Variability of the surface radiation budget over the United States from 1996 through 2011 from high‐quality measurements

Abstract

Sixteen years of high‐quality surface radiation budget (SRB) measurements over seven U.S. stations are summarized. The network average total surface net radiation increases by +8.2 Wm−2 per decade from 1996 to 2011. A significant upward trend in downwelling shortwave (SW‐down) of +6.6 Wm−2 per decade dominates the total surface net radiation signal. This SW brightening is attributed to a decrease in cloud coverage, and aerosols have only a minor effect. Increasing downwelling longwave (LW‐down) of +1.5 Wm−2 per decade and decreasing upwelling LW (LW‐up) of −0.9 Wm−2 per decade produce a +2.3 Wm−2 per decade increase in surface net‐LW, which dwarfs the expected contribution to LW‐down from the 30 ppm increase of CO2 during the analysis period. The dramatic surface net radiation excess should have stimulated surface energy fluxes, but, oddly, the temperature trend is flat, and specific humidity decreases. The enigmatic nature of LW‐down, temperature, and moisture may be a chaotic result of their large interannual variations. Interannual variation of the El Niño/Southern Oscillation (ENSO) ONI index is shown to be moderately correlated with temperature, moisture, and LW‐down. Thus, circulations associated with ENSO events may be responsible for manipulating (e.g., by advection or convection) the excess surface energy available from the SRB increase. It is clear that continued monitoring is necessary to separate the SRB's response to long‐term climate processes from natural variability and that collocated surface energy flux measurements at the SRB stations would be beneficial.