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A. W. Hogan

Abstract

A series of aerosol measurements were made over the Antarctic Continent, during the austral spring of 1977. Vertical profiles indicate that the highest aerosol concentrations are present in the mixing layer, around the edges of the continent where some open water is present, and in a moist layer just above the surface over the South Polar Plateau. Aerosol concentrations, with respect to both mass and volume of air, decrease continuously with increasing altitude above these layers, and this smooth decrease continues without discontinuity when the local tropopause is crossed.

Aerosol concentrations were distributed uniformly at all latitudes between 75 and 90°S in the 500–450 mb layer. A small systematic decrease in aerosol concentration with increasing latitude was detected above 400 mb. This decrease is attributed to the relatively calm and very cold conditions present above 400 mb, which allow small ice crystals to form by homogeneous nucleation which scavenge particles from the air by Brownian coagulation during their slow fall.

These measurements indicate that the lower troposphere, especially those layers below 450 mb, is the most probable level at which particulate material is transported to the South Polar Plateau.

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A. W. Hogan

Abstract

Aerosol concentrations were measured at sea along the international date line from 40 to 75°S latitude, and over pack ice, fast ice and unvegetated land surfaces in a 100 km circle immediately south of the Ross Sea. Surface aerosol concentrations measured were similar to oceanic concentrations measured in the Northern Hemisphere in westerly and northwesterly winds, but lower concentrations were observed in the polar easterlies. Turbidity measurements indicated a relatively greater total aerosol burden in the vicinity of the antarctic convergence than near New Zealand or over the Ross Sea. Similar turbidity measurements showed a progressively smaller relative aerosol burden over fast ice, Ross Island, and the Antarctic continent. Comparison of these turbidity measurements with those made 13 and 31 years previously show no systematic change in turbidity.

Surface aerosol measurements in the dry valleys of Victoria Land indicated higher concentrations that may result from natural production mechanism.

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A. W. Hogan

Abstract

Aitken nuclei and ozone concentrations were measured, in concert with meteorological variables, while flying beneath the core of a jet stream at the 400 mb level. Stratospheric air which subsided to the flight level was richer in ozone, but similar in particle concentration when compared with adjacent tropospheric air. Concurrent carbon tetrachloride measurements by E. Robinson showed the tropospheric air to be richer in CCL4.

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A. Hogan and M. Ferrick

Abstract

Results of temperature measurements, which may be applied to inference of winter temperatures in data-sparse areas, are presented. The morning air temperatures during three winters were measured at 80 places in a 10 km × 30 km area along the Connecticut River. NOAA climatologies show this region to have complex spatial variation in mean minimum temperature. Frequency analysis techniques were applied to evaluate the differences in daily local temperature.

Temperature lapse or temperature inversion in the study area was inferred from the difference of surface temperature measurements 100 and 300 m above river level. The frequency of inferred temperature lapse and the inferred lapse rate diminished as snow cover increased. The frequency of inferred temperature inversion and inversion strength increased as snow cover increased. When more than 20 cm of snow covered the ground, an additional surface inversion was frequent in the layer less than 100 m above river level, and two-thirds of river level temperatures less than −20°C occurred concurrent with these conditions.

The daily temperature differences at the individual points, with respect to a defined point, were lognormally distributed. The magnitude and geometric standard deviation of temperature differences throughout the study area were larger on mornings when inversion was inferred. With respect to topography, temperature differences and the geometric standard deviation of temperature differences were smaller along flats or among basins than along or atop slopes on mornings when inversion was inferred.

It is proposed that some meteorologically prudent inferences of surface temperature and near-surface temperature lapse or temperature inversion can be made for similar data-sparse areas.

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W. Winters, S. Barnard, and A. Hogan

Abstract

A modern replica of the Aitken counter for detection of aerosol particles smaller in diameter than a half wavelength of visible light has been constructed using modern materials. The instrument employs photographic recording, rather than visual observations, of the cloud drops formed on these particles. This feature eliminates observer bias and provides a permanent record of the observation. Comparison of this instrument with a Pollak photoelectric nucleus counter indicated correspondence in the concentration sensed to well within experimental sampling error.

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A. Hogan, W. Winters, and G. Gardner

Abstract

A portable photoelectric nucleus counter, with similar sensitivity to the Pollak photoelectric nucleus counter with convergent light beam, has been developed and calibrated. This instrument has been incorporated into a packaged measurement system which allows the experimenter to determine the effective diffusion coefficient and fraction charged, of the natural aerosol, in uncontaminated areas. The photoelectric counter has comparable accuracy to the absolute (Aitken, Scholz) counters in the concentration range of interest, and is capable of determining the concentration once per minute.Field tests of the prototype instrument were conducted near sea level in Greenland. The concentration of natural aerosol in this area ranged from 150 to 200 particles cm−3. The instrumentation had sufficient sensitivity to detect a gradual increase in particle size at this low concentration.

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A. W. Hogan and M. G. Ferrick

Abstract

The urban heat island is a well-known and well-described temperature anomaly, but other types of heat islands are also infrequently reported. A 10 km × 30 km data field containing more than 100 individual winter morning air temperature measurement points was examined for areas characteristically warmer than surrounding areas. The very small “downtown” of Hanover, New Hampshire, was found to be 1°–2°C warmer than nearby open areas at the same elevation. The same technique was applied to examine the morning air temperature within a nearby hamlet consisting of about 60 wooden buildings within an area less than 0.3 km2. The bulk of observations and observations stratified by snow and sky cover showed no systematic difference between hamlet air temperatures and those obtained in surrounding terrain. Morning air temperatures along a freezing river were measured and found to be systematically warmer than nearby air temperatures for several days, until a significant snowfall diminished the ice growth rate. A thorough examination of temperature profiles near the river showed that the increase in air temperature beneath the overnight inversion during this freezing period was proportional to the heat release resulting from river ice growth.

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A. W. Hogan, V. A. Mohnen, and V. J. Schaefer

Abstract

No abstract available.

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A. W. Hogan, C. P. Edwards, and C. Robertson

Abstract

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Melinda S. Peng, James A. Ridout, and Timothy F. Hogan

Abstract

The convective parameterization of Emanuel has been employed in the forecast model of the Navy Operational Global Atmospheric Prediction System (NOGAPS) since 2000, when it replaced a version of the relaxed Arakawa–Schubert scheme. Although in long-period data assimilation forecast tests the Emanuel scheme has been found to perform quite well in NOGAPS, particularly for tropical cyclones, some weaknesses have also become apparent. These weaknesses include underprediction of heavy-precipitation events, too much light precipitation, and unrealistic heating at upper levels. Recent research efforts have resulted in modifications of the scheme that are designed to reduce such problems. One change described here involves the partitioning of the cloud-base mass flux into mixing cloud mass flux at individual levels. The new treatment significantly reduces a heating anomaly near the tropopause that is associated with a large amount of mixing cloud mass flux ascribed to that region in the original Emanuel scheme. In another modification, the selection of the updraft source level is changed in a manner that takes into consideration the assumed connection between updraft mass flux and parcel buoyancy at cloud-base level in the Emanuel scheme. Test results suggest that the modified scheme may in some cases better represent precipitation during the middle and latter stages of convective events. The scheme has also been modified to eliminate cloud-top overshooting. The parameterization changes are supported in part by diagnostic tests, including semiprognostic model tests using observed data and single-column model tests using cloud-resolving-scale simulation data. The modifications showed significant positive impacts in forecast experiments over the original designs and have been implemented into the operational NOGAPS.

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