Strong aerosol effects on cloud amount based on long-term satellite observations over the East Coast of the United States
<p>Here we use 16-year satellite and reanalysis data in combination with a multivariate regression model to investigate how aerosols affect cloud fraction (CF) over the East Coast of the United States. Cloud droplet number concentrations (N<sub>d</sub>), cloud geometrical thickness, lower tropospheric stability, and relative humidity at 950 hPa (RH<sub>950</sub>) are identified as major cloud controlling parameters that explain 97% of the variability in CF. N<sub>d</sub> is shown to play an important role in regulating the dependence of cloud fraction on RH<sub>950</sub>. The observed annual-mean CF shows no significant trend due to the cancelation from the opposite trends in N<sub>d</sub> and RH<sub>950</sub>. The multivariate regression model revealed that the decline in N<sub>d</sub> alone would lead to a about 20% relative decline in CF. Our results indicate the significant aerosol effects on CF and suggest the need to account for pollution-induced cloud changes in quantifying cloud feedback based on long-term observations.</p>