170 research outputs found
Characterisation and airborne deployment of a new counterflow virtual impactor inlet
A new counterflow virtual impactor (CVI) inlet is introduced with details of its design, laboratory characterisation tests and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i) negligible organic contamination; (ii) a significant sample flow rate to downstream instruments (∼15 l min^(−1)) that reduces the need for dilution; and (iii) a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterised the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7–13.1 μm. The mean percentage error between cut size measurements and predictions from aerodynamic drag theory is 1.7%. The CVI was deployed on the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California in July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream of the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter
Constraining the aerosol influence on cloud liquid water path
The impact of aerosols on cloud properties is one of the largest uncertainties in the anthropogenic radiative forcing of the climate. In recent years, significant progress has been made in constraining this forcing using observations, but uncertainty still remains, particularly in the adjustments of cloud properties to aerosol perturbations. Cloud liquid water path (LWP) is the leading control on liquid-cloud albedo, making it important to observationally constrain the aerosol impact LWP. Previous modelling and observational studies have shown that multiple processes play a role in determining the LWP response to aerosol perturbations, but that the aerosol effect can be difficult to isolate. Following previous studies using mediating variables, this work investigates use of the relationship between cloud droplet number concentration (Nd) and LWP for constraining the role of aerosols. Using joint probability histograms to account for the non-linear relationship, this work finds a relationship that is broadly consistent with previous studies. There is significant geographical variation in the relationship, partly due to role of meteorological factors (particularly relative humidity) in the relationship. However, the Nd-LWP relationship is negative in the majority of regions, suggesting that aerosol induced LWP reductions could offset a significant fraction of the radiative forcing from aerosol-cloud interactions (RFaci). However, variations in the Nd-LWP relationship in response to volcanic and shipping aerosol perturbations indicate that the Nd-LWP relationship overestimates the Nd impact on LWP. As such, the estimate of LWP changes due to aerosol in this work provides an upper bound to the radiative forcing from aerosol-induced changes in the LWP
Silicon Pad Detectors for the PHOBOS Experiment at RHIC
The PHOBOS experiment is well positioned to obtain crucial information about
relativistic heavy ion collisions at RHIC, combining a multiplicity counter
with a multi-particle spectrometer. The multiplicity arrays will measure the
charged particle multiplicity over the full solid angle. The spectrometer will
be able to identify particles at mid-rapidity. The experiment is constructed
almost exclusively of silicon pad detectors. Detectors of nine different types
are configured in the multiplicity and vertex detector (22,000 channels) and
two multi-particle spectrometers (120,000 channels). The overall layout of the
experiment, testing of the silicon sensors and the performance of the detectors
during the engineering run at RHIC in 1999 are discussed.Comment: 7 pages, 7 figures, 1 table, Late
Cloud base height retrieval from multi-angle satellite data
Clouds are a key modulator of the Earth energy budget at the top
of the atmosphere and at the surface. While the cloud top height is
operationally retrieved with global coverage, only few methods have been
proposed to determine cloud base height (zbase) from satellite
measurements. This study presents a new approach to retrieve cloud base
heights using the Multi-angle Imaging SpectroRadiometer (MISR) on the Terra
satellite. It can be applied if some cloud gaps occur within the chosen
distance of typically 10 km. The MISR cloud base height (MIBase) algorithm
then determines zbase from the ensemble of all MISR cloud top
heights retrieved at a 1.1 km horizontal resolution in this area. MIBase
is first calibrated using 1 year of ceilometer data from more than 1500
sites within the continental United States of America. The 15th percentile of
the cloud top height distribution within a circular area of 10 km radius
provides the best agreement with the ground-based data. The thorough
evaluation of the MIBase product zbase with further ceilometer
data yields a correlation coefficient of about 0.66, demonstrating the
feasibility of this approach to retrieve zbase. The impacts of
the cloud scene structure and macrophysical cloud properties are discussed.
For a 3-year period, the median zbase is generated globally
on a 0.25∘ × 0.25∘ grid. Even though overcast cloud
scenes and high clouds are excluded from the statistics, the median
zbase retrievals yield plausible results, in particular over ocean
as well as for seasonal differences. The potential of the full 16 years of
MISR data is demonstrated for the southeast Pacific, revealing interannual
variability in zbase in accordance with reanalysis data. The
global cloud base data for the 3-year period (2007–2009) are available
at https://doi.org/10.5880/CRC1211DB.19.</p
The effect of rapid adjustments to halocarbons and N2O on radiative forcing
Rapid adjustments occur after initial perturbation of an external climate driver (e.g., CO2) and involve changes in, e.g. atmospheric temperature, water vapour and clouds, independent of sea surface temperature changes. Knowledge of such adjustments is necessary to estimate effective radiative forcing (ERF), a useful indicator of surface temperature change, and to understand global precipitation changes due to different drivers. Yet, rapid adjustments have not previously been analysed in any detail for certain compounds, including halocarbons and N2O. Here we use several global climate models combined with radiative kernel calculations to show that individual rapid adjustment terms due to CFC-11, CFC-12 and N2O are substantial, but that the resulting flux changes approximately cancel at the top-of-atmosphere due to compensating effects. Our results further indicate that radiative forcing (which includes stratospheric temperature adjustment) is a reasonable approximation for ERF. These CFCs lead to a larger increase in precipitation per kelvin surface temperature change (2.2 ± 0.3% K−1) compared to other well-mixed greenhouse gases (1.4 ± 0.3% K−1 for CO2). This is largely due to rapid upper tropospheric warming and cloud adjustments, which lead to enhanced atmospheric radiative cooling (and hence a precipitation increase) and partly compensate increased atmospheric radiative heating (i.e. which is associated with a precipitation decrease) from the instantaneous perturbation
Ratios of charged antiparticles to particles near mid-rapidity in Au+Au collisions at sqrt(s_NN) = 130 GeV
We have measured the ratios of antiparticles to particles for charged pions,
kaons and protons near mid-rapidity in central Au+Au collisions at sqrt(s_NN) =
130 GeV. For protons, we observe pbar/p = 0.60 +/- 0.04 (stat.) +/- 0.06
(syst.) in the transverse momentum range 0.15 < p_T < 1.0 GeV/c. This leads to
an estimate of the baryo-chemical potential mu_B of 45 MeV, a factor of 5-6
smaller than in central Pb+Pb collisions at sqrt(s_NN) = 17.2 GeV.Comment: 4 page
Centrality Dependence of Charged Particle Multiplicity at Mid-Rapidity in Au+Au Collisions at sqrt(s_NN) = 130 GeV
We present a measurement of the pseudorapidity density of primary charged
particles near mid-rapidity in Au+Au collisions at sqrt(s_NN) = 130 GeV as a
function of the number of participating nucleons. These results are compared to
models in an attempt to discriminate between competing scenarios of particle
production in heavy ion collisions.Comment: 5 pages, 4 figures, revtex (submitted to Phys. Rev. Letters
Scientific data from precipitation driver response model intercomparison project
This data descriptor reports the main scientific values from General Circulation Models (GCMs) in the Precipitation Driver and Response Model Intercomparison Project (PDRMIP). The purpose of the GCM simulations has been to enhance the scientific understanding of how changes in greenhouse gases, aerosols, and incoming solar radiation perturb the Earth's radiation balance and its climate response in terms of changes in temperature and precipitation. Here we provide global and annual mean results for a large set of coupled atmospheric-ocean GCM simulations and a description of how to easily extract files from the dataset. The simulations consist of single idealized perturbations to the climate system and have been shown to achieve important insight in complex climate simulations. We therefore expect this data set to be valuable and highly used to understand simulations from complex GCMs and Earth System Models for various phases of the Coupled Model Intercomparison Project
Hygroscopic properties of smoke-generated organic aerosol particles emitted in the marine atmosphere
During the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE), a plume of organic aerosol was produced by a smoke generator and emitted into the marine atmosphere from aboard the R/V Point Sur. In this study, the hygroscopic properties and the chemical composition of the plume were studied at plume ages between 0 and 4 h in different meteorological conditions. In sunny conditions, the plume particles had very low hygroscopic growth factors (GFs): between 1.05 and 1.09 for 30 nm and between 1.02 and 1.1 for 150 nm dry size at a relative humidity (RH) of 92%, contrasted by an average marine background GF of 1.6. New particles were produced in large quantities (several 10 000 cm^−3), which lead to substantially increased cloud condensation nuclei (CCN) concentrations at supersaturations between 0.07 and 0.88%. Ratios of oxygen to carbon (O : C) and water-soluble organic mass (WSOM) increased with plume age: from < 0.001 to 0.2, and from 2.42 to 4.96 μg m^−3, respectively, while organic mass fractions decreased slightly (~ 0.97 to ~ 0.94). High-resolution aerosol mass spectrometer (AMS) spectra show that the organic fragment m/z 43 was dominated by C_(2)H_(3)O^+ in the small, new particle mode and by C_(3)H_(7)^+ in the large particle mode. In the marine background aerosol, GFs for 150 nm particles at 40% RH were found to be enhanced at higher organic mass fractions: an average GF of 1.06 was observed for aerosols with an organic mass fraction of 0.53, and a GF of 1.04 for an organic mass fraction of 0.35
Eastern Pacific Emitted Aerosol Cloud Experiment
Aerosol–cloud–radiation interactions are widely held to be the largest single source of uncertainty in climate model projections of future radiative forcing due to increasing anthropogenic emissions. The underlying causes of this uncertainty among modeled predictions of climate are the gaps in our fundamental understanding of cloud processes. There has been significant progress with both observations and models in addressing these important questions but quantifying them correctly is nontrivial, thus limiting our ability to represent them in global climate models. The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) 2011 was a targeted aircraft campaign with embedded modeling studies, using the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft and the research vessel Point Sur in July and August 2011 off the central coast of California, with a full payload of instruments to measure particle and cloud number, mass, composition, and water uptake distributions. EPEACE used three emitted particle sources to separate particle-induced feedbacks from dynamical variability, namely 1) shipboard smoke-generated particles with 0.05–1-μm diameters (which produced tracks measured by satellite and had drop composition characteristic of organic smoke), 2) combustion particles from container ships with 0.05–0.2-μm diameters (which were measured in a variety of conditions with droplets containing both organic and sulfate components), and 3) aircraft-based milled salt particles with 3–5-μm diameters (which showed enhanced drizzle rates in some clouds). The aircraft observations were consistent with past large-eddy simulations of deeper clouds in ship tracks and aerosol– cloud parcel modeling of cloud drop number and composition, providing quantitative constraints on aerosol effects on warm-cloud microphysics
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