title: Black carbon solar absorption suppresses turbulence in the atmospheric boundary layer.
abstract: The introduction of cloud condensation nuclei and radiative heating by sunlight-absorbing aerosols can modify the thickness and coverage of low clouds, yielding significant radiative forcing of climate. The magnitude and sign of changes in cloud coverage and depth in response to changing aerosols are impacted by turbulent dynamics of the cloudy atmosphere, but integrated measurements of aerosol solar absorption and turbulent fluxes have not been reported thus far. Here we report such integrated measurements made from unmanned aerial vehicles (UAVs) during the CARDEX (Cloud Aerosol Radiative Forcing and Dynamics Experiment) investigation conducted over the northern Indian Ocean. The UAV and surface data reveal a reduction in turbulent kinetic energy in the surface mixed layer at the base of the atmosphere concurrent with an increase in absorbing black carbon aerosols. Polluted conditions coincide with a warmer and shallower surface mixed layer because of aerosol radiative heating and reduced turbulence. The polluted surface mixed layer was also observed to be more humid with higher relative humidity. Greater humidity enhances cloud development, as evidenced by polluted clouds that penetrate higher above the top of the surface mixed layer. Reduced entrainment of dry air into the surface layer from above the inversion capping the surface mixed layer, due to weaker turbulence, may contribute to higher relative humidity in the surface layer during polluted conditions. Measurements of turbulence are important for studies of aerosol effects on clouds. Moreover, reduced turbulence can exacerbate both the human health impacts of high concentrations of fine particles and conditions favorable for low-visibility fog events.
authors.affiliation: Division of Atmospheric Sciences, Desert Research Institute, Reno, NV 89512; Eric.Wilcox@dri.edu r.thomas@bham.ac.uk vramanathan@ucsd.edu.
authors.forename: Eric M
authors.initials: EM
authors.lastname: Wilcox
authors.affiliation: Center for Clouds, Chemistry and Climate, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093; School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, United Kingdom; Eric.Wilcox@dri.edu r.thomas@bham.ac.uk vramanathan@ucsd.edu.
authors.forename: Rick M
authors.initials: RM
authors.lastname: Thomas
authors.affiliation: Center for Clouds, Chemistry and Climate, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093; International Centre for Integrated Mountain Development, Kathmandu, Nepal.
authors.forename: Puppala S
authors.initials: PS
authors.lastname: Praveen
authors.affiliation: Center for Clouds, Chemistry and Climate, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093; Ames Research Center, Universities Space Research Association, National Aeronautics and Space Administration (NASA), Moffett Field, CA 94035.
authors.forename: Kristina
authors.initials: K
authors.lastname: Pistone
authors.affiliation: Department of Meteorology, Stockholm University, 106 91 Stockholm, Sweden; Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden.
authors.forename: Frida A-M
authors.initials: FA
authors.lastname: Bender
authors.affiliation: Center for Clouds, Chemistry and Climate, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093; Eric.Wilcox@dri.edu r.thomas@bham.ac.uk vramanathan@ucsd.edu.
authors.forename: Veerabhadran
authors.initials: V
authors.lastname: Ramanathan
keywords: aerosols; atmospheric turbulence; autonomous unmanned aerial vehicles; cloud cover; radiative forcing