Estimating cloud field coverage using morphological analysis

Abstract

The apparent cloud-free atmosphere in the vicinity of clouds (‘the twilight zone’) is oftenaffected by undetectable weak signature clouds and humidified aerosols. It is suggested hereto classify the atmosphere into two classes: cloud fields, and cloud-free (away from a cloudfield), while detectable clouds are included in the cloud field class as a subset. Since thedefinition of cloud fields is ambiguous, a robust cloud field masking algorithm is presentedhere, based on the cloud spatial distribution. The cloud field boundaries are calculated thenon the basis of the Moderate Resolution Imaging Spectroradiometer (MODIS) cloudmask 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 AtlanticOcean during July is 53%, the cloud field fraction may reach 97%, suggesting that cloudfield properties should be considered in climate studies. A comparison between aerosoloptical depth values inside and outside cloud fields reveals differences in the retrievedradiative properties of aerosols depending on their location. The observed mean aerosoloptical depth inside the cloud fields is more than 10% higher than outside it, indicatingthat such convenient cloud field masking may contribute to better estimations of aerosoldirect and indirect forcing.