Significant decadal brightening of downwelling shortwave in the continental United States

Abstract

[1] 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 À2 /decade for the all-sky shortwave and 5 W m À2 /decade for the clear-sky shortwave. This all-sky increase is substantially greater than the 2 W m À2 /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 À2 /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.

Aerosol trends as a potential driver of regional climate in the central United States: evidence from observations

“…The RRTMG_SW results in Bondville show that changing aerosols influences SWdn trends. Taken together, the observations and modeled results suggest that aerosol-radiation interactions are significant at Bondville, a conclusion doubted in previous studies (Long et al, 2009;Augustine and Dutton, 2013). We find evidence that these interactions play a role in the observed climate trends at Bondville, similar in response to modelling studies that probed the influence of aerosols on the warming hole (Leibensperger et al, 2012b).…”

“…However, the diffuse component of clear-sky SWdn increases at both Bondville (+6.6 Wm −2 ) and Goodwin Creek (+5.2 Wm −2 ), consistent with previous studies (e.g., Gan et al, 2014) and the cause of these increases remains an open question. We do not find evidence of aircraft contrails influencing observed diffuse SWdn at SURFRAD sites in the aftermath of 11 September 2001, and thus caution against attributing increasing clear-sky diffuse SWdn trends to increasing US air traffic as has been done in previous studies (e.g., Long et al, 2009;Augustine and Dutton, 2013).…”

“…We extend previous analyses of SURFRAD trends (Long et al, 2009;Augustine and Dutton, 2013;Gan et al, 2014) by using more recent observations and by focusing on two central and eastern US sites, where emission controls have had the largest influence on AOD. To gain knowledge of the potential influence of changing AOD on regional meteorology, we apply the observed AOD and cirrus cloud variables to a radiative transfer model and a simple statistical model.…”

“…After eliminating cloudy measurements using various selection criteria, the power law model is refit following an iterative process until its coefficients converge, giving an estimate of clear-sky fluxes. A weakness of this fitting algorithm is that it may not remove the influence of thin cirrus clouds on the clear-sky flux (Long et al, 2009), and trends in cirrus cloud cover may potentially influence estimates of trends in clear-sky SWdn.…”

Contribution of solar radiation to decadal temperature variability over land

“…Aerosol loading over North America was relatively light (35) and rather stable during the past few decades (8). Observations at six stations in the United States showed that R s significantly increased from 1995 to 2007 (36,37), primarily in the 1990s (38).…”