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Lieut. J. B. ANDERSON

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J. B. Anderson
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Lieut. J. B. Anderson
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B. J. Anderson and J. Hallett

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Nucleation of individual ice crystals on large (3.0 mm) cleaved crystals of solution-grown silver iodide and covellite is investigated by microscopy. The environmental vapor pressure is controlled by saturating two air streams by passage through ice labyrinths at different temperatures and mixing them in known proportion. This enables the vapor pressure to be changed over a period of about 10 s.

Ice crystals do not usually appear immediately when a supersaturation is imposed. Nucleation, defined as the appearance of crystals of 1 µm radius, is delayed between zero and 70 s near water saturation and between 20 and 400 s at a few percent ice supersaturation, the longer times occurring at higher temperature. This time decreases only marginally when the crystal is exposed to a period of higher supersaturation which ends a few seconds prior to the time crystals would appear at this higher value. The number of crystals per unit area increases with ice supersaturation at a given temperature; for CuS at −16°C, it increases by a factor of 3 between 3% and water saturation. Number concentrations on silver iodide are comparable, but increase with time when the surface is exposed to light. The absolute crystal concentration varies over the substrate surface. Large areas fail to nucleate at all; some areas give high concentrations, 500 mm−2. Crystals form at specific nucleation sites. Each requires a different critical ice supersaturation for nucleation which remains unchanged in sequential tests. This property disappears for AgI after exposure to light; then nucleation sites do not repeat. Nucleation events per unit area are fewer than on particulates which are inferred to contain a proportionately greater surface concentration

of nucleation sites.

Results are applied to crystal nucleation in the atmosphere and the characterization of ice nuclei in laboratory instruments.

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B. J. Anderson, J. D. Sutkoff, and J. Hallett

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Ice crystals grow from the vapor in the presence of Eastman9 10 cement (containing methyl 2-cyanoacrylate monomer) in the form of fine fibres a few microns in diameter, in the c axis direction of the nucleating crystal. A previous suggestion that similar fibers observed using this material for the replication process occur undernatural conditions and are a source of atmospheric ice crystals appears unlikely.

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E. M. Weinstock, J. B. Smith, D. Sayres, J. V. Pittman, N. Allen, and J. G. Anderson

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This paper describes the performance and in-flight validation of an instrument mounted in a pallet on the NASA WB-57 research aircraft that measures the sum of gas phase and solid phase water, or total water, in cirrus clouds. Using a heated isokinetic inlet and a Lyman-α photofragment fluorescence technique for detection, measurements of total water have been made over three orders of magnitude. During the Cirrus Regional Study of Tropical Anvils and Cirrus Layers Florida Area Cirrus Experiment (CRYSTAL FACE), the instrument operated at duct temperatures sufficiently warms to completely evaporate particles up to 150-μm diameter. Laboratory calibrations, in-flight diagnostics, intercomparison with water vapor measured by absorption in flight, and intercomparisons in clear air with the Harvard water vapor instrument validate the detection sensitivity of the instrument and illustrate the minimal hysteresis from instrument surface contamination. The Harvard total water and water vapor instruments together provide measurements of the ice water content of cirrus clouds in the mid- and upper troposphere with an uncertainty of 18%.

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J. Ström, B. Strauss, T. Anderson, F. Schröder, J. Heintzenberg, and P. Wendling

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In situ measurements made in cold (−35° to −60°C) cirrus clouds over southern Germany in March 1994 are presented. The clouds appeared to be in an early stage of their life cycle and their properties in many ways resemble those reported for ice fogs. Crystal concentrations were high (median 2.5 cm−3, STP) and sizes small with a diameter of mean mass of typically 16 μm. The cloud on 18 March presents an interesting case for modeling studies of cirrus formation. On that particular day, the bulk properties of the cloud appeared to be connected to wave structures in the vertical wind field consistent with the Brunt–Väisälä frequency, which gave a corresponding wavelength of 40–50 km. Furthermore, analyses of potential temperature and vertical wind suggested that the vertical displacement producing these clouds was less than 100 m. Size distribution measurements of interstitial particles and crystal residues (particles remaining after evaporation of the crystals) show that small aerosol particles (diameters <0.5 μm) participate in the nucleation of cirrus crystals at low temperatures. Because the aerosol in this small size range is readily perturbed by anthropogenic activity, it is important to study the link between upper tropospheric aerosol properties and cirrus cloud microphysics. While the observations presented here are not adequate to quantify this link, they pave the way for modeling studies and would be interesting to compare to cirrus observations from cleaner regions.

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E. M. Weinstock, J. B. Smith, D. Sayres, J. R. Spackman, J. V. Pittman, N. Allen, J. Demusz, M. Greenberg, M. Rivero, L. Solomon, and J. G. Anderson

Abstract

This paper describes an instrument designed to measure the sum of gas phase and solid phase water, or total water, in cirrus clouds, and to be mounted in a pallet in the underbelly of the NASA WB-57 research aircraft. The ice water content of cirrus is determined by subtracting water vapor measured simultaneously by the Harvard water vapor instrument on the aircraft. The total water instrument uses an isokinetic inlet to maintain ambient particle concentrations as air enters the instrument duct, a 600-W heater mounted directly in the flow to evaporate the ice particles, and a Lyman-α photofragment fluorescence technique for detection of the total water content of the ambient air. Isokinetic flow is achieved with an actively controlled roots pump by referencing aircraft pressure, temperature, and true airspeed, together with instrument flow velocity, temperature, and pressure. Laboratory calibrations that utilize a water vapor addition system that adds air with a specific humidity tied to the vapor pressure of water at room temperature and crosschecked by axial and radial absorption of Lyman-α radiation at the detection axis are described in detail. The design provides for in-flight validation of the laboratory calibration by intercomparison with total water measured by radial absorption at the detection axis. Additionally, intercomparisons in clear air with the Harvard water vapor instrument are carried out. Based on performance of the Harvard water vapor instrument, this instrument has the detection capability of making accurate measurements of total water with mixing ratios in the mid- to upper troposphere of up to 2500 ppmv and mixing ratios in the lower stratosphere of about 5 ppmv, corresponding to almost three orders of magnitude in measurement capability.

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Wiebe A. Oost, Christopher W. Fairall, James B. Edson, Stuart D. Smith, Robert J. Anderson, John A.B. Wills, Kristina B. Katsaros, and Janice DeCosmo

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Several methods are examined for correction of turbulence and eddy fluxes in the atmospheric boundary layer, two of them based on a potential-flow approach initiated by Wyngaard. If the distorting object is cylindrical or if the distance to the sensor is much greater than the size of the body, the undisturbed wind stress can be calculated solely from measurements made by the sensor itself; no auxiliary measurements or lengthy model calculations are needed. A more general potential-flow correction has been developed in which distorting objects of complex shape are represented as a number of ellipsoidal elements.

These models are applied to data from three turbulence anemometers with differing amounts of flow distortion, operated simultaneously in the Humidity Exchange over the Sea (HEXOS) Main Experiment. The results are compared with wind-stress estimates by the inertial-dissipation technique; these are much less sensitive to local flow distortion and are consistent with the corrected eddy correlation results. From these comparisons it is concluded that the commonly used “tilt correction” is not sufficient to correct eddy wind stress for distortion by nearby objects, such as probe supports and neighboring sensors.

Neither potential-flow method is applicable to distortion by larger bodies of a scale comparable to the measuring height, such as the superstructure of the Meetpost Noordwijk (MPN) platform used in HEXOS. Flow distortion has been measured around a model of MPN in a wind tunnel study. The results were used to correct mean winds, but simulation of distortion effects on turbulence levels and wind stress turned out not to be feasible.

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P. O. Wennberg, T. F. Hanisco, R. C. Cohen, R. M. Stimpfle, L. B. Lapson, and J. G. Anderson

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Recent aircraft and balloon borne measurements of OH and H02 are reviewed. The authors demonstrate the ability of the laser-induced fluorescence technique to provide accurate, high signal to noise ratio measurements of OH throughout the upper troposphere and stratosphere. HO2 is measured as OH after gas phase chemical titration with nitric oxide. The addition of the HOx measurement capability to the suite of instruments aboard the NASA ER-2 aircraft has provided a wealth of new information about the processes that determine the concentration of ozone in the lower stratosphere. These simultaneous, in situ measurements provide a unique test of our understanding of the mechanisms that control the odd-hydrogen chemistry of the lower atmosphere.

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