to modification in a future climate. © 2012 Author(s).JB was funded by a studentship from the
Natural Environment Research Council and by the Met Office
through a CASE partnership. KC is a Royal Society Wolfson
Merit Award holder. We would like to thank Neil Gordon for
providing low cloud satellite climatologies from the MODIS
satellite and Dr Graham Mann for his comments and assistance.
The authors acknowledge the Canadian National Atmospheric
Chemistry (NAtChem) Database and its data contributing agencies/
organizations for the provision of the Sulphate mass data
for the years 2000â2002, used in this publication. The agency
responsible for all data contributions from the the NAtChem
Database is the Canadian Arctic aerosol programme. The authors
acknowledge and thank the scientists and data-providers of the
Norwegian institute of air research (NILU), the National ocean and
atmospheric administration (NOAA) and the EMEP observation
network for the provision of BC and sulphate mass data used in this
publication
Spin-Peierls instability in a quantum spin chain with Dzyaloshinskii-Moriya interaction
We analysed the ground state energy of some dimerized spin-1/2 transverse XX
and Heisenberg chains with Dzyaloshinskii-Moriya (DM) interaction to study the
influence of the latter interaction on the spin-Peierls instability. We found
that DM interaction may act either in favour of the dimerization or against it.
The actual result depends on the dependence of DM interaction on the distortion
amplitude in comparison with such dependence for the isotropic exchange
interaction.Comment: 12 pages, latex, 3 figure
- âŠ