Estimating cloud field coverage using morphological analysis

Rotem Z. Bar-Or, Ilan Koren, Orit Altaratz

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


The apparent cloud-free atmosphere in the vicinity of clouds ('the twilight zone') is often affected by undetectable weak signature clouds and humidified aerosols. It is suggested here to classify the atmosphere into two classes: cloud fields, and cloud-free (away from a cloud field), while detectable clouds are included in the cloud field class as a subset. Since the definition of cloud fields is ambiguous, a robust cloud field masking algorithm is presented here, based on the cloud spatial distribution. The cloud field boundaries are calculated then on the basis of the Moderate Resolution Imaging Spectroradiometer(MODIS) cloud mask products and the total cloud field area is estimated for the Atlantic Ocean (50°S-50°N). The findings show that while the monthly averaged cloud fraction over the Atlantic Ocean during July is 53%, the cloud field fraction may reach 97%, suggesting that cloud field properties should be considered in climate studies. A comparison between aerosol optical depth values inside and outside cloud fields reveals differences in the retrieved radiative properties of aerosols depending on their location. The observed mean aerosol optical depth inside the cloud fields is more than 10% higher than outside it, indicating that such convenient cloud field masking may contribute to better estimations of aerosol direct and indirect forcing.

Original languageAmerican English
Article number014022
JournalEnvironmental Research Letters
Issue number1
StatePublished - 2010
Externally publishedYes


  • Aerosol
  • Cloud
  • Cloud field
  • Remote sensing
  • Spatial analysis


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