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David A. Robinson and George Kukla

Abstract

Albedos of surfaces covered with 50 cm of fresh dry snow following a major U.S. East Coast storm on 11–12 February 1983 ranged from 0.20 over a mixed coniferous forest to 0.80 over open farmland. As the snow cover dissipated, albedo decreased in a quasi-linear fashion over forests. It dropped rapidly at first, then slowly, over shrubland; while the opposite was observed over farmland.

Following the melt, the albedo of snowfree surfaces ranged from 0.07 over a predominantly wet peat field to 0.20 over a field covered with corn stubble and yellow grass. The difference between snow-covered and snowfree albedo was 0.72 over the peaty field and 0.10 over the mixed forest.

Visible band (0.28–0.69 μm) reflectivities of snow-covered fields and shrubland were higher than those in the near-infrared (0.69–2.80 μm), whereas the opposite was true over mixed coniferous forests. Visible and near-infrared reflectivities were approximately equal over deciduous forests.

Data were collected in a series of low-altitude flights between 10 February and 24 March 1984 in northern New Jersey and southeastern New York with Eppley hemispheric pyranometers mounted on the wingtip of a Cessna 172 aircraft.

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David A. Robinson and George Kukla

Abstract

No abstract available.

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David A. Robinson and George Kukla

Abstract

Areally weighted clear sky surface albedo of snow-covered land in the middle and high latitudes of the Northern Hemisphere was measured from satellite imagery in A 1×1° latitude-longitude cells. The study area included 87% of the land polewards of 25°N, where Dickson and Posey found the probability of the seasonal occurrence of snow cover over −2.5 cm deep to be greater than zero. Albedo is 0.60 in Eurasia and 0.56 in North America, approximately 3.5 times greater than snow-free conditions. The highest average value for a 5° latitudinal zone is 0.77 at 70–75°N. The lowest is 0.43 at 60–75°N, which includes 0.36 in Eurasia and 0.58 in North America. The low albedo is due to the masking of snow covered ground by the canopy of coniferous forests.

Data were obtained by image processor analyses of Defense Meteorological Satellite Program imagery. Scene brightness was converted to surface albedo by linear interpolation between bright and dark snow-covered surfaces with known albedo.

The resulting chart is a refinement of an earlier product. The 1 × 1° digital data set is available for use in climate modeling.

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Beate G. Liepert and George J. Kukla

Abstract

A statistically significant decrease in mean annual global solar radiation between 1964 and 1990 under completely overcast skies was found at five out of eight studied locations in Germany. A decrease of global solar radiation is also evident in partly cloudy conditions. The mean annual cloud cover fraction and sunshine duration did not significantly change, while the visually assessed mean annual horizontal visibility increased at six of the eight stations. The authors’ findings point to a decrease of the cloud transmissivity, which in turn could be explained by an increased frequency of multilevel cloudiness, changing cloud types, or by indirect aerosol effects on clouds. The decreasing trend of global solar radiation in clear skies was most expressed at high and intermediate solar zenith angles, whereas a slight increase of global radiation was found at low sun zenith angles. A decline of the diffuse component of the global radiation over time was also detected. It is shown that the observed changes in clear-sky radiation were probably related to the recovery from the effects of major volcanic eruptions in the mid-1960s and 1980s. Increase of submicron aerosol particles with simultaneous reduction of aerosol mass concentrations reported by others and increasing absorption by urban aerosol may also contribute to the observed changes. The results are based on statistical analyses of hourly data of solar radiation, sunshine duration, cloud cover, and horizontal visibility stratified by solar zenith angle.

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Thomas R. Karl, George Kukla, and Joyce Gavin

Abstract

Previous work has shown significant decreases of the diurnal temperature range (1941–80) across a network of 130 stations in the United States and Canada. In the present study, changes in monthly total precipitation at these same stations were related to the decrease in temperature range using various Monte Carlo. These tests indicate that factors other than those related to precipitation contributed to the decrease of daily temperature range. Further study of the mechanisms responsible for the decreased temperature range is warranted, based on these results. The decreased range may be one of the few pieces of evidence available in North America that is consistent with potential impacts of increased greenhouse gases and/or anthropogenic aerosols.

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Thomas R. Karl, Henry F. Diaz, and George Kukla

Abstract

Several equations were developed that related the effect of urban growth, measured by increasing population, to the mean seasonal and annual temperature: the diurnal maximum, minimum, average, and range. These equations were derived from a network of 1219 stations across the United States, which were analyzed for the years 1901–84. The results indicate that urban effects on temperature are detectable even for small towns with populations under 10 000. Stations with populations near 10 000 are shown to average 0.1°C warmer for the mean annual temperature than nearby stations located in rural areas with populations less than 2000. Urbanization decreases the daily maxima in all seasons except winter and the temperature range in all seasons. It increases the diurnal minima and the daily means in all seasons.

The equations indicate that, for the annual mean temperature, urbanization during the twentieth century accounts for a warm bias of about 0.06°C in the U.S. Historical Climatology Network (HCN). Due to the large number of stations located in sparsely populated arms [(over 85% (70%) of all stations had a 1980 population of less than 25 000 (10 000)], the impact of urbanization is not large in relation to decadal changes of temperature in the United States. The average heat island impact during the period 1901–84 for the HCN is largest for the daily minima (0.13°C) and the temperature range (−0.14°C), while the impact on the daily maxima (−0.01°C) is an order of magnitude smaller.

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David A. Robinson, Mark C. Serreze, Roger G. Barry, Greg Scharfen, and George Kukla

Abstract

Visible-band satellite imagery is used to manually map surface brightness changes over sea ice throughout the Arctic Basin from May to mid-August over a 10-yr period. These brightness changes are primarily due to snowmelt atop the ice cover. Using image processor techniques, parameterized albedos are estimated for each brightness class. Snowmelt begins in May in the marginal seas, progressing northward with time, finally commencing near the pole in late June. large year-to-year differences are found in the timing of melt, exceeding one month in some regions. Parameterized albedo for most regions of the pack ice exceed 0.70 during May, declines rapidly during June, and reaches a seasonal low of between 0.40 and 0.50 by late July. For August, regional albedos, which also include areas of open water beyond the southern pack ice limit, are up to 0.16 lower than the corresponding values for pack ice areas only.

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Keith P. Shine, David A. Robinson, Ann Henderson-Sellers, and George Kukla

Abstract

Recent work has emphasized the potential importance of atmospheric aerosols in the Arctic. This paper presents results indicating the large-scale presence of arctic aerosols during late spring. Their screening effect may be sufficient to alter significantly the shortwave radiation budget. The ratios of brightness over sea and snow covered ice surfaces are shown to be considerably lower, using DMSP shortwave imagery, than those calculated for clear skies using a radiative transfer scheme. Our analysis shows that aerosols are the most likely cause of the discrepancy. With additional calibration the method offers the potential for remote sensing of the aerosol distribution and concentration over the Arctic.

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Thomas R. Karl, Philip D. Jones, Richard W. Knight, George Kukla, Neil Plummer, Vyacheslav Razuvayev, Kevin P. Gallo, Janette Lindseay, Robert J. Charlson, and Thomas C. Peterson

Monthly mean maximum and minimum temperatures for over 50% (10%) of the Northern (Southern) Hemisphere landmass, accounting for 37% of the global landmass, indicate that the rise of the minimum temperature has occurred at a rate three times that of the maximum temperature during the period 1951–90 (0.84°C versus 0.28°C). The decrease of the diurnal temperature range is approximately equal to the increase of mean temperature. The asymmetry is detectable in all seasons and in most of the regions studied.

The decrease in the daily temperature range is partially related to increases in cloud cover. Furthermore, a large number of atmospheric and surface boundary conditions are shown to differentially affect the maximum and minimum temperature. Linkages of the observed changes in the diurnal temperature range to large-scale climate forcings, such as anthropogenic increases in sulfate aerosols, greenhouse gases, or biomass burning (smoke), remain tentative. Nonetheless, the observed decrease of the diurnal temperature range is clearly important, both scientifically and practically.

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