Variability of Surface Radiation Budget Components Over the U.S. From 1996 to 2019—Has Brightening Ceased?

Abstract

The record of downwelling solar irradiance and other surface radiation budget components for the U.S. has been extended through 2019 using SURFRAD Network data. Brightening of surface solar irradiance of +7.36 Wm−2/decade occurred from 1996 through 2012. In 2013, surface solar radiation sharply decreased to the long-term mean (representing 1996–2019) and remained near that level through 2017. Successive decreases in 2018 and 2019 yielded a dimming trend of −3.90 Wm−2/decade after 2012, but with a high uncertainty owing to the observed variability and brief period covered. Individually, all stations but Penn State showed brightening trends consistent with the network average, and surface solar irradiance decreased at all stations after 2012. Total surface net radiation showed similar tendencies but the reversal from increasing to decreasing was more gradual because of the response of surface net longwave to the changing solar input. Aerosol optical depth decreased continuously throughout the tenure of the network but accounted for only 3% of the variability of surface solar irradiance, while cloud fraction explained 62%. The mean cloud fraction was 2.4% greater during the dimming period than the brightening period but showed no trends due to high interannual variability. However, annual anomalies of direct-normal solar radiation, which relate to sun duration and clouds, generally increased to 2012 and then decreased thereafter. Collectively, these results indicate that changing cloud cover was the primary source of brightening and dimming over the U.S. from 1996 to 2019.