Significant Decadal Brightening of Downwelling Shortwave in the Continental U.S.

Chuck Long Pacific Northwest National Laboratory
Ellsworth Dutton NOAA/OAR/ESRL
John Augustine National Oceanic and Atmospheric Administration
Warren Wiscombe Brookhaven National Laboratory
Martin Wild Institute for Atmospheric and Climate Science - ETH Zurich
Sally McFarlane U.S. Department of Energy
Connor Flynn Pacific Northwest National Laboratory

Category: Radiation

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The aggregate 1996-2007 increasing trend in all-sky downwelling SW (top) for the ACRF Southern Great Plains (SGP) Central Facility (CF) and six SURFRAD sites across the continental U.S. is large: 8 W/m^2 per decade. Bottom panel shows the corresponding decrease in daylight sky cover is 2.3% per decade.

We have conducted analyses of all-sky and clear-sky surface downwelling shortwave (SW) radiation and bulk cloud properties using data from several ARM Climate Research Facility (ACRF) Southern Great Plains (SGP) and National Oceanic and Atmospheric Administration (NOAA) Surface Radiation Research (SURFRAD) sites spanning the years 1995 through 2007. Five ACRF sites are aggregated to study downwelling shortwave tendencies on global circulation model grid scales, and then six SURFRAD sites plus the central ACRF site are aggregated to study the wider scale of the continental U.S. We show that wide-spread brightening has occurred over the continental U.S. as represented by these measurements over the 12 years of the study, averaging about 8 Wm-2/decade for the all-sky shortwave and 5 Wm-2/decade for the clear-sky shortwave. This all-sky increase is substantially greater than the 2 Wm-2/decade previously reported over much more of the globe as represented by data from the Global Energy Balance Archive spanning 1986-2000, and is more than twice the magnitude of the corresponding 1986-2000 2-3 Wm-2/decade increase in downwelling longwave. Our results show that changes in dry aerosols and/or direct aerosol effects alone cannot explain the observed changes in surface SW radiation, but it is likely that changes in cloudiness play a significant role. These shortwave increases are accompanied by decreasing tendencies in cloudiness, and an increasing tendency in the clear-sky SW diffuse/direct ratio that is often associated with atmospheric turbidity. We will present an update on the most recent results of our investigations into the causes and characteristics of the U.S. decadal brightening.

This poster will be displayed at ARM Science Team Meeting.

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