摘要：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.
