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  • Author or Editor: Ellsworth G. Dutton x
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Ellsworth G. Dutton

Abstract

The radiation balance consisting of upward and downward components of solar and thermal infrared broadband irradiances is continuously measured from the top of a 300-m tower situated on the Colorado high plains. The data are representative of a weighted areal average over a variety of surface and vegetation types within about a 1.5-km radius of the tower. Data from a three-year period, 1986–88, appears to be sufficient to define smooth annual cycles in monthly averages and 1-h resolution diurnal cycles in seasonal averages. It is found that even though infrared cycles are out of phase with cycles of corresponding solar components, the overall net radiation balance is in phase with surface solar forcing. The latter follows closely the extraterrestrial forcing but with some phase modifications by clouds and surface reflectance variations. The value of the correlation coefficient squared between the extraterrestrial radiation and the measured surface radiation balance quickly increases from 0.89–0.99 as averaging time increases from 1–90 days, respectively.

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Ellsworth G. Dutton

Abstract

No abstract available.

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Ellsworth G. Dutton
and
Barry A. Bodhaine

Abstract

The clear-sky transmission of the atmosphere contributes to determining the amount of solar irradiance that reaches various levels in the atmosphere, which in turn is fundamental to defining the climate of the earth. As of the end of 1999, sustained clear-sky solar transmission over the mid-Pacific, as viewed from Mauna Loa, Hawaii, reached its highest level of clarity since before the eruption of Mount Pinatubo in 1991 and appears to be continuing to increase toward baseline levels established during 1958–62 and not sustained since. This record is used to answer the question as to impact of transmission variations, which can be attributed to either upward scattering or absorption above the station, on the net solar irradiance at 3.4 km, the altitude of the isolated mountain-top observing site. Net solar irradiance at a given level describes the total solar irradiance absorbed below that level. Monthly mean net solar anomalies caused by transmission variations, relative to the 1958–62 baseline, range from −14 to 2 W m−2 and averaged −1.45 W m−2 (−0.7%) between 1963 and 1999. Because of inherent attributes of this transmission record, the observed fluctuations in the record are of unusually high precision over the entire period of record and are also representative of an extended surrounding region. Irradiance anomalies have a long-term precision of better than 0.1 W m−2 (∼0.05%) per decade. Any possible linear trend for the entire 42 yr is limited by the data to between about 0.0 and −0.1 W m−2 decade−1, or any net shift over the 42 yr must be in the range of about 0.0 to −0.35 W m−2 (0.0% to −0.15%). The transmission fluctuations are potentially caused by various atmospheric constituents, primarily aerosols, ozone, and water vapor, but the role of a specific constituent cannot be uniquely isolated on the basis of the transmission record alone. Aerosols have the greatest potential influence on the record and in general have the ability to cause both scattering and absorption such that the net radiative heating effect in the entire atmospheric column cannot be determined from the transmission data alone. However, because the largest anomalies in the record are known to be due to volcanic eruptions that produce predominantly conservative scattering aerosols, those large anomalies resulted in net radiative cooling tendencies in the entire associated atmospheric column.

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Ellsworth G. Dutton
,
Robert S. Stone
,
Donald W. Nelson
, and
Bernard G. Mendonca

Abstract

Incoming global solar irradiance measured at the surface at the South Pole unexpectedly decreased steadily by 15% from 1976 through 1987 during the late austral summer season, whereas no trend is apparent for September through December. February's irradiance trend, − 1.24% yr−1 on the average, is statistically significant at greater than the 99.9% confidence level. The irradiance observations were made continuously with the same calibrated sensor and are confirmed by a second simultaneous solar irradiance measurement series. Associated changes in seasonal sky cover (clouds) and surface air temperature were also observed. Seasonally increasing cloud cover is directly associated with the decreasing irradiance trends, whereas temperatures show a warming trend significant only in March, followed by a cooling trend significant only in May. Cloudiness and temperature records for 32 years suggest that the observed cloudiness trend began in the late 1970s, while the temperature trends become apparent only in the early 1980s. The observed sensitivity of total global solar irradiance to the change in sky cover is roughly six percent per one-tenth and is shown to vary spectrally. Although the annual averages of solar irradiance at the South Pole display an overall decrease between 1976 and 1989, the most recent years in this period show some recovery from earlier declines. Likewise, the downward trends in January and February irradiance diminished in 1988 and 1989.

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Robert D. Cess
,
Ellsworth G. Dutton
,
John J. Deluisi
, and
Feng Jiang

Abstract

Two separate datasets both of which provide measurements of net downward shortwave radiation have been combined, so as to provide a means of critically examining methods for transferring satellite measurements to the surface. This is further facilitated through interfacing the two datasets with an atmospheric shortwave-radiation model. One dataset comprises near-surface measurements made at the Boulder Atmospheric Observatory Tower while the other consists of collocated satellite pixel measurements from the Earth Radiation Budget Experiment.

This study amplifies previous suggestions that surface-shortwave absorption is a more meaningful quantity, for climate studies, than is surface insolation. The former should not, however, be evaluated from the latter through use of a surface albedo, since surface albedo is not solely a surface property nor can it easily be evaluated from satellite measurements. It is further demonstrated that a direct evaluation of surface shortwave absorption can be more accurately obtained from satellite measurements than can surface insolation. Specifically, a linear slope-offset relationship exists between surface and surface-atmosphere shortwave absorption, and an algorithm is suggested for transferring satellite shortwave measurements to surface-shortwave absorption. The present study is directed solely at clear-sky conditions because the clear-sky top-to-surface transfer serves as a necessary prerequisite towards treating both clear and overcast conditions.

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Seth Nemesure
,
Robert D. Cess
,
Ellsworth G. Dutton
,
John J. Deluisi
,
Zhanqing Li
, and
Henry G. Leighton

Abstract

Recent data from the Earth Radiation Budget Experiment (ERBE) have raised the question as to whether or not the addition of clouds to the atmospheric column can decrease the top-of-the-atmosphere (TOA) albedo over bright snow-covered surfaces. To address this issue, ERBE shortwave pixel measurements have been collocated with surface insolation measurements made at two snow-covered locations: the South Pole and Saskatoon, Saskatchewan. Both collocated datasets show a negative correlation (with solar zenith angle variability removed) between TOA albedo and surface insulation. Because increased cloudiness acts to reduce surface insulation, these negative correlations demonstrate that clouds increase the TOA albedo at both snow-covered locations.

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Robert D. Cess
,
Seth Nemesure
,
Ellsworth G. Dutton
,
John J. Deluisi
,
Gerald L. Potter
, and
Jean-Jacques Morcrette

Abstract

Two datasets have been combined to demonstrate how the availability of more comprehensive datasets could serve to elucidate the shortwave radiative impact of clouds on both the atmospheric column and the surface. These datasets consist of two measurements of net downward shortwave radiation: one of near-surface measurements made at the Boulder Atmospheric Observatory tower, and the other of collocated top-of-the-atmosphere measurements from the Earth Radiation Budget Experiment. Output from the European Centre for Medium-Range Weather Forecasts General Circulation Model also has been used as an aid in interpreting the data, while the data have in turn been employed to validate the model's shortwave radiation code as it pertains to cloud radiation properties. Combined, the datasets and model demonstrate a strategy for determining under what conditions the shortwave radiative impact of clouds leads to a heating or cooling of the atmospheric column. The datasets also show, in terms of a linear slope-offset algorithm for retrieving the net downward shortwave radiation at the surface from satellite measurements, that the clouds present during this study produced a modest negative bias in the retrieved surface flux relative to that inferred from a clear-sky algorithm.

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