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Retrieval of cloud droplet size from visible and microwave radiometric measurements during INDOEX: Implication to aerosols’ indirect radiative effect

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Abstract
  • The effective radius of water cloud droplets is retrieved using remotely sensed passive microwave and visible data collected by aircraft during the Indian Ocean Experiment. The purpose of this study is to assess the aerosols’ effect on cloud microphysical and radiative properties. To study this effect, we investigate the relationships among effective radius, liquid water path and number concentration of cloud droplets. The effective radius retrieval uses imager observations of reflected sunlight at 0.64 mm and liquid water path derived frommicrowave measurements. Results of an error analysis show that retrieval error is the largest for thin clouds having small visible reflectances and small liquid water path. For this reason, only pixels with visible reflectances greater than 0.2 are used in our data analysis, so that the maximum RMS error in effective radius is limited to about 4 mm. The relation between liquid water path and effective radius is examined for four different latitudinal regions. Results show that for the same liquid water path, effective radii are significantly smaller in the north than in the south, in correspondence to the north-south gradient of aerosol concentration in this region. In situ aircraft observations reveal larger cloud droplet number concentrations in the north than in the south. The north-south gradient of these variables are consistent with the aerosols’ effect on cloud microphysics, that is, higher aerosol concentration increases the number concentration of cloud droplets, which, in turn, reduces droplet sizes given the same liquid water path and cloud thickness. Results based on comparison between data collected from northern and southern hemispheres suggest that the increase in aerosol number concentration alters cloud droplet numbers and sizes while leaving liquid water contents approximately the same.
  • Keywords: aerosol indirect effect, liquid water path, Indian Ocean Experiment, cloud effective radius
  • Keywords: aerosol indirect effect, liquid water path, Indian Ocean Experiment, cloud effective radius
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Citation
  • Liu, G., H. Shao, J. A. Coakley Jr., J. A. Curry, J. A. Haggerty, and M. A. Tschudi, Retrieval of cloud droplet size from visible and microwave radiometric measurements during INDOEX: Implication to aerosols’ indirect radioactive effect, J. Geophys. Res., 108(D1), 4006, doi:10.1029/2001JD001395, 2003.
Journal Title
Journal Volume
  • 108
Journal Issue/Number
  • D1
Academic Affiliation
Déclaration de droits
Funding Statement (additional comments about funding)
  • This research has been supported by NSF Grant ATM-0002860 and NASA Grant NAG5-8647. J. Curry’s participation was supported by the NASA FIRE project. J. Coakley’s participation was supported by NSF through the Center for Clouds, Chemistry, and Climate (C4), a national science and technology center at the Scripps Institution of Oceanography and by NSF Grant ATM-9612886.
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  • 0148-0227

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