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Owen B. Toon and James B. Pollack

Abstract

A global average model of the size distribution, chemical composition and optical thickness of stratospheric and tropospheric aerosols is proposed. The uncertainties involved in making the model are emphasized, and some of the model's implications are discussed. The model is designed for, and biased toward, global average radiative transfer calculations.

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Peter J. Gierasch and Owen B. Toon

Abstract

If Mars has permanent CO2 polar caps, atmospheric heat transport may cause the atmospheric pressure to be extremely sensitive to variations of solar heating at the poles. This could happen because atmospheric heating depends on density, which depends strongly on the polar temperature through the vapor pressure relation. A simple climatological model is used to study the question.

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Douglas L. Westphal, Owen B. Toon, and Toby N. Carlson

Abstract

Numerical models of the atmosphere and aerosols are used to investigate mobilization and transport of Saharan dust over West Africa and the tropical Atlantic Ocean for 23–28 August 1974. We have found that mobilization during this period was related to the passage of a shallow easterly wave and was not initiated by dry convective mixing of a midlevel easterly jet, as has been previously suggested, since high static stability beneath the midlevel easterly jet inhibited significant boundary layer development and transport of momentum in the jet down to the surface. Instead, mobilization was done by dry convective mixing of low-level jets associated with the easterly wave. Another easterly wave present in the domain during the period did not contribute significantly to dust mobilization while over Africa yet became a strong tropical storm over the Atlantic Ocean in early September. The periodicity of the outbreak was reinforced by scavenging of dust by precipitation associated with the easterly waves.

The model simulations show that the aerosol at any one point can be a complicated mixture of particles lifted at different times and different places. Bimodal size distributions developed when dust was mobilized within a dust plume that was generated on a previous day. An elevated layer of dust developed over the ocean as the northeast trade winds advected clean air underneath the dust-laden air as it moved westward. The size and spatial distributions of aerosol in the marine layer depended upon the undercutting process, the amount of background mineral aerosol present, and transport across the marine layer inversion by sedimentation and turbulent mixing.

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Andrew S. Ackerman, Peter V. Hobbs, and Owen B. Toon

Abstract

A detailed 1D model of the stratocumulus-topped marine boundary layer is described. The model has three coupled components: a microphysics module that resolves the size distributions of aerosols and cloud droplets, a turbulence module that treats vertical mixing between layers, and a multiple wavelength radiative transfer module that calculates radiative heating rates and cloud optical properties.

The results of a 12-h model simulation reproduce reasonably well the bulk thermodynamics, microphysical properties, and radiative fluxes measured in an ∼500-m thick, summertime marine stratocumulus cloud layer by Nicholls. However, in this case, the model predictions of turbulent fluxes between the cloud and subcloud layers exceed the measurements. Results of model simulations are also compared to measurements of a marine stratus layer made under gale conditions and with measurements of a high, thin marine stratocumulus layer. The variations in cloud properties are generally reproduced by the model, although it underpredicts the entrainment of overlying air at cloud top under gale conditions.

Sensitivities of the model results are explored. The vertical profile of cloud droplet concentration is sensitive to the lower size cutoff of the droplet size distribution due to the presence of unactivated haze particles in the lower region of the modeled cloud. Increases in total droplet concentrations do not always produce less drizzle and more cloud water in the model. The radius of the mean droplet volume does not correlate consistently with drizzle, but the effective droplet radius does. The greatest impacts on cloud properties predicted by the model are produced by halving the width of the size distribution of input condensation nuclei and by omitting the effect of cloud-top radiative cooling on the condensational growth of cloud droplets. The omission of infrared scattering produces noticeable changes in cloud properties. The collection efficiencies for droplets <30-µm radius, and the value of the accommodation coefficient for condensational droplet growth, have noticeable effects on cloud properties. The divergence of the horizontal wind also has a significant effect on a 12-h model simulation of cloud structure.

Conclusions drawn from the model are tentative because of the limitations of the 1D model framework. A principal simplification is that the model assumes horizontal homogeneity, and, therefore, does not resolve updrafts and downdrafts. Likely consequences of this simplification include overprediction of the growth of droplets by condensation in the upper region of the cloud, underprediction of droplet condensational growth in the lower region of the cloud, and underprediction of peak supersaturations.

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Donald M. Hunten, Richard P. Turco, and Owen B. Toon

Abstract

A height profile of ablated mass from meteors is calculated, assuming an incoming mass of 10−16 g cm−2 s−1 (44 metric tons per day) and the velocity distribution of Southworth and Sekanina, which has a mean of 14.5 km s−1. The profile peaks at 84 km. The fluxes of micrometeorites and residual meteoroids are also calculated. The coagulation of the evaporated silicates into “smoke” particles is then followed by means of a model adapted from a previous study of the stratospheric sulfate layer. Numerous sensitivity tests are made. Features of the results are a sharp cutoff of the particle distribution above 90 km, and a surface area close to 10−9 cm2 cm−3 all the way from 30 to 85 km. Some confirmation is obtained from balloon studies of condensation nuclei, although the various measurements differ greatly. The optical scattering and extinction am shown to be undetectable. Several potential applications are suggested: nucleation of sulfate particles and noctilucent clouds, scavenging of metallic ions and atoms, and perhaps other aeronomical effects. The latter are limited to processes that can be influenced by a collision time of the order of a day.

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James B. Pollack, Owen B. Toon, Andrey Summers, Warren Van Camp, and Betty Baldwin

Abstract

Aircraft and Space Shuttles flying through the stratosphere over the next several decades will add sulfuric acid and aluminum oxide particles, respectively, to this region of the atmosphere. To evaluate the effect of these additional aerosols on the global heat balance, we have performed solar and terrestrial radiative transfer calculations. The solar calculations employed an accurate numerical method for solving the multiple-scattering problem for unpolarized light to determine the dependence of the global (spherical) albedo on the optical depth perturbation Δτ. Correct allowance was made for absorption by gases. Using these results, and those obtained from calculations of the terrestrial thermal flux at the top of the atmosphere, we determined the resulting change in the mean surface temperature, ΔT, as a function of Δτ. In both calculations, we used the measured optical constants of the aerosol species.

To apply these results to the problem of interest, we used engine exhaust properties of the various types of vehicles to estimate their optical depth perturbation and examined the record of past climate changes to set a threshold value, 0.1 K, on the mean surface temperature change, below which no significant impact is to be expected. Using the above information, we find that no significant climate change should result from the aerosols produced by Space Shuttles, SST's, and other high flying aircraft, operating at traffic levels projected for the next several decades. However, the effect of SST's is sufficiently close to our threshold limit to warrant a reevaluation as their characteristics are updated.

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Owen B. Toon, R. P. Turco, P. Hamill, C. S. Kiang, and R. C. Whitten

Abstract

We have performed sensitivity tests on a one-dimensional physical-chemical model of the unperturbed stratospheric aerosols and have compared model calculations with observations. The sensitivity tests and comparisons with observations suggest that coagulation controls the particle number mixing ratio, although the number of condensation nuclei at the tropopause and the diffusion coefficient at high altitudes are also important. The sulfate mass and large particle number (r > 0.15 µm) mixing ratios are controlled by growth, sedimentation, evaporation at high altitudes and washout below the tropopause. The sulfur gas source strength and the aerosol residence time are much more important than the supply of condensation nuclei in establishing mass and large particle concentrations. The particle size is also controlled mainly by gas supply and residence time. OCS diffusion (not SO2diffusion) dominates the production of stratospheric H2SO4 particles during unperturbed times, although direct injection of SO2 into the stratosphere could be significant if it normally occurs regularly by some transport mechanism. We suggest a number of in-situ observations of the aerosols and laboratory measurements of aerosol parameters that can provide further information about the physics and chemistry of the stratosphere and the aerosols found there.

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Andrew S. Ackerman, Owen B. Toon, Jonathan P. Taylor, Doug W. Johnson, Peter V. Hobbs, and Ronald J. Ferek

Abstract

Airborne measurements from the Meteorological Research Flight’s Hercules C-130 and the University of Washington’s Convair C-131A during the Monterey Area Ship Track field project are used to evaluate Twomey’s analytic expression for cloud susceptibility, which describes the sensitivity of cloud albedo to changes in droplet concentrations. This expression incorporates assumptions about cloud physics, such as the independence of the cloud liquid water content and the width of the droplet size distribution on droplet concentrations. Averaged over all 69 ship track penetrations, cloud liquid water content decreased slightly and the droplet size distributions broadened from the ambient values. For the 17 cases for which albedos were measured during overflights, Twomey’s parameterization represents the trend of albedo changes with droplet concentrations remarkably well, passing through the midpoints of the considerable spread in the data. The fortuitous agreement results from compensating changes in cloud properties. Together with the albedo changes, the changes in cloud liquid water content and droplet size distributions imply that cloud thickness usually increased in the ship tracks. Such an increase was observed on the occasions that changes in cloud thickness were recorded (in the Sanko Peace ship track during very clean ambient conditions). Unfortunately systematic measurements of cloud thickness were not made for most of the ship tracks observed. The greatest outlier in the data corresponds to measurements made under horizontally inhomogeneous ambient conditions; possible explanations for its divergence include an increase in cloud thickness or an error in matching above-cloud albedo measurements with in-cloud microphysics measurements.

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Eric J. Jensen, Leonhard Pfister, David E. Jordan, Thaopaul V. Bui, Rei Ueyama, Hanwant B. Singh, Troy D. Thornberry, Andrew W. Rollins, Ru-Shan Gao, David W. Fahey, Karen H. Rosenlof, James W. Elkins, Glenn S. Diskin, Joshua P. DiGangi, R. Paul Lawson, Sarah Woods, Elliot L. Atlas, Maria A. Navarro Rodriguez, Steven C. Wofsy, Jasna Pittman, Charles G. Bardeen, Owen B. Toon, Bruce C. Kindel, Paul A. Newman, Matthew J. McGill, Dennis L. Hlavka, Leslie R. Lait, Mark R. Schoeberl, John W. Bergman, Henry B. Selkirk, M. Joan Alexander, Ji-Eun Kim, Boon H. Lim, Jochen Stutz, and Klaus Pfeilsticker

Abstract

The February–March 2014 deployment of the National Aeronautics and Space Administration (NASA) Airborne Tropical Tropopause Experiment (ATTREX) provided unique in situ measurements in the western Pacific tropical tropopause layer (TTL). Six flights were conducted from Guam with the long-range, high-altitude, unmanned Global Hawk aircraft. The ATTREX Global Hawk payload provided measurements of water vapor, meteorological conditions, cloud properties, tracer and chemical radical concentrations, and radiative fluxes. The campaign was partially coincident with the Convective Transport of Active Species in the Tropics (CONTRAST) and the Coordinated Airborne Studies in the Tropics (CAST) airborne campaigns based in Guam using lower-altitude aircraft (see companion articles in this issue). The ATTREX dataset is being used for investigations of TTL cloud, transport, dynamical, and chemical processes, as well as for evaluation and improvement of global-model representations of TTL processes. The ATTREX data are publicly available online (at https://espoarchive.nasa.gov/).

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