PNNL

Abstract

We conduct analyses of all-sky and clear-sky surface downwelling shortwave (SW) radiation and bulk cloud properties using data from several DOE Atmospheric Radiation Measurement (ARM) Program and NOAA SURFRAD Network sites spanning the years 1995 through 2007. We aggregate some ARM sites in an attempt to represent global climate model grid scales, and the SURFRAD sites to represent the wider-spread scale of the continental US. Our results show that changes in dry aerosols and/or direct aerosol effects alone cannot explain the changes in surface SW radiation, but likely changes in cloudiness play a role. A hypothesis for the conundrum regarding the clear-sky results is put forth, but it is noted that further study is needed. We do show that wide-spread brightening has occurred over the continental US over the years of the study averaging about 10 Wm-2/decade for the all-sky SW and about 7 Wm-2/decade for the clear-sky SW. These SW increases are accompanied by decreasing tendencies in cloudiness, and increasing tendencies in the clear-sky SW diffuse/direct ratio that is often associated with atmospheric turbidity.