Lawrence F. Radke

Direct and remote sensing observations of the effects of ships on clouds

Under certain conditions ships can affect the structure of shallow layer clouds. Simultaneous observations of two ship track signatures in stratus clouds from a satellite and in situ from an aircraft show that in the ship tracks the droplet sizes were reduced and total concentrations of both droplets and particles were substantially increased from those in adjacent clouds. In situ measurements of the upwelling radiance within the ship tracks was significantly enhanced at visible wavelengths, whereas radiance at 2.2 micrometers was significantly reduced. Cloud reflectivity along the tracks was enhanced at 0.63 and 3.7 micrometers. These observations support the contention that ship track signatures in clouds are produced primarily by particles emitted from ships.

Determination of the Optical Thickness and Effective Particle Radius of Clouds from Reflected Solar Radiation Measurements. Part II: Marine Stratocumulus Observations

A multispectral scanning radiometer has been used to obtain measurements of the reflection function of marine stratocumulus clouds at 0.75, 1.65 and 2.16 pm. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP [International Satellite Cloud Climatology Project] Regional Experiment (FIRE), conducted off the coast of southern California during July 1987. Multispectral images of the reflection function were used to derived the optical thickness and effective particle radius of stratiform cloud layers on four days. In addition to the radiation measurements, in situ microphysical measurements were obtained from the University of Washington Convair C- 13 I A aircraft. In this paper we compare remote sensing results with in situ observations, which show a good spatial correlation for both optical thickness and effective radius. These comparisons further show systematic differences between remote sensing and in situ values, with a tendency for remote sensing to overestimate the effective radius by -2-3 pm, independent of particle radius. The optical thickness, in contrast, is somewhat overestimated for small optical thicknesses and underestimated for large optical thicknesses. An introduction of enhanced gaseous absorption at a wavelength of 2.16 pm successfully explains some of these observed discrepancies. Marginal probability density functions of optical thickness, liquid water path and effective radius have been derived from our remote sensing results. The joint probability density function of liquid water path and effective radius shows that the effective radius increases as the liquid water path increases for optically thin clouds, in contrast to optically thick clouds for which the effective radius decreases with increasing liquid water path.

A Summary of the Physical Properties of Cirrus Clouds

Abstract A review of existing literature is made to determine typical values for the physical properties cirrus clouds. The properties examined (with typical values and measured ranges) are cloud-center altitude (9 km, 4 to 20 km), cloud thickness (1.5 km, 0.1 to 8 km), crystal number density (30 L−1, 10−4 to 10−4 L−1), condensed water content (0.025 g m −3, 10−4 to 1.2 g m −3), and crystal size (250 μm, 1 to 8000 μm). A typical crystal size distribution is also reported.

Measurements of Aitken nuclei and cloud condensation nuclei in the marine atmosphere and their relation to the DMS-cloud-climate hypothesis

New airborne measurements provide support for the hypothesis that layers of high concentrations of Aitken nuclei near the tops of marine clouds are due to photochemical nucleation. They also reveal a significant correlation between cloud condensation nucleus (CCN) concentrations in the boundary layer and mean cloud droplet concentration in stratus clouds topping a marine boundary layer. Nonsea salt sulfate mass and the concentration of CCN active at 1% supersaturation are also significantly correlated. These results provide quantitative support for some facets of the DMS-cloud-climate hypothesis.

Airborne measurements of particle and gas emissions from the 1990 volcanic eruptions of Mount Redoubt

Airborne in situ and remote sensing (lidar and correlation spectrometer) measurements are described for the volcanic emissions from Mount Redoubt, Alaska, in January and June 1990. The lidar provided excellent real-time information on the distribution of the volcanic effluents. In postanalysis the lidar observations were used to determine cross-sectional areas of the plumes of emissions which, together with the airborne in situ measurements, were used to derive the fluxes of particles and gases from the volcano. For the intraeruptive emissions the ranges of the derived fluxes were for water vapor, ∼160–9440 kg s−1; for CO2, ∼30–1710 kg s−1; for SO2, ∼1–140 kg s−1; for particles (<48 μm diameter), ∼1–6 kg s−1; for SO4=, <0.1–2 kg s−1; for HCl, <0.01–2 kg s−1; and for NOx, <0.1–2 kg s−1;. Independent measurements of SO2 from a correlation spectrometer during the period of active dome growth between late March and early June 1990 gave fluxes from 12 to 75 kg s−1;. The particles in the intraeruptive emissions consisted primarily of silicate rock and mineral fragments devoid of any sulfuric acid coating. Very little of the SO2 (∼0.1%) was oxidized to sulfate in the cold, dark conditions of the Arctic atmosphere. During a large eruption of Mount Redoubt on January 8, 1990, the particle (<48 μm diameter) emission flux averaged ∼104 kg s−1. During posteruptive emissions on June 11, 1990, the fluxes of both particles and gases were either close to or less than our lower detection limits (except for water vapor, which had a flux of ∼6×103 kg s−1).

Scavenging of Aerosol Particles by Precipitation

Abstract Airborne measurements have been made of aerosol particle size distributions (>0.01 μm) in aged air masses, in the plumes from several coal power plants and a large Kraft paper mill, and in the emissions from a volcano, before and after rain or snow showers. These measurements have been used to deduce the precipitation scavenging collection efficiencies of aerosol particles ranging in size from ∼0.01 to 10 μm diameter. Despite large variations in the nature of the aerosol particles and the precipitation, the scavenging collection efficiencies as a function of particle size showed marked similarities, with some well-defined maxima and minima values. The measurements agree well with theoretical calculations for aerosol particles >1 μm, but for the submicron aerosol particles the scavenging collection efficiencies are generally much higher, and the region of very low scavenging efficiencies (the “scavenging gap”) much narrower, than current theories predict. Some possible explanations for these discr...

Mercury in smoke from biomass fires

Litter and green vegetation were collected in 7 locations in the contiguous United States, analyzed for mercury, and burned under controlled conditions at the US Forest Service Fire Science laboratory in Missoula, MT. Among fuels, leaf and 3needle litter contained the highest concentration (up to 71ng/g on dry weight) of mercury. The combustion of litter and green vegetation resulted in essentially complete release of mercury stored in the fuel. Mercury is emitted primarily as elemental mercury, >95% for most burns, with particulate mercury (TPM) accounting for the remainder. From the laboratory experiments we project that mercury emitted from temperate/boreal forest fires and from all biomass burning is an important source components for the atmospheric mercury budget.

Particle emissions and the production of ozone and nitrogen oxides from the burning of forest slash

Airborne measurements in the plumes from three prescribed burns of conifer slash showed the number concentration-size spectra to be bimodal with peaks at ~0.1 and ~0.5 μm. The mass distribution was sharply peaked at ~0.3 μm where over 80% of the mass of the particles in the plume resided. Most of the particles in the plumes were of primary rather than secondary origin. The burns emitted particulate mass into the atmosphere at rates of 0.1–15 kg s−1 during the active burning phase. The average density of the particles ranged from 0.75 to 1.34 g cm−3. Estimates of particle emission factors for the burns ranged from 0.2 to 2%, higher values being associated with higher fuel consumption rates. The burns were prolific sources of cloud condensation nuclei (CCN), producing ~1010−1011 CCN per gram of wood consumed. The CCN resulted in anomalously high concentrations of water droplets < 10 μm diameter in the cumulus clouds produced by the burns. Ozone concentrations near the tops of the plumes reached values as high as 44 ppb above ambient values, the higher values generally being associated with high u.v. intensities. Peak concentrations of NO2 and NO in the plumes were ~60 ppb; the ratio of NOxNO ranged from 1 to 3. SO2 was not found in the plumes.

Airborne Studies of the Smoke from the Kuwait Oil Fires

Airborne studies of smoke from the Kuwait oil fires were carried out in the spring of 1991 when ∼4.6 million barrels of oil were burning per day. Emissions of sulfur dioxide were ∼57% of that from electric utilities in the United States; emissions of carbon dioxide were ∼2% of global emissions; emissions of soot were ∼3400 metric tons per day. The smoke absorbed ∼75 to 80% of the suns radiation in regions of the Persian Gulf. However, the smoke probably had insignificant global effects because (i) particle emissions were less than expected, (ii) the smoke was not as black as expected, (iii) the smoke was not carried high in the atmosphere, and (iv) the smoke had a short atmospheric residence time.

Humidity and particle fields around some small cumulus clouds

Abstract Aircraft-borne measurements showed that five small cumulus clouds were surrounded by regions of high humidity out to distances of several cloud radii from their centers. Total particle concentrations in the regions of high humidity were about twice those in the air at the same level but well removed from the cloud boundaries. The regions of high humidity and particle concentrations also coincided with regions of high turbulence surrounding the clouds. In addition to affecting particle production, regions of high humidity surrounding clouds can be expected to affect chemical process and atmospheric radiation transfer.