11 research outputs found
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Measurements of wave-cloud microphysical properties with two new aicraft probes
Measurements of ice water content (IWC) and
mean ice-crystal size and concentration made by two in-situ
probes, C, VI and PVM, were compared on the DC-8 aircraft
during SUCCESS flights in orographic ice clouds. The
comparison of 1WC in these wave clouds, that formed at
temperatures of about -38 °C on April 30 and -62 °C on
May 2, 1996, showed good agreement. The comparison of
ice crystal concentrations agreed better for the April-30
clouds than for the May-2 clouds ; and the effective radius
compared for both probes and for remote retrievals from
aircraft and satellite for a segment of the Berthoud wave
cloud (May 2) agreed within 30%. The measured
parameters of the ice crystals were similar to earlier
measurements and recent modeling of cold wave clouds.Copyrighted by American Geophysical Union
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Environmental conditions required for contrail formation and persistance
The ambient temperatures and humidities required for contrail formation
and persistence are determined from in situ measurements during the
Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) experiment.
Ambient temperatures and water vapor concentrations were measured
with the meteorological measurement system, a laser hygrometer, and a cryogenic
hygrometer (all onboard the DC-8). The threshold temperatures are compared
with theoretical estimates based on simple models of plume evolution. Observed
contrail onset temperatures for contrail formation are shown to be 0-2 K below
the liquid-saturation threshold temperature, implying that saturation with respect
to liquid water must be reached at some point in the plume evolution. Visible
contrails observed during SUCCESS persisted longer than a few minutes only when
substantial ambient supersaturations with respect to ice existed over large regions.
On some occasions, contrails formed at relatively high temperatures (> -50°C) due
to very high ambient supersaturations with respect to ice (of the order of 150%).
These warm contrails usually formed in the presence of diffuse cirrus. Water vapor
from sublimated ice crystals that entered the engine was probably necessary for
contrail formation in some of these cases. At temperatures above about -50°C,
contrails can only form if the ambient air is supersaturated with respect to ice, so
these contrails should persist and grow.Copyrighted by American Geophysical Union
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Uptake of NOy on wave-cloud ice particles
In flight through a wave cloud during SUCCESS on 2 May 1996, simultaneous forward- and aft- facing NOy inlets were used to infer the amount of condensed-phase NOy present on ice particles that were up to a few minutes old. Condensed-phased amounts were 25-75 pptv, or 10-20% of gas-phase NOy. Given the rapid HNO₃ uptake on ice observed in the laboratory, a model calculation implies that virtually all of the gas-phase HNO₃ will be depleted in the first 1-2 minutes after the appearance of ice. Thus the NOy observations are consistent with the laboratory results only of the ambient HNO₃/NOy ratio is 10-20%.Copyright 1998 by the American Geophysical Union
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Prevalence of ice-supersaturated regions in the upper troposphere : implications for optically thin ice cloud formation
In situ measurements of water vapor and temperature from recent aircraft campaigns have provided evidence that the upper troposphere is frequently supersaturated with respect to ice. The peak relative humidities with respect to ice (RHI) occasionally approached water saturation at temperatures ranging from -40°C to -70°C in each of the campaigns. The occurrence frequency of ice supersaturation ranged from about 20% to 45%. Even on flight segments when no ice crystals were detected, ice supersaturation was measured about 5-20% of the time. A numerical cloud model is used to simulate the formation of optically thin, low ice number density cirrus clouds in these supersaturated regions. The potential for scavenging of ice nuclei (IN) by these clouds is evaluated. The simulations suggest that if less than about 5 x 10¯³ to 2 x 10¯² cm¯³ ice nuclei are present when these supersaturations are generated, then the cirrus formed should be subvisible. These low ice number density clouds scavenge the IN from the supersaturated layer, but the crystals sediment out too rapidly to prevent buildup of high supersaturations. If higher numbers of' ice nuclei are present, then the clouds that form are visible and deposition growth of the ice crystals reduces the RHI down to near 100%. Even if no ice clouds form increasing the RHI from 100% to 150% between 10 and 10.5 km results in a decrease in outgoing longwave radiative flux at the top of the atmosphere of about 8 W m¯². If 0.02-0.1 cm¯³ IN are present, the resulting cloud radiative forcing reduces the net radiative flux several watts per square meter further. Given the high frequency of supersaturated regions without optically thick clouds in the upper troposphere, there is a potential for a climatically important class of optically thin cirrus with relatively low ice crystal number densities. The optical properties of these clouds will depend very strongly on the abundance of ice nuclei in the upper troposphere
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Ice nucleation processes in upper tropospheric wave-clouds observed during SUCCESS
We have compared in situ measurements
near the leading-edges of wave-clouds observed during
the SUCCESS experiment with numerical simulations.
Observations of high supersaturations with respect to ice (> 50%) near the leading edge of a very cold wave cloud (T < -60°C) are approximately consistent
with recent theoretical and laboratory studies suggesting
that large supersaturations are required to homogeneously freeze sulfate aerosols. Also, the peak ice crystal
number densities observed in this cloud (about 4 cm¯³)
are consistent with the number densities calculated in
our model. In the warmer wave-cloud (T ~ -37°C) relatively
large ice number densities were observed (20-40
cm¯³). Our model calculations suggest that these large
number densities are probably caused by activation of
sulfate aerosols into liquid droplets followed by subsequent
homogeneous freezing. If moderate numbers of
effective heterogeneous freezing nuclei (0.5-1cm¯³) had
been present in either of these clouds, then the number
densities of ice crystals and the peak relative humidities
should have been lower than the observed values
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DC-8-based observations of aircraft CO, CH₄, N₂O, H₂O(G) emission indices during SUCCESS
We report the first measurements of CO₂, CH₄,
N₂O, CO₂, and H₂O(g) in the exhaust trails of T-39, B-757, and
DC-8 aircraft at cruise conditions. Emission indices (El)
derived from these in-situ measurements are presented.
Results are in agreement with ground-based tests indicating
aircraft act as a net sink for CH₄ and recent airborne in-situ
measurements that N₂O is not an important exhaust
constituent. Condensation of H₂O(g) on exhaust particles
resulted in EI(H₂O(g)) values less than those expected from the
combustion of fuel alone. Observed apparent negative
EI(H₂O(g)) values suggest that aircraft aerosol emissions,
under unique atmospheric conditions, seed cloud formation
and lead to dehydration of the exhaust-influenced air parcel.
Such conditions may induce the formation of cirrus clouds
from persistent contrails. Comparisons with the Boeing EMIT
Code show measurement-derived CO emission index values
consistent with model evaluations
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Deep convection as a source of new particles in the midlatitude upper troposhere
A case study of new particle formation in the region downwind of a mesoscale convective system stretching across much of the central United States is presented. Airborne measurements were made of condensation nuclei (CN), cloud particle surface area, water vapor, and other gases. CN concentrations were greatly enhanced above and downwind of the cirrus anvil, with maximum concentrations of 45,000 per standard cm³. Volatility and electron microscope measurements indicated that most of the particles were likely to be small sulfate particles. The enhancement extended over at least a 600‐km region. Multivariate statistical analysis revealed that high CN concentrations were associated with surface tracers, as well as convective elements. Convection apparently brings gas‐phase particle precursors from the surface to the storm outflow region, where particle nucleation is favored by the extremely low temperatures. Simple calculations showed that deep convective systems may contribute to a substantial portion of the background aerosol in the upper troposphere at midlatitudes