Radiative properties of humidified aerosols in cloudy environment

R. Z. Bar-Or, I. Koren*, O. Altaratz, E. Fredj

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

38 Scopus citations


The remotely sensed aerosol retrievals in the transition zone between clouds and clear sky ("the twilight zone") are affected by aerosol humidification, undetectable clouds, and complex cloud radiative 3D effects. Recent studies that have estimated this total effect, found a strong exponential dependence of the aerosol retrievals on the distance from the nearest cloud, up to 30. km. In this study, we estimate the net effect of aerosol humidification on the aerosol optical depth (AOD) and aerosol fine-mode fraction (FMF). For this purpose, a new parameterization of the relative humidity (RH) as a function of the distance from the nearest cloud is presented, and calculated for shallow warm Cumulus cloud fields, based on large eddy simulation results. The results show an exponential increase in the relative humidity near clouds, from its background values (far from clouds), with an e-fold exponential distance scale of 90-300. m from the cloud edge. This finding suggests that at least for warm Cumulus cloud fields, the variations of the mean RH values are negligible at distances larger than ~. 0.5. km from clouds, when taking into account humidification effects and therefore, the total effect of the twilight zone on aerosol retrievals is not dominated by aerosol humidification at distances of 0.5-30. km from clouds.Closer examination of the aerosol humidification effect on aerosol retrievals near clouds is then presented, using a radiative transfer model (SHDOM). We estimate the sensitivity of humidified aerosol retrievals to the physical aerosol properties. We show that the humidified aerosol optical depth (AOD) is constantly increasing near clouds, for all aerosol types and size distributions, due to aerosol hygroscopic growth. On the contrary, the humidified aerosol fine-mode fraction (FMF) is found to be sensitive to the aerosol physical properties, showing variant shapes near clouds.

Original languageAmerican English
Pages (from-to)280-294
Number of pages15
JournalAtmospheric Research
StatePublished - 15 Nov 2012
Externally publishedYes

Bibliographical note

Funding Information:
This research was supported in part by the Israel Science Foundation (grant no. 1172\10 ), and by the Israel Ministry of Science and Technology (grant no. 3–8163 ). The authors would like to thank Graham Feingold for useful and engaging discussions, and the two anonymous reviewers for providing constructive comments.


  • Aerosols
  • Cloud field
  • Clouds
  • Radiative transfer
  • Remote sensing


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