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Peter V. Hobbs
and
Lawrence F. Radke

Abstract

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David R. Dowling
and
Lawrence F. Radke

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.

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Lawrence F. Radke
and
Anthony C. Delany

Abstract

During the Cooperative Atmospheric–Surface Exchange Study in 1999 (CASES99), an intensive investigation of the stable nocturnal boundary layer, a versatile and sensitive cryogenically cooled thermal imaging radiometer, the Inframetrics PM 380 Thermal Camera, was deployed. The 60-m-high instrumented tower at the central CASES99 site provided the perch from which to survey the instrumented research field. The field of view of 16° (0.28 sr) and an angular resolution of 1/16° (0.0011 sr) enabled a segment of landscape 500 m distant, of approximately 150-m width, to be viewed with a resolution of approximately 0.5 m. Or, looking down from the 50-m level of the tower, a section of the ground surface 15 m on a side could be viewed with a resolution of 5 cm. The surface brightness temperature of any area could be surveyed with a temperature resolution of 0.1°C and a time resolution of 30 Hz. The information obtained from analysis of these thermal images uniquely complemented the data acquired by the more conventional radiometric and meteorological instrumentation. The thermal imager provided valuable information on the landscape-scale changes of surface temperature after sunset and yielded insights into the development of surface drainage flow and its initiation. Also during periods of nocturnal stability the movement of thermal features across the standing vegetation could be traced, allowing the propagation of eddies to be investigated. The examination of apparently uniform land surfaces enabled a quantified analysis of the inhomogeneities of longwave emission. This information is critically important for the understanding of errors in the surface energy balance. The nocturnal thermal images of mature deciduous trees illustrated the extent to which trees modulate local airflow patterns. Finally, the tantalizing prospect of utilizing the thermal image of the 60-m tower itself to achieve a surrogate air temperature profile was examined.

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Lawrence F. Radke
and
Peter V. Hobbs

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.

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Peter V. Hobbs
and
Lawrence F. Radke

Abstract

Airborne, ground and radar techniques used for evaluating the effects of artificial seeding on winter clouds and precipitation over the Cascade Mountains are described. The clouds were seeded for 1 or 2 h with silver iodide and/or Dry Ice, dispersed from an aircraft, at locations which particle trajectory analysis, based on field data, indicated would affect precipitation in a small (90 km2) predetermined target area straddling the Cascade crest. The effects of seeding on the clouds were determined from the aircraft through visual observations, ice nuclei measurements, and measurements of the type and concentrations of cloud particles. A Doppler radar located near the Cascade crest was used to measure the spectra of fallspeeds of the precipitation particles. At manned stations within the target area on the ground, measurements and observations were made before, during and after seeding of precipitation rates, the types, concentrations and degrees of riming of snow crystals, and the concentrations of freezing nuclei and silver in the snowfall.The effects of heavy seeding on the clouds were generally pronounced and measurable. Good physical evidence for artificial modifications of snowfall on the ground within the target area was not as common, but was obtained in a number of detailed case studies.

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Peter V. Hobbs
,
Lawrence F. Radke
, and
Marcia K. Politovich

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Lawrence F. Radke
,
Peter V. Hobbs
, and
Mark W. Eltgroth

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 discrepancies are suggested.

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David A. Bowdle
,
Peter V. Hobbs
, and
Lawrence F. Radke

Abstract

One hundred and fifty-five ice nucleus (IN) spectra were measured from an aircraft at various locations over the High Plains of the United States during the springs and summers of 1975 and 1976. Frequency distributions of the IN concentrations at each test temperature reveal two modes, each of which is apparently lognormally distributed. The dominant mode occurs at high IN concentrations, and the other mode at low concentrations.

The characteristic IN spectrum was represented by N(T) = 2 × 10−4 exp(−0.3ΔT), where N(T) is the concentration (per liter) of IN at temperature T and supercooling ΔT.

The IN generally appear to derive from a widespread and fairly uniform ground source, which accounts for the dominant mode. However, concentrations can be increased by duststorms and industrial pollution and decreased in the vicinity of localized IN sinks (e.g., clouds and precipitation). As a consequence, IN concentrations can vary markedly during the course of a day, from day-to-day, and cyclically over periods of days to weeks.

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Lawrence F. Radke
,
Peter V. Hobbs
, and
Dean A. Hegg

Abstract

Airborne measurements over periods of several hours were made in the effluents that collected in the boundary layer in the form of “ground clouds” when an Atlas/Centaur and Titan III rocket were launched at night-time from Cape Canaveral, Florida. The ground cloud produced by the ATLAS was dry, whereas that produced by the TITAN was initially wet, then dry, and finally wet again. Both clouds dispersed primarily in the horizontal plane. Their volumes at time t (min) were given by V = V 0 t n where V 0 = 1.3 × 106 m3 and n = 0.98 for the ATLAS and V 0 &equals 1.76 × 107 m3 and n = 0.94 for the TITAN.

The ATLAS ground cloud initially contained elevated concentrations of NO, N02, hydrocarbons and particulate mass. However, dispersion of the cloud quickly reduced these concentrations and the light-scattering coefficient of the cloud. Gas-to-particle conversion (postulated to be the result of the oxidation of NO to NO2 followed by the Formation of NH4NO3) produced smoke particles at a rate of - ∼1016 s−1 in the ATLAS ground cloud but these did not contribute significantly to the total mass of particles in the cloud.

Gas-to-particle conversion in the TITAN ground cloud during its dry phase (probably produced by the reaction of HCI, from the rocket exhausts, with NH3, from the ambient air, to produce NH4Cl) created mass at a sufficient rate (∼0.1 μg m−3 min−1) to provide the potential for a significant source of pollution for several days.

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Edward E. Hindman
,
Lawrence F. Radke
, and
Mark W. Eltgroth

Abstract

Airborne measurements of cloud nuclei [cloud condensation nuclei (CCN) and ice nuclei (IN)] were made in the stabilized ground clouds resulting from the launches of a liquid-fueled ATLAS/Centaur rocket and a solid-fueled TITAN III rocket. Concentrations of CCN in both types of clouds were greater than ambient values for the ∼2 h duration of the measurements. The initial production of CCN active at 0.5% supersaturation in the ATLAS and TITAN clouds was equivalent to a 20 and 700 s emission, respectively, by the city of Denver, Colorado. Thereafter, the clouds continued to generate CCN at a rate of ∼1 cm−3 s−1. Concentrations of IN in the ATLAS cloud were greater than ambient values for only a short period after launch; the nuclei were probably from entrained launch pad and ground debris. The concentrations of IN in the TITAN cloud were mainly at or below ambient values (possibly due to the presence of high concentrations of HCI) until ∼2 h after launch when they increased substantially above ambient values. Estimates of the IN activity of the ground cloud material have large uncertainties due to unresolved discrepancies with previous laboratory measurements.

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