Aerosol Effects on Cloud Cover As Evidenced By Ground-Based and Space-Based Observations at Five Rural Sites in the United States

Previous studies of the second aerosol indirect (lifetime) effect on cloud cover have estimated the strength of the effect without correcting for near-cloud contamination and other confounding factors. Here we combine satellite-based observations with a multi-year ground-based data set across five rural locations in the United States to more accurately constrain the second indirect aerosol effect and quantify aerosol effects on radiative forcing. Results show that near-cloud contamination accounts for approximately 40% of the satellite-derived aerosol-cloud relationship. When contamination is removed and the effect of meteorological covariation is minimized, a strong physical aerosol effect on cloud cover remains. Averaged over all stations and after correcting for contamination, the daytime solar and total (solar + IR) radiative forcing is -52 W/m2 and -19 W/m2 , respectively, due to both direct and indirect aerosol effects for aerosol optical depths (τ) between 0 and 0.3.  Averaged diurnally, the average total radiative forcing is +16 W/m2.  This work was published in Geophysical Research Letters in 2016.