1. 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 2 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 3 1 liquid water path (g/m 2 ) due to faster evaporation of the smaller cloud droplets 101001000 courtesy of G. Feingold clean (100 cm -3 ) 0 km6.4 km polluted (1000 cm -3 ) 0 km6.4 km Koran et al. (2009)

4. Our aim 4 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 5 polluted cloud optical depth 23 0 km6.4 km clean 0 km6.4 km

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

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

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

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

10. We focus on low, non-precipitating clouds 10 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 11 ~ 200 cases Cu Sc Sc+Cu courtesy of Jasmine Rémillard, Pavlos Kollias

12. Compare cloud edge width statistics between simulations and observations 12

13. Cloud edge width as a function of aerosol light scattering 13 Range of cloud edge distance (m) 110 80 120 80 235 170 85 78 LESCOPSAzoresChina 7300580014514013010413 y-axis: Range of cloud edge width (m) observations: threshold in aerosol light scattering 50 Mm -1 significantnot significant

14. COPS: 95% clouds have cloud edges within 250-m in polluted cases; 350-m in clean cases 14 COPS cumulative probability (%) polluted clean cloud edge width (m) LES

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

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 16