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G. Kukla
and
J. Gavin

Abstract

Autumns in the Northern Hemisphere during the 1974–78 pentad were substantially cooler than in the pentad ending in 1938. Zonally averaged surface air temperature in October along latitude 80°N was 4.8°C lower, while summers were 0.6°C warmer. The recent pentad is cooler between 20 and 80°N in all seasons except spring when virtually no change was detected. The largest temperature difference was observed in autumn and winter in the high latitudes, which is a region of negative surface heat balance. In latitudinal bands along 80, 65, 55 and 40°N, the average October temperatures were lower in the recent pentad than during 87% or more of the 1891–1978 period. However, July and August along 80°N were warmer than 91% of the 88-year long interval. Zonal mean temperatures used in the study were obtained from a recent publication of Gruza and Ran'kova (1979). Monthly surface air temperature anomalies during 1974–78 in the latitudinal belt close to the snowline were inversely related to anomalies of the total hemispheric snow and ice extent.

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G. Kukla
and
D. Robinson

Abstract

Monthly mean surface albedo for the full spectra range of incoming solar radiation and average atmospheric properties is estimated for the period 1 April 1974–31 March 1975. It can be considered approximately representative of average conditions in the last decade. The monthly global surface albedo, averaged without weighting for radiation income, ranges from 15.7% in August to 17.9% in December, with an annual average of 17.0%. This is higher than most previously published figures. The difference is partly due to the averaging method, and partly to our more accurate satellite based information on snow and ice covers. Parameterized albedo values of characteristic surface types were compiled from published references. Estimates are zonally averaged separately for land and ocean in 2° latitudinal bells.

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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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G. Kukla
,
J. Gavin
, and
T. R. Karl

Abstract

Meteorological stations located in an urban environment in North America warmed between 1941 and 1980, compared to the countryside, at an average rate of about 0.12°C per decade. Secular trends of surface air temperature computed predominately from such station data are likely to have a serious warm bias.

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T. R. Karl
,
G. Kukla
, and
J. Gavin

Abstract

A network of 14 nonurban stations was used as a first step toward understanding the character of change of daily temperature between 1948 and 1983 across the United States. Overall, in the eastern two-thirds of the country more stations tend to reflect statistically significant cooler weather than would be expected due to chance alone. In the western one-third of the country the opposite is found. Despite the cooling in the east, air masses with comparable characteristics (dewpoints, cloud cover) affecting these stations have warmed. This finding is in qualitative agreement with the expected impact of rising concentrations of greenhouse gases.

Considering all stations regardless of location, a marked increase of daily minimum temperature was found in comparable air masses under overcast skies, which, when combined with the tendency for decreasing maxima, results in a highly significant (at 0.1% level) decrease of an implied diurnal temperature range. Both maxima and minima increased in clear weather after stratification by air mass characteristics. It is highly unlikely that the reported changes are related to boundary layer humidity or direction of air flow since the data were stratified for air mass identification by both surface dew-point temperature and wind direction.

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T. R. Karl
,
G. Kukla
, and
J. Gavin

Abstract

An appreciable number of nonurban stations in the United States and Canada have been identified with statistically significant (at the 90% level) decreasing trends in the monthly mean diurnal temperature range between 1941–80. The percentage of stations in the network showing the decrease is higher than expected due to chance throughout the year, with a maximum reached during late summer and early autumn and a minimum in December. Monte Carlo tests indicate that during five months the field significance of the decreasing range is above the 99% level, and in 12 months above the 95% level. There is a negligible probability that such a result is due to chance. In contrast, trends of increasing or decreasing monthly mean maximum or minimum temperatures have at most only two months with field significance at or above the 90% level. This is related to the tendency toward increasing temperature in the western portions of North America and decreasing temperature in the east.

The physical mechanism responsible for the observed decrease in the diurnal range is not known. Possible explanations include greenhouse effects such as changes in cloudiness, aerosol loading, atmospheric water vapor content, or carbon dioxide. Change in circulation is also a possibility, but it will be difficult to isolate since the patterns of the decreased diurnal temperature range have high field significance throughout much of the year, relatively low spatial coherence, and occur at many stations where individual trends in the maximum and minimum temperature are not statistically significant. Our data show that the trends in the maximum and minimum temperatures may differ considerably from trends in the mean.

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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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