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S. J. Birstein
and
C. E. Anderson

Abstract

A careful study has been made of the nucleating ability of various chemicals. The nuclei were prepared in a nitrogen atmosphere, rather than air, to prevent a reaction at the hot filament with atmospheric oxygen. With use of these carefully controlled conditions, numerous materials previously reported as effective were found to be poor nucleating agents. The discrepancies among the various sets of data were found to be due to reaction at the filament of the solid material and oxygen in previous investigations. The results are examined to determine how they support prevailing theories on ice-crystal formation.

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R. C. Anderson
,
D. R. Keefer
, and
O. E. Myers

Abstract

Air pressure and temperature measurements were made during the 7 March 1970 solar eclipse. A Fourier analysis showed a primary wave with a period of 89 min and an amplitude of 250 μb. Smaller peaks were found with periods of 57, 51, 45, 38, 20.3, 18.2, 15.7 and 12.3 min. The primary wave agreed reasonably well in magnitude and phase with five earlier eclipse measurements dating back as far as 1887. The temperature decreased 3C with a minimum slightly after totality. This occurred under a thick cloud blanket.

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F. G. FINGER
,
H. M. WOOLF
, and
C. E. ANDERSON

Abstract

A method of numerical objective analysis has been developed for application to stratospheric constant-pressure data at the 100-, 50-, 30-, and 10-mb. levels (approximately 16, 20, 24, and 31 km., respectively). This system evolved from successive modifications of the programs employed for operational objective analysis of lower-level charts at the National Meteorological Center. For use with stratospheric data, the Automatic Data Processing portion of these programs was expanded to correct for the errors in high-level rawinsonde temperatures and heights caused by short and long-wave radiational effects on the temperature sensor. In addition, procedures for vertical extrapolation of rawinsonde reports and merging of off-time data were incorporated to compensate for the scarcity of reports at a given observation time.

General degradation of stratospheric data with increasing height necessitated more stringent data rejection criteria within the entire system. It was also essential that increased emphasis be placed on wind observations as an analysis parameter. The resulting charts have shown that the objective system employed produces analyses of acceptable quality. Improvements are continually being developed and incorporated to increase the efficacy and objectivity of the procedures and the quality and usefulness of the product.

The main purposes of the computerized system are to provide good quality stratospheric analyses for anticipated operational requirements and to satisfy the needs of research. Daily analysis of Northern Hemisphere charts is being performed during the IQSY and is expected to continue after the end of the period. These maps are recorded for distribution on microfilm and also on punched-card decks containing grid-point data.

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Rodney J. Anderson
,
Ronald C. Miller
,
James L. Kassner Jr.
, and
Donald E. Hagen

Abstract

Observations of the homogeneous nucleation of water vapor in an expansion cloud chamber have been carried out for the temperature range −50 to +17°C in the carrier gases argon and helium. We have found that the onset of the ice phase in freshly nucleated drops always occurs in the form of a two-stage process, condensation followed by homogeneous freezing at temperatures near −40°C. Ice particles appear as brilliant spherical particles in the cloud of liquid drops which scatter much less light. The critical gas temperature associated with the observation of ice nucleation depends on the type of carrier gas, the duration of the minimum final temperature, and whether there are ions or re-evaporation nuclei present. These effects and the analysis of the total homogeneous nucleation rate (liquid drops plus ice particles) strongly support the conclusion that the ice particles result from the freezing of liquid water drops which have been nucleated homogeneously from the vapor phase. A somewhat higher critical freezing temperature is observed in the absence of an electric clearing field. This probably is an indication that ice particles preferentially form on ions or simply that droplets which nucleate slightly earlier on ions have a chance to grow to a larger size, thus increasing the droplets’ probability of freezing. An ice memory effect has also been observed in nucleation which occurs on re-evaporation nuclei remaining from previous expansions. lens and re-evaporation nuclei raise the threshold temperature of ice nucleation about 1 and 2°C, respectively, above the critical spontaneous freezing temperature (−41°C). Consequently, they would be expected to have little impact on atmospheric processes.

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S. A. Vay
,
B. E. Anderson
,
K. L. Thornhill
, and
C. H. Hudgins

Abstract

Results are reported from an experiment conducted aboard the NASA DC-8 research aircraft to determine whether cabin air vented upstream of investigator's inlets had potentially contaminated ambient air samples obtained aboard the aircraft during previous airborne scientific expeditions. For the study, three multiport inlet rakes were mounted in windows downstream of an exhaust vent in locations forward, above, and aft of the right wing. These were used to make impact pressure measurements for determining boundary layer thickness (δ) as well as to collect ambient air samples at various distances outward from the airframe. The fraction of cabin air in the samples was determined by doping the vent air with a metered amount of CO2, then monitoring air at the inlet ports for differential CO2 enhancements. Data were collected at altitudes ranging from the surface to 12 km, at various indicated airspeeds, pitch and yaw angles, and during vertical ascents and descents. Results indicate that δ varies from about 13 to 37 cm and depends on inlet position, as well as the aircraft velocity, altitude, and pitch angle. The CO2-doped vent air was observed to mix throughout the depth of the boundary layer, but to be confined vertically to a narrow stream so that its interception by any particular inlet probe was highly dependent upon the aircraft-indicated airspeed and pitch angle. The inlet located forward of the wing was the most highly impacted, as samples collected there contained up to 0.8% cabin air at cruise altitudes under typical aircraft operating conditions. The implications of these findings on previous datasets are discussed, and a modified formula for calculating δ values appropriate for the DC-8 is proposed.

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C. W. Fairall
,
P. O. G. Persson
,
E. F. Bradley
,
R. E. Payne
, and
S. P. Anderson

Abstract

The calibration and accuracy of the Eppley precision infrared radiometer (PIR) is examined both theoretically and experimentally. A rederivation of the fundamental energy balance of the PIR indicates that the calibration equation in common use in the geophysical community today contains an erroneous factor of the emissivity of the thermopile. If a realistic value (0.98) for the emissivity is used, then this leads to errors in the total flux of 5–10 W m−2. The basic precision of the instrument is found to be about 1.5% of the total IR irradiance when the thermopile voltage and both dome and case temperatures are measured. If the manufacturer’s optional battery-compensated output is used exclusively, then the uncertainties increase to about 5% of the total (20 W m−2). It is suggested that a modern radiative transfer model combined with radiosonde profiles can be used as a secondary standard to improve the absolute accuracy of PIR data from field programs. Downwelling IR fluxes calculated using the Rapid Radiative Transfer Model (RRTM), from 55 radiosondes ascents in cloud-free conditions during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment field program, gave mean agreement within 2 W m−2 of those measured with a shipborne PIR. PIR data from two sets of instrument intercomparisons were used to demonstrate ways of detecting inconsistencies in thermopile-sensitivity coefficients and dome-heating correction coefficients. These comparisons indicated that pairs of PIRs are easily corrected to yield mean differences of 1 W m−2 and rms differences of 2 W m−2. Data from a previous field program over the ocean indicate that pairs of PIRs can be used to deduce the true surface skin temperature to an accuracy of a few tenths of a kelvin.

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S. H. Suck
,
J. L. Kassner Jr.
,
R. E. Thurman
,
P. C. Yue
, and
R. A. Anderson

Abstract

The clustering of water vapor about ions is important because of its relevance to atmospheric electrical processes. For this reason we have placed our emphasis particularly on the description of the size distribution (concentrations) and mobilities of the small ion clusters at various humidities. From our present theoretical study, we find that most of the hydronium ions H3O+ tend to associate with a small number of water molecules to form a hydrated ion cluster even at extremely low humidities in the range of 5 × 10−3 to 1%. At atmospherically more realistic humidities and at the room temperature, our computed number of water molecules in the hydrated ion clusters is predicted to be relatively small. It is then conjectured that ion-induced nucleation process (if it occurs) starts rather from the small hydrated ion clusters which initially existed even at extremely low humidities in the atmosphere. In addition, we also find that, in general, the hydrated ion clusters of small sizes corresponding to the mass range of 2–5 water molecules are responsible for the ion mobility range of 2–2.5 cm−2 (V s)−1. For reduced mobility below 2.0 cm2 (V s)−1, the mass of the hydrated ion cluster is predicted to be greater than that of approximately five water molecules. The simultaneous estimation of size distribution and mobility aids us in better understanding observed mobility spectra and the nature of atmospherically important prenucleation clusters, including the information of their electric conductivities in the atmosphere.

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L. A. Sromovsky
,
J. R. Anderson
,
F. A. Best
,
J. P. Boyle
,
C. A. Sisko
, and
V. E. Suomi

Abstract

An untended instrument to measure ocean surface heat flux has been developed for use in support of field experiments and the investigation of heat flux parameterization techniques. The sensing component of the Skin-Layer Ocean Heat Flux Instrument (SOHFI) consists of two simple thermopile heat flux sensors suspended by a fiberglass mesh mounted inside a ring-shaped surface float. These sensors make direct measurements within the conduction layer, where they are held in place by a balance between surface tension and float buoyancy. The two sensors are designed with differing solar absorption properties so that surface heat flux can be distinguished from direct solar irradiance. Under laboratory conditions, the SOHFI measurements agree well with calorimetric measurements (generally to within 10%). Performance in freshwater and ocean environments is discussed in a companion paper.

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L. A. Sromovsky
,
J. R. Anderson
,
F. A. Best
,
J. P. Boyle
,
C. A. Sisko
, and
V. E. Suomi

Abstract

The Skin-Layer Ocean Heat Flux Instrument (SOHFI) described by Sromovsky et al. (Part I, this issue) was field-tested in a combination of freshwater and ocean deployments. Solar irradiance monitoring and field calibration techniques were demonstrated by comparison with independent measurements. Tracking of solar irradiance diurnal variations appears to be accurate to within about 5% of full scale. Preliminary field tests of the SOHFI have shown reasonably close agreement with bulk aerodynamic heat flux estimates in freshwater and ocean environments (generally within about 20%) under low to moderate wind conditions. Performance under heavy weather suggests a need to develop better methods of submergence filtering. Ocean deployments and recoveries of drifting SOHFI-equipped buoys were made during May and June 1995, during the Combined Sensor Program of 1996 in the western tropical Pacific region, and in the Greenland Sea in May 1997. The Gulf Stream and Greenland Sea deployments pointed out the need for design modifications to improve resistance to seabird attacks. Better estimates of performance and limitations of this device require extended intercomparison tests under field conditions.

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Douglas C. Morton
,
Ruth S. DeFries
,
Yosio E. Shimabukuro
,
Liana O. Anderson
,
Fernando Del Bon Espírito-Santo
,
Matthew Hansen
, and
Mark Carroll

Abstract

The Brazilian government annually assesses the extent of deforestation in the Legal Amazon for a variety of scientific and policy applications. Currently, the assessment requires the processing and storing of large volumes of Landsat satellite data. The potential for efficient, accurate, and less data-intensive assessment of annual deforestation using data from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) at 250-m resolution is evaluated. Landsat-derived deforestation estimates are compared to MODIS-derived estimates for six Landsat scenes with five change-detection algorithms and a variety of input data—Surface Reflectance (MOD09), Vegetation Indices (MOD13), fraction images derived from a linear mixing model, Vegetation Cover Conversion (MOD44A), and percent tree cover from the Vegetation Continuous Fields (MOD44B) product. Several algorithms generated consistently low commission errors (positive predictive value near 90%) and identified more than 80% of deforestation polygons larger than 3 ha. All methods accurately identified polygons larger than 20 ha. However, no method consistently detected a high percent of Landsat-derived deforestation area across all six scenes. Field validation in central Mato Grosso confirmed that all MODIS-derived deforestation clusters larger than three 250-m pixels were true deforestation. Application of this field-validated method to the state of Mato Grosso for 2001–04 highlighted a change in deforestation dynamics; the number of large clusters (>10 MODIS pixels) that were detected doubled, from 750 between August 2001 and August 2002 to over 1500 between August 2003 and August 2004. These analyses demonstrate that MODIS data are appropriate for rapid identification of the location of deforestation areas and trends in deforestation dynamics with greatly reduced storage and processing requirements compared to Landsat-derived assessments. However, the MODIS-based analyses evaluated in this study are not a replacement for high-resolution analyses that estimate the total area of deforestation and identify small clearings.

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