Significant decadal brightening of downwelling shortwave in the continental United States

We conduct analyses of all-sky and clear-sky surface downwelling shortwave radiation and bulk cloud properties using data from several Department of Energy Atmospheric Radiation Measurement (ARM) Program and National Oceanic and Atmospheric Administration Surface Radiation (SURFRAD) network sites spanning the years 1995 through 2007. Five ARM sites are aggregated to study downwelling shortwave tendencies on global circulation model grid scales, and then six SURFRAD sites plus the central ARM site are aggregated to study the wider scale of the continental United States. We show that widespread brightening has occurred over the continental United States as represented by these measurements over the 12 years of the study, averaging,about 8 W m(2)/decade for the all-sky shortwave and 5 W m2/decade for the clear-sky shortwave. This all-sky increase is substantially greater than the 2 W m(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 W m(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 shortwave (SW) radiation, but it is likely that changes in cloudiness play a significant role. These SW 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. However, given the many local influences, evidence presented here suggests that the determination of the causes of decadal changes in the downwelling solar radiation at the surface are better studied locally and regionally, rather than on a global or continental scale.