89 research outputs found
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Measurements of Saharan Dust in Convective Clouds over the Tropical Eastern Atlantic Ocean
Mineral dust particles have been shown to act as cloud condensation nuclei, and they are known to interact
with developing tropical storms over the Atlantic downwind of the Sahara. Once present within liquid
droplets, they have the potential to act as freezing ice nuclei and further affect the microphysics, dynamics,
and evolution of tropical storms. However, few measurements of mineral dust particles in tropical convective
clouds exist. This study indicates that about one-third of droplets sampled in small convective clouds in the
tropical eastern Atlantic contained dust particles, and dust was the dominant residual particle type sampled in
ice crystals from anvil outflow. However, estimated number and mass concentrations of dust in anvil ice were
small compared to the amount of dust available within the Saharan air layer itself.Keywords: Cirrus clouds, Ice crystals, Aerosols, Dust or dust storms, Cumulus clouds, Deep convectio
Evidence of nitric acid uptake in warm cirrus anvil clouds during the NASA TC4 campaign
Uptake of HNO3 onto cirrus ice may play an important role in tropospheric NOx cycling. Discrepancies between modeled and in situ measurements of gas-phase HNO3 in the troposphere suggest that redistribution and removal mechanisms by cirrus ice have been poorly constrained. Limited in situ measurements have provided somewhat differing results and are not fully compatible with theory developed from laboratory studies. We present new airborne measurements of HNO3 in cirrus clouds from anvil outflow made during the Tropical Composition, Cloud, and Climate Coupling Experiment (TC4). Upper tropospheric (\u3e9 km) measurements made during three flights while repeatedly traversing the same cloud region revealed depletions of gas-phase HNO3 in regions characterized by higher ice water content and surface area. We hypothesize that adsorption of HNO3 onto cirrus ice surfaces could explain this. Using measurements of cirrus ice surface area density and some assumptions about background mixing ratios of gas-phase HNO3, we estimate molecular coverages of HNO 3 on cirrus ice surface in the tropical upper troposphere during the TC4 racetracks to be about 1 × 1013 molecules cm-2. This likely reflects an upper limit because potential dilution by recently convected, scavenged air is ignored. Also presented is an observation of considerably enhanced gas-phase HNO3 at the base of a cirrus anvil suggesting vertical redistribution of HNO3 by sedimenting cirrus particles and subsequent particle sublimation and HNO3 evaporation. The impact of released HNO3, however, appears to be restricted to a very thin layer just below the cloud. Copyright 2010 by the American Geophysical Union
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Chemical characteristics of ice residual nuclei in anvil cirrus clouds: evidence for homogeneous and heterogeneous ice formation
A counterflow virtual impactor was used to collect residual particles larger than about 0.1 μm diameter from anvil cirrus clouds generated over Florida in the southern United States. A wide variety of particle types were found. About one-third of the nuclei were salts, with varying amounts of crustal material, industrial metals, carbonaceous particles, and sulfates. Ambient aerosol particles near the anvils were found to have similar compositions, indicating that anvils act to redistribute particles over large regions of the atmosphere. Sampling occurred at a range of altitudes spanning temperatures from −21 to −56°C. More insoluble (crustal and metallic) particles typical of heterogeneous ice nuclei were found in ice crystals at warmer temperatures, while more soluble salts and sulfates were present at cold temperatures. At temperatures below about −35 to −40°C, soluble nuclei outnumbered insoluble nuclei, evidently reflecting the transition from primarily heterogeneous to primarily homogeneous freezing as a source of anvil ice
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Electron microscope analysis of residual particles from aircraft contrails
Ice crystal larger than about 5 μm diameter were separated from interstitial particles in aircraft contrails and evaporated. Residual particles larger than 0.1 μm were analyzed by electron microscopy. Soot, metals, and volatile organic substances, apparently from the aircraft exhaust, were found. However, the residual particles also contained high percentages of minerals, thought to be crustal in origin, that were often mixed with sulfur. The percentages of particles in our samples (representing the larger residual particles from relatively large ice crystals) identified as exhaust-derived and the percentage apparently derived from the ambient aerosol were roughly equal, suggesting that ambient particles may be important in contrail formation. Possible explanations for this are presented
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Refinements to Ice Particle Mass Dimensional and Terminal Velocity Relationships for Ice Clouds. Part I: Temperature Dependence
This two-part study attempts to find appropriate mass dimension and terminal velocity relationships that,
when considered together with particle size distributions (PSD), agree with coincident measurements of ice
water content (IWC), and with variables related to higher moments such as the mean mass-weighted fall
speed. Reliable relationships are required for improving microphysical parameterizations for weather forecast
models and developing methods for evaluating them, subjects addressed in detail in Part II of this study.
Here, a range of values from 1.5 to 2.3 is assumed for the exponent b in the mass dimension relationship,
m = aD[superscript]b, where D is the maximum particle dimension, to bound its likely value for sizes above about 100
m. Measured IWC and size spectra are used to find appropriate values for the coefficient a. It is demonstrated
that all values of the exponent b, with appropriate a coefficients, can fit the IWC measurements.
Coincident information on particle cross-sectional areas with the m(D) relationships is used to develop
general fall velocity relationships of the form Vt[subscript]t = AD[superscript]B. These assessments use five midlatitude, synoptically
generated ice layers, and 10 low-latitude, convectively generated ice cloud layers, spanning the temperature
range from -60° to 0°C.
The coefficients a and A and exponent B are represented in terms of the exponent b and are shown to
be temperature-dependent for the synoptic clouds and relatively independent of it in the convective clouds,
a result of particle mixing through the cloud column. Consistency is found with earlier results and with
analytic considerations. It is found that the fall velocity is inversely proportiona
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Performance of a Counterflow Virtual Impactor in the NASA Icing Research Tunnel
A counterflow virtual impactor (CVI) designed for aircraft use was evaluated at the NASA Icing Research Tunnel in Cleveland, Ohio. Tests were conducted for tunnel speeds of 67 and 100 m s⁻¹, for liquid water contents of 0.23–1.4 g m⁻³, and for a wide range of droplet median volume diameters (MVDs). For droplet distributions with MVDs between about 30 and 240 μm, liquid water content (LWC) measured by the CVI agreed with reference values within the uncertainty of the measurements. For a range of LWCs at 30-μm MVD, the relationship was near 1:1, and no systematic dependence of CVI results on LWC or airspeed was observed. For smaller MVDs, the CVI underestimated LWC. Decreased collection efficiency for small droplets can partially explain this effect, but the difference from reference values was larger than expected based on previous calibrations and comparisons with in situ data. Tunnel runs conducted with a flow-straightening shroud around the CVI inlet produced approximately 20% enhancements in LWC at small MVDs, which are expected for these speeds based on previous modeling studies. The effect of large drop breakup on CVI droplet number concentration was evaluated both theoretically and experimentally; drop breakup was predicted to occur for drops larger than 169 μm at 67 m s⁻¹ and larger than 76 μm at 100 m s⁻¹. Enhancement in number concentration measured by the CVI was found to be strongly related to observed large drop concentrations, particularly to those in the 312–700-μm-diameter range
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Measurement of Condensed Water Content in Liquid and Ice Clouds Using an Airborne Counterflow Virtual Impactor
Condensed water content (CWC) measured using a counterflow virtual impactor (CVI) with a Lyman-α hygrometer downstream is compared with that measured by other airborne instruments (a hot-wire probe, a PMS FSSP, and a PMS 2D-C). Results indicate that the CVI system provides a reliable measurement of CWC in both liquid- and ice-phase clouds and that the CVI measures CWC contained in both large and small hydrometeors; this means that the condensed water present in both phases and virtually all hydrometeor sizes can be measured with a single device. Small ice contents of a few milligrams per cubic meter present in cirrus clouds can also be measured by the technique
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Nitrogenated organic aerosols as cloud condensation nuclei
One important role of anthropogenic aerosol particles is their influence on climate by acting as cloud condensation nuclei. However, these particles are diverse in composition and mixing state, and our knowledge of which particle types act as cloud condensation nuclei is incomplete. Here we present direct measurements of individual organic particles that nucleated cloud droplets in the atmosphere. These results indicate that nitrogenated organic aerosol particles can act as cloud condensation nuclei without being mixed with inorganic material, and thus influence climate through cloud formation
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Aircraft measurements of high average charges on cloud drops in layer clouds
The first reliable aircraft measurements of characteristic cloud drop charges were obtained by utilizing a counterflow virtual impactor to substantially increase charge sensitivity and eliminate spurious contact charging that contaminated previous aircraft measurements. We find average drop charges more than an order of magnitude larger than expected from mountain surface measurements in similar clouds. Our evaluation of the data indicates that the high average charges on cloud drops originate in charge layers at the cloud boundaries and are carried into the cloud layer by vertical motions. These initial aircraft results demonstrate that cloud drop charges in layer clouds may be high enough to influence microphysical processes that promote precipitation
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