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Do Polluted Clouds Have Sharper Cloud Edges?

Do Polluted Clouds Have Sharper Cloud Edges?. Christine Chiu, Julian Mann, Robin Hogan University of Reading Alexander Marshak, Warren Wiscombe NASA Goddard Space Flight Center Graham Feingold, Allison McComiskey NOAA Earth System Research Laboratory.

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Do Polluted Clouds Have Sharper Cloud Edges?

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  1. Do Polluted Clouds Have Sharper Cloud Edges? Christine Chiu, Julian Mann, Robin Hogan University of Reading Alexander Marshak, Warren Wiscombe NASA Goddard Space Flight Center Graham Feingold, Allison McComiskey NOAA Earth System Research Laboratory

  2. Importance of understanding aerosol-to-cumulus transition • Cumulus clouds increase heat and moisture transport from the surface to the free troposphere, and strongly influence atmospheric state variables and cloud cover. • It remains challenging to model their transitions to stratocumulus or deep convection • Cumulus clouds are also strongly affected by ambient aerosols

  3. Clouds in more polluted air have sharper edges • due to faster evaporation of the smaller cloud droplets clean (100 cm-3) polluted (1000 cm-3) 6.4 km 6.4 km 0 km 6.4 km 0 km 6.4 km Koran et al. (2009) courtesy of G. Feingold liquid water path (g/m2) 1 10 100 1000

  4. Our aim • To investigate whether observations support this finding that clouds in polluted environments have sharper cloud edges

  5. Defining cloud edge width as the distance from clear sky to cloud optical depth of 2 clean polluted 6.4 km 6.4 km 0 km 6.4 km 0 km 6.4 km cloud optical depth 2 3

  6. Mean difference in cloud edge width between clean and polluted cases is statistically significant cloud edge width (m) significant mean=120 mean=80

  7. LES: 95% clouds have cloud edges within 200-m in the polluted case; 350-m in the clean case cumulative probability (%) polluted clean need high-resolution measurements cloud edge width (m)

  8. Measuring cloud edge width using cloud optical depth retrieved from 2NFOV 2-channel Narrow-Field-Of-View radiometer: 1-s, 1.2 FOV, 673 (RED) & 870 (NIR) nm 2NFOV

  9. We focus on three AMF campaigns COPS in Germany, 2007 China in 2008 Azores in 2009 and 2010 courtesy of ARM Climate Research Facility’s Photostream

  10. We focus on low, non-precipitating clouds • Radar/lidar data – cloud base height < 2 km • Microwave radiometer – liquid water path • Aerosol observing system – aerosol light scattering • Merged sounding – wind speed at cloud layers • 2NFOV – clouds should be big enough and away from each other with an at least 20-sec time interval thanks to people who involve and provide these data products

  11. Seasonal distribution of Azores cases ~ 200 cases Sc Cu Sc+Cu courtesy of Jasmine Rémillard, Pavlos Kollias

  12. Compare cloud edge width statistics between simulations and observations

  13. Cloud edge width as a function of aerosol light scattering Range of cloud edge distance (m) significant not significant 235 170 110 120 85 80 80 78 y-axis: Range of cloud edge width (m) observations: threshold in aerosol light scattering 50 Mm-1

  14. COPS: 95% clouds have cloud edges within250-m in polluted cases; 350-m in clean cases COPS cumulative probability (%) polluted clean LES cloud edge width (m)

  15. Azores: 95% clouds have cloud edges within 300-m in polluted cases; 450-m in clean cases Azores cumulative probability (%) polluted clean LES Azores cloud edge width (m)

  16. Summary • Observations show that clouds in more polluted air have sharper cloud edges, but statistical significance is not always met • The distribution of cloud edge width in simulations is similar to that observed in COPS, not Azores • We plan to relax criteria of case selections to allow us to stratify cases based on meteorological factors

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