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

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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. - PowerPoint PPT Presentation

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Do Polluted Clouds Have Sharper Cloud Edges?

Christine Chiu, Julian Mann, Robin Hogan University of Reading

Alexander Marshak, Warren WiscombeNASA Goddard Space Flight Center

Graham Feingold, Allison McComiskeyNOAA Earth System Research Laboratory

Importance of understanding aerosol-to-cumulus transition

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• 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

Clouds in more polluted air have sharper edges

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1liquid water path (g/m2)

• due to faster evaporation of the smaller cloud droplets

10 100 1000courtesy of G. Feingold

clean (100 cm-3)

0 km 6.4 km

6.4 kmpolluted (1000 cm-3)

0 km 6.4 km

6.4 km

Koran et al. (2009)

Our aim

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

Defining cloud edge width as the distance from clear sky to cloud optical depth of 2

5

polluted

cloud optical depth 2 3

0 km 6.4 km

6.4 kmclean

0 km 6.4 km

6.4 km

0

100

200

300

400

Clean Polluted

Mean difference in cloud edge width between

clean and polluted cases is statistically significant

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cloud edge width (m)

mean=120mean=80

significant

0 100 200 300 400 50010

20

30

50

70

80

90

95

LES: 95% clouds have cloud edges within 200-m in the polluted case; 350-m in the clean case

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cumulative probability (%)

polluted clean

cloud edge width (m)

need high-resolution

measurements

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

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Azores in 2009 and 2010

China in 2008

COPS in Germany, 2007

We focus on three AMF campaigns

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courtesy of ARM Climate Research Facility’s Photostream

We focus on low, non-precipitating clouds

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• 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

Seasonal distribution of Azores cases

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05

101520253035

6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12Month (2009-2010)

~ 200 cases

Cu

Sc

Sc+Cu

courtesy of Jasmine Rémillard, Pavlos Kollias

Compare cloud edge width statistics between simulations and observations

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Cloud edge width as a function of aerosol light scattering

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Range of cloud edge distance (m)

0

100

200

300

400

500

LES-clean polluted COPS-clean polluted Azores-clean polluted China-clean polluted

11080

12080

235

17085

78

LES COPS Azores China

7300 5800 145 140 130 10 4 13

y-axis: Range of cloud edge width (m)observations: threshold in aerosol light scattering 50 Mm-1

significant not significant

0 100 200 300 400 50010

20

30

50

70

80

90

95

COPS: 95% clouds have cloud edges within250-m in polluted cases; 350-m in clean cases

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COPS

cumulative probability (%)

polluted

clean

cloud edge width (m)

LES

0 100 200 300 400 50010

20

30

50

70

80

90

95

LES

Azores: 95% clouds have cloud edges within 300-m in polluted cases; 450-m in clean cases

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Azores

cumulative probability (%)

polluted

clean

cloud edge width (m)

Azores

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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